Оптические диэлектрические наноантенны на основе наноалмазов с азотно-вакансионными центрами окраски тема диссертации и автореферата по ВАК РФ 01.04.05, кандидат наук Залогина Анастасия Сергеевна

• Залогина Анастасия Сергеевна
• кандидат наук
• 2019

Оглавление диссертации кандидат наук Залогина Анастасия Сергеевна

Введение

Глава 1 Излучение точечного источника в диэлектрической оболочке

1. 1 Центры окраски в алмазе

1.1.1. Измерение времени жизни

1.1.2 Автокорреляционная функция

1.2 Методы контроля излучения

1.2.1. Плазмонные наноструктуры

1.2.2 Фотонные кристаллы

1.3 Основные проблемы контроля излучения

1.4 Диэлектрические наноструктуры для контроля оптического излучения

1.4.1 Оптические свойства диэлектрических наноантенн

1.4.2 Материалы и способы изготовления наноструктур

1.5 Контроль излучения наноразмерных источников

1.5.1 Активная наноантенна

1.5.2 Пассивная наноантенна

Выводы по главе

Глава 2. Экспериментальные и численные методики

2.1 Аналитические методы исследования

2.2 Методы численного моделирования

2.3 Методы создания диэлектрических наноструктур

2.3.1 Синтез наноалмазов из газовой фазы

2.3.2 Высокотемпературный синтез наноалмазов

2.3.3 Метод лазерной печати

2.3.4 Электронно-лучевая литографии

2.3.5 Кристаллизация кремниевых наноантенн

2.3.6 Размещение алмазов на кремниевые наноантенны методом переноса зондом

2.3.7 Размещение алмазов на кремниевые наноантенны методом центрифугирования

2.4 Методы исследования диэлектрических наноструктур

2.4.1 Сканирующая электронная микроскопии

2.4.2 Темнопольная микроскопия

2.4.3 Фотолюминесцентная спектроскопия

2.4.4 Спектроскопия комбинационного рассеяния

2.5 Определение автокорреляционной функции второго порядка

2.6 Метод единичного подсчета фотонов с корреляцией по времени

Выводы по главе

Глава 3. Исследование активной диэлектрической наноантенны на базе резонансных алмазных частиц с NV-центрами

3.1 Концепция активной алмазной наноантенны

3.2 Теоретическая модель активной диэлектрической наноантенны

3.2.1 Зависимость спектров рассеяния и коэффициента усиления поля от размера алмазных частиц

3.2.2 Зависимость фактора Парселла от размера алмазных частиц с одиночным NV-центром

3.2.3 Зависимость фактора Парселла от размера алмазных частиц с несколькими NV-центрами

3.3 Экспериментальная реализация активной диэлектрической наноантенны

3.4 Исследование времени жизни методом подсчета единичных фотонов

Выводы по главе

4 Контроль излучения алмазных наночастиц с азотно-вакансионными центрами окраски при помощи диэлектрических и гибридных наноантенн

4.1. Пассивная наноантенна на основе сферической кремниевой наноантенны объединенной с наноалмазом

4.1.1 Моделирование системы: резонансная диэлектрическая частица и наноалмаз

4.1.2 Методы изготовления кремниевых наноантенн

4.1.3 Реализация пассивной наноантенны на основе кремниевой сферической наноантенны

4.1.4 Экспериментальное исследование системы наноалмаз-кремниевая сферическая наноантенна

4.2 Пассивная наноантенна на основе цилиндрической кремниевой наноантенны объединенной с наноалмазом

4.2.1 Теоретическое исследование пассивной наноантенны на основе кремниевой цилиндрической наноантенны

4.2.3 Изготовление пассивной наноантенны на основе цилиндрической кремниевой наноантенны и наноалмаза

4.2.4 Исследование оптических свойств пассивной наноантенны на основе цилиндрической кремниевой наноантенны и наноалмаза

4.3 Пассивная наноантенна на основе гибридной наноантенны объединенной с наноалмазом

4.3.1 Исследование управленя излучением NV-центра за счет метллодиэлектрической наноантенны, состоящей из золотого диска и кремниевой наноантенны

4.3.2 Пассивная наноантенна на основе наноалмаза и олигомеров, состоящих из гибридных наноантенн

Выводы по главе

Заключение

Список литературы

Приложение A Свойства центров окраски в наноалмазе

Приложение Б Диаграммы направлености для системы: наноалмаз и

диэлектрическая наночастица

Приложение В Геометрические параметры олигомера

Приложение Г (обязательное) Тексты публикаций

Введение диссертации (часть автореферата) на тему «Оптические диэлектрические наноантенны на основе наноалмазов с азотно-вакансионными центрами окраски»

Реферат Общая характеристика работы

Актуальность темы. В условиях быстроразвивающихся информационных и коммуникационных технологий требуется разработка принципиально новых подходов к созданию систем обработки информации и реализация концепции полностью оптической передачи информации [1]. Трудности при создании таких инфокоммуникационных систем чаще всего вызваны фундаментальными проблемами: дифракционный предел, неконтролируемое взаимодействие квантовой системы с окружением, сложность управления излучением на наномасштабе. Поэтому создание нанофотонных систем, которые позволят эффективно управлять излучением наноразмерных источников, без сомнения является важной научной задачей мирового уровня, решение которой обеспечит качественный скачок в развитии оптических технологий и позволит создать многофункциональные устройства нанофотоники, основанные на распространении и взаимодействии оптических сигналов, в том числе оптические чипы, системы связи и квантовые компьютеры.

Диссертационная работа посвящена одному из актуальных направлений нанофотоники - исследованию резонансных диэлектрических наночастиц [2,3]. Оптически-индуцированные электрические и магнитные Ми резонансы в диэлектрических наночастицах с высоким показателем преломления открывают новые возможности для усиления многих оптических эффектов благодаря резонансному характеру рассеяния света в видимом диапазоне, отсутствию потерь на оптических частотах и возможности управления диаграммой направленности [4-6]. Важным этапом в развитии этого направления является разработка наносистем, обеспечивающих усиление и контроль излучения наноразмерного

оптического источника посредством резонансной связи с локализованными модами. Размещение оптических излучателей вне резонаторов и использование дополнительных диэлектрических структур может быть полезно в разработке различных приложений для магнитометрии и визуализации. Также известно, что Ми резонансы диэлектрических наночастиц возникают внутри структур в отличие от поверхностных плазмонных мод, поля которых локализованы на поверхности. Поэтому размещение оптических излучателей внутри диэлектрических наноструктур обеспечит дополнительные возможности для контроля и усиления их излучения.

Наиболее перспективным кандидатом на роль оптического излучателя для нанофотонных инфокоммуникационных устройств, является центр окраски в наноразмерном алмазе (наноалмазе). Алмаз является высокоиндексным диэлектрическим материалом (п ~ 2.4), в котором может быть создано множество излучающих центров (более 500 типов) с частотами излучения, охватывающими область от видимого до ближнего инфракрасного диапазона [7,8]. В настоящее время наиболее исследованными являются центры азот-вакансия (NV-центры), которые применяются, например, в квантовой физике [9,10], биомедицине [11] и магнитометрии [12,13]. При этом, МУ-центры в алмазе имеют большое значение как источники одиночных фотонов, которые обеспечивают излучение бесфононной линии (БФЛ) в спектре фотолюминесценции при 637 нм даже при комнатной температуре. Однако для таких источников одиночных фотонов существует проблема эффективного сбора излучения, так как при комнатной температуре БФЛ содержит около 4% люминесценции всей системы. Недавно было продемонстрировано, что алмазные наночастицы поддерживают собственные моды [14]. Поэтому потенциально такие наночастицы могут применяться для контроля излучения центров окраски. При этом также существует возможность управления излучением

нерезонансных наноалмазов при их объединении с резонансными диэлектрическими наноструктурами из других материалов.

Таким образом, в основе диссертации лежат две идеи: создание и исследование пассивной наноантенны, состоящей из нерезонансного наноалмаза с NV-центрами и кремниевой наноантенны, а также разработка концепции активной диэлектрической наноантенны, состоящей из оптически резонансного наноалмаза с интегрированными оптическими излучателями (NV-центрами) для эффективного усиления и контроля излучения последних.

Целью работы является разработка оптических диэлектрических наноантенн для управления излучением наноалмазов с азотно-вакансионными центрами окраски. Для достижения поставленных целей необходимо решить следующие задачи:

1. Разработка концепции и теоретическое исследование активной алмазной наноантенны с интегрированными множественными излучателями (NV-центрами).

2. Экспериментальная реализация активной алмазной наноантенны и изучение возможности управления временем жизни люминесценции NV-центров в такой системе.

3. Разработка пассивной диэлектрической наноантенны, состоящей из резонансной кремниевой наночастицы, объединенной с алмазной наночастицей с одиночным или множественными NV-центрами.

4. Экспериментальная реализация разработанных пассивных диэлектрических наноантенн и исследование их свойств.

Научная новизна работы заключается в том, что в работе изучены и описаны следующие эффекты:

1. Показана возможность увеличения мощности излучения множественных МУ-центров в алмазных сферических частицах за счет оптически индуцированных электрических и магнитных резонансов, которые эффективно локализуют возбуждающее электромагнитное поле;

2. Продемонстрирован эффект уменьшения времени жизни люминесценции множественных NV-центров в алмазных частицах обладающих резонансами Ми по сравнению с нерезонансными алмазными наночастицами с NV-центрами;

3. Обнаружено, что взаимодействие одиночных NV-центров в наноалмазах с резонансными кремниевыми наночастицами позволяет уменьшить время жизни их люминесценции;

4. Обнаружено увеличение интенсивности излучения NV-центров в нерезонасной алмазной наночастице при ее расположении в ближней волновой зоне кремниевой наноантенны.

Теоретическая и практическая значимость состоит в разработке способов управления оптическим излучением наноразмерных источников при помощи диэлектрических резонансных наноструктур. Результаты теоретических исследований, полученные в ходе выполнения диссертации, объясняют влияние Ми резонансов алмазных наночастиц и кремниевых наноантенн на направленность излучения и изменение скорости спонтанного излучения NV-центров. Предложенные в работе концепции активной и пассивной наноантенн имеют большое значение не только для нанофотоники в целом, но и для различных областей современной науки, начиная от квантовых систем обработки информации и заканчивая оптической биовизуализацией и магнитометрией. Кроме того, такие наноантенны могут служить основой для разработки перспективных платформ, использующих другие излучающие наносистемы и материалы с высоким показателем преломления.

Защищаемые положения:

1. Наличие Ми резонансов в алмазной сферической частице приводит к увеличению мощности излучения азотно-вакансионных центров окраски с учетом усреднения по их положению, ориентации дипольных моментов и спектру люминесценции в несколько раз по сравнению с алмазными сферическими наночастицами диаметром менее 100 нм.

2. В алмазных частицах диаметром 200-1600 нм, содержащих азотно-вакансионные центры окраски и обладающих Ми резонансами, время затухания люминесценции в несколько раз меньше по сравнению со временем затухания в нерезонансных алмазных наночастицах меньшего диаметра.

3. Взаимодействие одиночных азотно-вакансионных центров окраски с оптически резонансными сферическими наночастицами из кристаллического кремния, расположенными на стеклянной подложке, может уменьшить среднее время затухания люминесценции центров окраски более чем в 2 раза.

4. При расположении наночастицы алмаза, содержащей азотно-вакансионные центры окраски, в ближней волновой зоне резонансной наноантенны из кристаллического кремния мощность люминесценции, излучаемая в телесный угол, отвечающий числовой апертуре МА = 0.95 в верхнем полупространстве, может увеличиться более чем на 50% по сравнению с мощностью, излучаемой тем же наноалмазом в отсутствие наноантенны.

Степень достоверности и апробация результатов.

Основные результаты диссертации докладывались на 9 научных международных конференциях и 4 международных школах по оптике и нанофотонике: PIERS'17 (Санкт-Петербург, Россия, 2017); Metamaterials'17 (Марсель, Франция, 2017); Metanano'18 (Сочи, Россия, 2018); МАМОР'18 (Рим, Италия, 2018), и др.

Работы по разработке численных моделей диэлектрических наноантенн и их экспериментальным исследованиям методами темнопольной и рамановской спектроскопии проводились при поддержке Министерства Образования и Науки Российской Федерации (Проект 2.2267.2017/4.6), работы по численному моделированию оптических свойств резонансного металлодиэлектрического комплекса в форме олигомера для контроля

излучения NV-центра в наноалмазе выполнялись при поддержке Гранта Президента Российской Федерации МК-3669.2019.9.

Личный вклад автора заключается в постановке задач и формулировании целей исследования, написании научных статей. Все теоретические и экспериментальные исследования проведены автором лично или при его определяющем участии.

Достоверность научных достижений. Исследования проводились с применением современных численных и экспериментальных методик. Полученные экспериментальные данные находятся в хорошем согласии с результатами проведенных теоретических исследований и не противоречат литературным данным.

Внедрение результатов работы. По результатам работы получен патент на полезную модель (Активная диэлектрическая наноантенна // Заявка на полезную модель № 201924774 дата приоритета 01.08.2019), которая может быть внедрена в оптические информационно-коммуникационные системы, устройства для детектирования магнитных полей и температуры.

Публикации. Основное содержание диссертации опубликовано в 9 статьях, из них 9 публикаций в изданиях, индексируемых в базах цитирования Web of Science и/или Scopus.

Основное содержание работы

Во введении и первой главе обоснована актуальность проведенных исследований и аргументирована научная новизна работы.

Представлен обзор литературы, на основе которого сформулировано общее понятие азотно-вакансионного центра окраски в алмазной частице, и рассмотрена концепция резонансной диэлектрической наноантенны, которая способна управлять светом на наномасштабе. Также приведена классификация основных известных методов управления излучением центров окраски в наноалмазе. По каждой из групп методов управления

проведен актуальных литературных источников, и выделены основные проблемы современного состояния по управлению излучением наномасштабных источников. Дается общее представление о диэлектрических наноструктурах для контроля излучением центров окраски в наноалмазе. Обсуждаются основные достоинства и недостатки наноантенн, а также обосновывается необходимость разработки нового типа наноантенн на основе наноалмаза с азотно-вакансионными центрами окраски.

Вторая глава диссертации посвящена изложению теоретических и экспериментальных методик. Исследование влияния резонансов в алмазных частицах и кремниевых наноантеннах на направленность и скорость спонтанного излучения NV-центров в алмазах осуществлялось с помощью численных и аналитических методов. Подробно описана технология изготовления образцов. Наноалмазы были получены с помощью метода синтеза из газовой фазы и высокотемпературного синтеза. Диэлектрические наноантенны для исследования пассивной наноантенны были изготовлены с помощью двух методов: метод лазерной печати для получения кремниевых наноантенн сферической формы и метод электронно-лучевой литографии для получения кремниевых наноантенн цилиндрической формы, нанесение наноалмазов, в свою очередь, осуществлялось с помощью метода переноса зондом и методом центрифугирования.

Для исследования морфологии образцов применялся метод сканирующей электронной микроскопии (СЭМ). Исследование оптических свойств проводилось с помощью следующих методов: темнопольная микроскопия, фотолюминесцентная спектроскопия, спектроскопия комбинационного рассеяния. Исследование источников одиночных фотонов для наноструктур, где наноалмазы с NV-центрами находятся вблизи кремниевых сферических наноантенн, проводилось с помощью измерения автокорреляционной функции второго порядка методом фотонной корреляционной спектроскопией. Измерение времени жизни для демонстрации ускорения

спонтанного излучения NV-центров осуществлялось методом единичного подсчета фотонов с корреляцией по времени.

В третьей главе теоретически обосновывается и экспериментально реализуется концепция активной наноантенны.

Оптические свойства предложенных активных наноантенн исследованы методами темнопольной микроскопии, фотолюминесцентной спектроскопии, методом единичного подсчета фотонов с корреляцией по времени, а также изложены механизмы влияния мод высокого порядка в алмазных сферах на интенсивность излучения и поля возбуждения при определенных геометрических параметрах наноантенн. Так, форма и размер наноантенны влияют на тип резонанса, причем диаметр наноалмаза должен быть > 100 нм для возбуждения первого магнитного резонанса. На рисунке 1 показана зависимость фактора Парселла от радиуса алмазной частицы.

0 -!-,-!-!-.-,-.-

0 200 400 600 800

Радиус (нм)

Рисунок 1 - Зависимость фактора Парселла, усредненного по объему сферической диэлектрической частицы с диэлектрической проницаемостью и по спектральному диапазону 620-770 нм, от ее радиуса

Для того, чтобы учесть наличие большого количества случайно ориентированных излучателей, фактор Парселла был усреднен по объему наночастицы и нормирован на спектр фотолюминесценции в спектральном диапазоне 620-770 нм. Из представленных расчетов видно, что резонансная

алмазная наночастица способна уменьшить усредненное время жизни в несколько раз по сравнению с нерезонансными частицами, размер которых менее 100 нм.

Сравнение усредненнных значений времени жизни, измеренных для всех образцов со временем жизни NV-центров в субволновых нерезонансных алмазах, взятых из литературы показывает значительную разницу. Такое сравнение представлено на рисунке 2 (а) в виде гистограммы времен жизни для 40 наночастиц алмазов.

30 25 20

15 10 5 0

Резонансные наночастицы

Нерезонансные наночастицы

Объемный наноалмаз [15]

30-90 нм [16] 50 нм [17)0 60 нм [18]#Щ

11 13 15 17 Время жизни (не)

20 40 60

Время (не)

Рисунок 2 - (а) Гистограмма значений времени жизни для 40 алмазных частиц с различными спектральными положениями резонансов относительно спектра люминесценции NV-центра. Маркер в виде звезды показывает время

жизни NV-центра в объемном алмазе [15]. Маркеры в виде заполненных цветных кругов представляют литературные данные о типичных значениях времен жизни для МУ-центров в нерезонансных алмазах с размерами 30-90 нм, 50 нм и 60 нм [16-18]. (б) Измерения затухания фотолюминесценции с временным разрешением для резонансных алмазных

частиц с МУ-центрами

Время жизни в объемном алмазе и в субволновых алмазах (взятых из соответствующих ссылок) отмечено на том же рисунке звездой и заполненными кругами, соответственно. В то время как отрицательно

заряженные NV центры в оптически малых (<100 нм) алмазных частицах на стеклянной подложке согласно литературе [16-18] показывают время жизни не менее 20-25 нс, время жизни излучателей в больших резонансных частицах, измеренных в данной работе, находится в диапазоне 9-17 нс, что в 1,5-3 раза короче.На рисунке 2 (б) представлены кривые затухания, демонстрирующие ускорение спонтанного излучения NV-центров в два раза для резонансной и нерезонансной алмазной частицы с NV-центрами.

Отличие между экспериментальными данными и теоретическими результатами вызвано несовершенством измеренных образцов алмазных частиц с NV-центрами. Стоит отметить тот факт, что основные результаты достигнуты за счет резонансных свойств самого алмаза без дополнительных наноструктур.

В 4 главе изложена концепция управления излучением азотно-вакансионного центра окраски в наноалмазе с помощью дополнительной диэлектрической наноантенны. Предлагается, а также исследуется аналитически и при помощи численного моделирования, дизайн пассивной наноантенны, состоящей из кремниевой наноантенны и наноалмаза, содержащего NV-центры. Исследованы оптические свойства предложенных пассивных наноантенн, а также изложены физические механизмы формирования сильного электрического и магнитного отклика при определенных геометрических параметрах наноантенн, положении и ориентации КУ-центра. Были рассмотрены две формы диэлектрических наноантенн: сферическая (вставка на рисунке 3(г)) и цилиндрическая (рисунок 5(а)). Сферические кремниевые наноантенны были изготовлены методом лазерной печати при помощи фемтосекундного лазера на длине волны 1050 нм.

Рисунок 3 - Экспериментальные кривые, описывающие (а), (в) автокорреляционную функцию второго порядка и (б), (г) затухание люминесценции для наноалмаза с одиночным NV-центром (логарифмическая шкала по вертикали) (а), (б) в отсутствии кремниевой наноантенны и (в), (г)

вблизи кремниевой наноантенны. Вставки на (б) и (г): схематические изображения наноалмаза с единичным NV-центром в отсутствии и вблизи кремниевой наноантенны, соответственно

Высокий показатель преломления кремния позволяет эффективно локализовать электрическое поле и усиливать оптические свойства NV-центра в наноалмазе. На рисунках 3 (б,г) продемонстрированы кривые затухания фотолюминесценции для наноалмаза размером 40-60 нм с единичными NV-центром вблизи сферической кремниевой наноантенной размером 140-160 нм (рисунок 3 (г)) и в отсутствии кремниевой наноантенны (рисунок 3 (б)). Время жизни для наноалмаза со сферической наночастицей составило 12.5 нс, в то время как без сферической наночастицы 21 нс.

Объем выборки при измерении скорости затухания люминесценции составил 52 наноалмаза с кремниевой наноантенной и 17 наноалмазов без кремниевой наноантенны (рисунок 4). Как видно из рисунка 4 наноалмазы с одиночными ^У-центрами, объединенные с кремниевыми сферическими наноантеннами, имеют время жизни более чем в 2 раза меньше по сравнению с алмазами без кремниевых наноантенн. Полученные экспериментальные данные хорошо согласуется с проведенным численным моделированием и аналитическими расчетами.

5 10 15 20 25 30 35 40 5 10 15 20 25 30 35 40

Время жизни (не) Время жизни (не)

Рисунок 4 - Гистограммы, сравнивающая значения времен жизни для алмаза с одиночным центром окраски без (а) и с (б) кремниевой

наноантенной

На следующем этапе была разработана и изготовлена наноструктра пассивной наноантенны на основе цилиндрической кремниевой наноантенны (рисунок 5 (а)). Было проведено сравнение двух материалов для цилиндрической наноантенны и выбран кремний как наиболее подходящий материал для исследования ^У-центров в наноалмазе, где БФЛ центра окраски находится на длине волны 637 нм. Такая структура способна реализовать усиление интенсивности излучения и ускорение спонтанного излучения ^У-центра.

Рисунок 5 - (а) Схематическое представление исследования оптических свойств наноструктуры, состоящей из наноалмаза и сферической кремниевой наноантенны. (б) Экспериментально полученные кривые насыщения для наноалмаза и наноалмаза вблизи кремниевой наноантенны (в), (г). Экспериментально измеренные кривые затухания люминесценции для наноалмаза с NV-центрами в отсутствии (в) и вблизи (г) кремниевой наноантенны. (д), (е) СЭМ-изображения, иллюстрирующие метод размещения наноалмаза с помощью зонда к кремниевому цилиндру

Так, по результатам теоретических расчетов, усредненный по ориентации и положению точечного источника излучения фактор Парселла составил 1.8. Были проведены исследования оптических свойств предложенной наноструктуры: получены спектры рассеяния с помощью метода темнопольной микроскопии, спектры фотолюминесценции, спектры комбинационного рассеяния. Также были проведены исследования морфологии образца с помощью сканирующей электронной микроскопии. На рисунке 5 (д) представлена микрофотография, на которой изображена цилиндрическая наноантенна с диаметром 269 нм и высотой 223 нм, слева от которой расположен наноалмаз диаметром 160 нм. Также были проведены измерения времен жизни и насыщения (рисунок 5 (б-г)).

При исследовании наноструктур методом единичного подсчета фотонов с корреляцией по времени не было отмечено изменения времени жизни при добавлении цилиндрической наноантенны. Однако было обнаружено изменение интенсивности сигнала, так, на рисунке 5 (б) показано, что количество отсчетов в секунду для наноалмаза с несколькими КУ-центрами без диэлектрической наноантенны составило 1.03^ 106, в то время как для наноалмаза с цилиндрической наноантенной количество отсчетов в секунду составило 1.6^ 106. Таким образом, экспериментально было продемонстрировано, что цилиндрическая кремниевая наноантенна позволяет увеличить количество отсчетов от точечных источников в наноалмазе, собираемых воздушным объективом в угол 72.1°, в 1.6 раз по сравнению с наноструктурой, не содержащей диэлектрической наноантенны.

Основные результаты работы

В Заключении обобщены основные результаты работы:

1. Установлено, что наличие Ми резонансов в алмазной сферической частице приводит к увеличению скорости спонтанного излучения КУ-центров в несколько раз по сравнению с алмазными частицами диаметром менее 100 нм.

2. Показано, что алмазные частицы диаметром от 200 до 1600 нм, содержащие множественные КУ-центры и обладающие Ми резонансами, имеют время затухания люминесценции в несколько раз меньше по сравнению со временем затухания люминесценции в нерезонансных алмазных частицах меньшего диаметра.

3. Определено, что взаимодействие одиночных КУ-центров в наноалмазе с оптически резонансными сферическими наночастицами из кристаллического кремния уменьшает среднее время жизни затухания люминесценции КУ-центров в более чем два раза.

4. Показано, что взаимодействие множественных NV-центров в наноалмазе с оптически резонансными цилиндрическими наночастицами из кристаллического кремния увеличивает мощность люминесценции, излучаемую в телесный угол, отвечающий числовой апертуре NA=0.95 в верхнее полупространство, более чем на 50% по сравнению с мощностью, излучаемой тем же наноалмазом в отсутствии кремниевой наноантенны.

Основные результаты диссертации представлены в следующих публикациях, входящих в международные реферативные базы данных и системы цитирования (Scopus, Web of Science):

[A1] Sun Y., Sinev I., Zalogina A., Ageev E., Shamkhi H., Komissarenko F., Morozov I., Lepeshov S., Milichko V., Makarov S., Mukhin I. and Zuev D. Citation for: Reconfigurable Near-field Enhancement with Hybrid Metal-Dielectric Oligomers, Laser & Photonics Reviews, 1800274. - 2019.

[A2] Zalogina A., Saveliev R., Ushakova E., Zograf G., Komissarenko P., Milichko V., Makarov S., Zuev D.A. and Shadrivov I. Purcell effect in active diamond nanoantennas, Nanoscale, 8721-8727, - 2018.

[A3] Makarov S.V., Zalogina A.S.,Tajik M., Zuev D.A., Rybin M.V., Kuchmizhak A.A., Juodkazis S. and Kivshar Yu. Light-induced tuning and reconfiguration of nanophotonic structures, Laser & Photonics Reviews, 1700108. - 2017.

[A4] Zalogina A.S., Benemetskiy F.A., Pidgayko D., Kapitanova P., Yaroshenko V.V., Makarov S.V., Zuev D.A. and Shadrivov I.V. Emission rate control in nanodiamonds with embedded nitrogen vacancy centers, Journal of Physics: Conference Series, vol. 1092, no. 1, p. 012171. IOP Publishing. - 2018.

[A5] Savelev R., Zalogina A., Kudryashov S., Ivanova A., Levchenko A., Makarov S., Zuev D., and Shadrivov I. Control of spontaneous emission rate in

luminescent resonant diamond particles, Journal of Physics Conference Series 961(1):012007, - 2018.

[A6] Zalogina A.S., Javadzade J., Zuev D.A., Savelev R.S., Vorobyov V.V., Makarov S.V., Belov P.A., Akimov A.V. and Shadrivov I.V. Effect of dipole orientation on Purcell factor for the quantum emitter near silicon nanoparticle, AIP Conference Proceedings. Vol. 1874. No. 1. AIP Publishing, 040058. - 2017.

[A7] Zalogina A.S., Zograf G.P., Ushakova E.V., Komissarenko F.E., Savelev R.S., Kudryashov S.I., Makarov S.V., Zuev D.A. and Belov P.A. Control of luminescence in resonant nanodiamonds with NV-centers, IEEE, 609-611pp. -2017.

[A8] Zalogina A.S., Zograf G.P., Savelev R.S., Zuev D.A., Belov P.A., Shadrivov I.V. Zero Phonon Line Enhancement by Mie-type Resonances of Nanodiamonds with Nitrogen-vacancy Centers, 2017 Progress In Electromagnetics Research Symposium - Spring (PIERS) - 2017.

[A9] Zalogina A.S., Savelev R.S., Zuev D.A., Belov P.A., Shadrivov I.V. Purcell Factor Enhancement by Dielectric Nanoantennas for Nanodiamonds with NV-centers, 2017 Progress In Electromagnetics Research Symposium - Spring (PIERS) - 2017.

Список цитируемой литературы:

1. Vivien L. Computer technology: Silicon chips lighten up // Nature. - 2015. -

Vol. 528. - No. 7583. - P.483.

2. Optically resonant dielectric nanostructures / A. Kuznetsov,

A. E. Miroshnichenko, M. L. Brongersma, Y. S. Kivshar et al. // Science. -

2016. - Vol. 354. - No. 6314. - P. aag2472.

3. Staude I., Schilling J. Metamaterial-inspired silicon nanophotonics // Nature

Photonics. - 2017. - Vol. 11. - No. 5. - P. 274.

4. Tuning of magnetic optical response in a dielectric nanoparticle by ultrafast

photoexcitation of dense electron-hole plasma / S. Makarov,

S. Kudryashov, I. Mukhin, A. Mozharov et al. // Nano letters. - 2015. -Vol. 15. - No. 9. - P. 6187-6192.

5. Magnetic light / A. I. Kuznetsov, A. E. Miroshnichenko, Y. H. Fu,

J. B. Zhang et al. // Scientific reports. - 2012. - Vol. 2. - P. 492.

6. Ultrafast all-optical switching with magnetic resonances in nonlinear

dielectric nanostructures / M. R. Shcherbakov, P. P Vabishchevich, A. S. Shorokhov, K. E. Chong et al. // Nano Letters. - 2015. - Vol. 15. -No. 10. - C. 6985-6990.

7. Diamond-based single-photon emitters / I. Aharonovich, S. Castelletto,

D. A. Simpson, C. H. Su et al. // Reports on progress in Physics. - 2011. -Vol. 74, - No. 7. - P. 076501.

8. Aharonovich I., Neu E. Diamond nanophotonics // Advanced Optical

Materials. - 2014. - Vol. 2. - No. 10. - P. 911-928.

9. The nitrogen-vacancy colour centre in diamond / M. W. Doherty,

N. B. Manson, P. Delaney, F. Jelezko et al. // Physics Reports. - 2013. -Vol. 528. - No. 1. - P. 1-45.

10. Electron spin contrast of Purcell-enhanced nitrogen-vacancy ensembles in nanodiamonds / S. Bogdanov, M.Y. Shalaginov, A. Akimov, A. S. Lagutchev et al. // Physical Review B. - 2017. - Vol. 96. -No. 3. -P. 035146.

11. In vivo imaging and toxicity assessments of fluorescent nanodiamonds in Caenorhabditis elegans / N. Mohan, C. S. Chen, H. H. Hsieh, Y. C. Wu et al. // - 2010. - Vol. 10. - No. 9. - P. 3692-3699.

12. G. Kucsko, P. C. Maurer, N. Y. Yao, M. N. Kubo h gp. Nanometre-scale thermometry in a living cell // Nature. - 2013. - Vol. 500. - No. 7460 -P. 54.

13. All-optical thermometry and thermal properties of the optically detected spin resonances of the NV--center in nanodiamond / T. Plakhotnik, M. W. Doherty, J. H. Cole, R. Chapman et al. // Nano letters. - 2014. -Vol. 14. - No.. - P. 4989-4996.

Список литературы диссертационного исследования кандидат наук Залогина Анастасия Сергеевна, 2019 год

- c /

570 630 690 750 Wavelength (nm)

1200

1400 1600

Raman shift (cm'1)

1800

Fig. 3 Raman spectra of a nanodiamond. Insets: left - SEM image of a typical diamond nanoparticle; right - luminescence spectrum of a nanodiamond with NV-centers.

monds that do not exhibit such resonances. To study the corre-Iation of the lifetime with the presence of the particle's resonances in the PL range, we perform both the Iifetime measurements by the time-correIated singIe photon counting method (for detaiIs, see the NumericaI methods section), and the measurements of the dark-fieId scattering spectra of seIected nanodiamonds, as shown in Fig. 4(a-c). The spectraI range of diamond photoIuminescence (580-710 nm) is shown in red. Dark fieId scattering spectra indicate that some of the measured particIes have stronger resonances in the PL range; therefore, the corresponding averaged Iifetimes are expected to decrease. Comparison of the spectra in Fig. 4(a-c) and averaged Iifetimes measured for these particIes indicate that indeed, the reIative spectraI position of the Iuminescence spectrum and resonances in the dark-fieId spectrum of the diamond particIe affect the averaged emission rate of the NV-centers: particIes with stronger resonances exhibit shorter Iife-times. However, the range of Iifetimes is quite smaII (9-16 ns)

Photoluminescence

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.(b) '/..... 12.1 ns

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Fig. 4 Dark-field scattering spectra and maps of the photoluminescence decay lifetime for nanodiamonds with NV-centers. Right: Confocal luminescence maps with pseudocolors representing lifetime: red and yellow correspond to longer lifetimes and green and blue to shorter lifetimes.

Paper

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9 11 13 15 17 25 0 20 40 60 80

Lifetime (ns) Time (ns)

Fig. 5 Lifetime measurements. (a) Lifetime histogram for 40 nanodiamonds with different spectral positions of resonances relative to the NV-center luminescence spectrum. The star shows the lifetime of the NV-center in a bulk diamond.37 The circles represent the literature data on typical lifetimes for NV-centers in nonresonant nanodiamonds with the sizes of: 30-90 nm, 50 nm, and 60 nm.34-36 A vertical coordinate for the references shouldn't be taken into account. (b) Time-resolved PL measurements for two cases of overlapping of Mie-type resonances with the NV-center luminescence spectrum.

due to the presence of multiple emitters and the broad luminescence spectrum of NV-centers as compared to line-widths of nanoantenna resonances. This qualitatively agrees with theoretical predictions made for spherical particles, which showed that for large particles the difference in average lifetimes can be of the order of 2.

Next, we compare the averaged lifetimes measured for all of our samples with the lifetimes of NV-centers in subwavelength non-resonant nanodiamonds taken from the literature. We found that the difference is more pronounced. Such a comparison is presented in Fig. 5(a) in the form of the lifetime histogram for 40 nanodiamonds. The lifetimes in a bulk diamond and in subwavelength nanodiamonds (taken from the corresponding references) are shown in the same figure with a star and solid circles, respectively. While negatively charged NV-centers in optically small (<100 nm) nanodiamonds on a glass substrate according to the literature12'34-36 exhibit the lifetime of at least 20-25 ns, the lifetime of emitters in large resonant particles, measured in this work, was in the range of 9-17 ns, which is 1.5-3 times shorter. We also notice that the lifetime of emitters in large particles is almost unaffected by the presence of a glass substrate, while it reduces in the case of subwavelength nanodiamonds. Measurements of the lifetime of NV-centers in nanodiamonds placed on a porous silica substrate (with the refractive index close to 1), carried out in ref. 35, demonstrated that the lifetime increases to more than 30 ns without a substrate. Therefore, more than threefold reduction of the lifetime of NV-centers can be achieved if one simply uses resonant diamond nanoparticles instead of the subwavelength ones.

4 Discussion and outlook

The nonlinear character in the logarithmic scale of the time dependence of the signal indicates that the lifetimes of different NV-centers vary continuously in a wide range, from

nanoseconds to tens of nanoseconds. This is due to the presence of a large number of emitters randomly oriented and approximately uniformly distributed inside the samples. These values are larger than what one would expect from Fig. 2. However, much smaller values can be achieved only for certain positions of the NV-centers inside the nanodiamond and at certain wavelengths. The experimentally studied samples include multiple NV-centers, randomly distributed over the volume of nanodiamonds. Therefore, to explain our experimental results, the averaged Purcell factor should be considered (see the ESI, section 1f). Calculations of the average Purcell factor reveal that for a large resonant diamond particle the lifetime is «5 times shorter than in subwavelength nano-diamonds, while in experiments we observe only two- or threefold decrease of lifetime (if one takes into account the effect of a substrate). Such a difference can partially stem from the non-radiative contribution in the case of subwavelength nanodi-amonds,38 which decreases the emitters' lifetime. It should also be mentioned that, in spite of a reduction of the lifetime, the PL intensity of the nanoantennas cannot be compared in our experiments, because the volumes of the diamond crystals and quantity of NV-centers differ for various nanoparticles.

The experimental demonstration of the Purcell factor enhancement with the NV-centers usually implies the fabrication of high Q-factor photonic cavity with a natural NV-center.3^40 For example, a photonic-crystal cavity can reduce the lifetime to 4 ns.24 Similar systems work well for a limited range of applications, and they are often too complex for the fabrication and integration into existing and future nanopho-tonic devices or biological objects. The active nanoantenna utilizing resonant properties of the nanodiamond itself is a relatively simple system, and it can provide higher values of the Purcell factor compared to the experimentally demonstrated values in this work.

Our experimental results demonstrate the applicability of the concept of active diamond-based nanoantennas to control the NV-center fluorescence lifetime via interaction with

Nanoscale

diamond particIe eigenmodes. AnaIyticaI caIcuIations show that high vaIues of the PurceII factor can be achieved at the ZPL waveIength that can be combined with the enhancement of the eIectricaI fieId inside the particIe at the waveIength of the nanoantenna excitation (532 nm for the NV-centers). This can be reaIized in the case of the appropriate engineering of active diamond nanoantennas (e.g. shape, size and position of a singIe NV-center within the nanodiamond). If two former parameters can be controIIed (e.g. using the verticaI orientation of the substrate during nanodiamond growth41) the engineering of the NV-center position in the nanocrystaI is a serious chaIIenge, that can be soIved potentiaIIy by Iaser writing.42 Thus, the proposed and verified concept possesses great fIexibiIity for NV-center Iuminescence controI.

5 Conclusion

We have proposed and deveIoped the concept of active aII-dieIectric nanoantennas based on nanodiamonds with embedded NV-centers, providing the Iuminescence rate enhancement. We have verified this concept for diamond nanoparticIes with sizes ranging from 300 x 300 nm to 1.5 x 2 pm, supporting Mie-type resonances. We have demonstrated experimentaIIy that the resonances of Iarge diamond particIes affect the photoIuminescence properties of the NV-centers providing acceIeration of their emission. We have observed a muI-tifoId decrease of the Iifetime of the NV-centers in the studied diamond particIes, as compared to subwaveIength nanodi-amonds, being in a good agreement with theoreticaI caIcu-Iations for the average PurceII factor in the case of muItipIe NV-centers within a nanoparticIe. A huge potentiaI for engineering, simpIicity of the design compared to existing photonic cavity systems, and appIicabiIity for a wide range of coIor centers in diamond make the active diamond nanoantenna a promising tooI for creating controIIabIe emitting eIements in the visibIe range for future nanophotonic devices, incIuding fast switching opticaI diodes for opticaI communication systems, as weII as efficient singIe photon sources for quantum information systems and bioimaging.

6 Experimental section

6.1 Numerical methods

The emission rate of an NV-center (modeIIed as a point eIectric dipoIe) inside a sphericaI diamond particIe is caIcuIated using the theory deveIoped in ref. 28. CaIcuIations of the scattering cross-section map for nanodiamonds are carried out anaIyti-caIIy by using the Mie theory. The sizes of particIes are taken from experimentaIIy measured nanodiamond sizes obtained from the SEM images.

6.2 Nanodiamond fabrication

We investigate sampIes that are prepared in the Lebedev PhysicaI Institute by growing a nanodiamond fiIm using

a pIasma-enhanced chemicaI vapour deposition (PECVD) method and subsequent miIIing. The NV-centers are incorporated into a sampIe during PECVD.43 High-resoIution morphoIogy characterization is performed by using a scanning eIectron microscope (SEM, CarI Zeiss, Neon 40). AII nanodi-amonds can be approximated by spheroids to estimate their sizes, which are found to be in the range from 300 x 300 nm to 1.5 x 2 pm.

6.3 Optical characterization

The photoIuminescence (PL), Raman and dark-fieId scattering spectra are obtained at room temperature on a confocaI spectrometer Horiba LabRam HR with a cooIed CCD camera (Andor DU 420A-0E 325). 600 g mm-1 and 150 g mm-1 diffraction gratings were appIied for Raman and PL/dark-fieId spectra registration, respectiveIy. For PL measurements, we excite the sampIes by the supercontinuum source Fianium SC400-6 with an integrated tunabIe fiIter yieIding Iaser puIses with a waveIength of 530 nm, a repetition rate of 60 MHz, a puIse duration of 6 ps, and a bandwidth of 20 nm. The He-Ne Iaser (632.8 nm) is utiIized for Raman spectroscopy. The objective Mitutoyo PIan Apo VIS (10x, NA = 0.28) for the PL excitation and Mitutoyo PIan Apo NIR (50x, NA = 0.42) are used for the PL excitation and DF signaI coIIection, respectiveIy. The objective Mitutoyo PIan Apo NIR (10x, NA = 0.26) is used in a dark-fieId scheme for sampIe iIIumination at the angIe of 67° to the surface normaI by a white Iight source (HL-2000 haIogen Iamp). The micrometer transIation stage (ThorIabs MBT616D) and atomic force microscope stage (SmartSPM AIST-NT) are used for sampIe positioning, which was monitored by using a CCD camera (Cannon 400 D).

6.4 Lifetime measurements

The measurements are carried out by the time-correIated singIe photon counting (TCSPC) method in the range of 430-700 nm. PL decay is observed on the Iaser scanning confocaI microscope MicroTime 100 (PicoQuant) equipped with an objective MPIanFL N 100x (NA = 0.9) and a picosecond puIsed diode Iaser (405 nm, 50 ps) is used for the excitation. The experimentaIIy obtained curves are fitted by a biexponen-tiaI function; exampIes of fitting parameters are presented in the tabIe in the ESI, section 4.f

Conflicts of interest

There are no confIicts to decIare.

Acknowledgements

CaIcuIations of the scattering cross-section map for diamond nanoparticIes were supported by the Ministry of Education and Science of Russian Federation (Project 2.2267.2017/4.6). The theoreticaI studies on PurceII factor and eIectricaI fieId enhancement in diamond nanoantenna as weII as experi-

Paper

mental part of this work were supported by the Russian Science Foundation (Grant 16-19-10367). The authors thank A. Ivanova, S. Kudryashov, and A. Levchenko (Lebedev Physical Institute, Moscow, Russia) for providing nanodiamond samples, and Yu. S. Kivshar for useful discussion.

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Light-Induced Tuning and Reconfiguration of Nanophotonic Structures

Sergey V. Makarov,* Anastasia S. Zalogina, Mohammad Tajik, Dmitry A. Zuev, Mikhail V. Rybin, Aleksandr A. Kuchmizhak, SauliusJuodkazis, and Yuri Kivshar

Interaction of light pulses ofvarious durations and intensities with nanoscale photonic structures plays an important role in many applications of nanophotonics for high-density data storage, ultra-fast data processing, surface coloring and sensing. A design of optically tunable and reconfigurable structures made from different materials is based on many important physical effects and advances in material science, and it employs the resonant character of light interaction with nanostructures and strong field confinement at the nanoscale. Here we review the recent progress in physics of tunable and reconfigurable nanophotonic structures of different types. We start from low laser intensities that produce weak reversible changes in nanostructures, and then move to the discussion of non-reversible changes in photonic structures. We focus on three platforms based on metallic, dielectric and hybrid resonant photonic structures such as nanoantennas, nanoparticle oligomers and nanostructured metasurfaces. Main challenges and key advantages of each of the approaches focusing on applications in advanced photonic technologies are also discussed.

S. V. Makarov, A. S. Zalogina, M. Tajik, D. A. Zuev, M. V. Rybin, Y. Kivshar

ITMO University

St. Petersburg, 197101, Russia

[e-mail]

M. V. Rybin

Ioffe Institute

St. Petersburg, 194021, Russia A. A. Kuchmizhak Far East Federal University Vladivostok, 690091, Russia A. A. Kuchmizhak

Institute ofAutomatics and Control Processes Far Eastern Branch of the Russian Academy of Science Vladivostok, 690091, Russia S. Juodkazis

Swinburne University ofTechnology Melbourne, Vic 3122, Australia

S. Juodkazis

Melbourne Centre for Nanofabrication Melbourne, Vic 3122, Australia Y. Kivshar

Nonlinear Physics Center Australian National University Canberra, ACT 2601, Australia

DOI: 10.1002/lpor.201700108

1. Introduction

Nanophotonics has exhibited a great potential for disruptive technologies aiming to complement or even replace the existing semiconductor photonics.!1-5! One of the basic elements of many advanced nanophotonic systems is a resonant nanoparticle that makes it possible to manipulate effectively light at the nanoscale, expanding many functionalities of conventional optical devices.

Optical antennas based on nanopar-ticles are utilized for many applications of modern photonics including single-molecule detection via enhanced fluorescence,!6,7! Raman scattering,!8! infrared absorption!9! as well as optoelectronic applications.!10! Specific arrangements of nanoparticles create a novel, recently emerged class of planar photonic nanostructures, metasurfaces.!11-19!

In a broader context, the recent developments of novel technologies need a substantial increase in photonic integration capacity and energy efficiency far surpassing functionalities of current photonic systems. Such dense integration can be achieved only with subwavelength structures, and they require the development oflow-cost fabrication technologies that would allow tuning and reconfigurating such structures. The development of efficient methods to change optical properties of such advanced nanostructures in a controllable manner would revolutionize modern optical technology, by creating a new type of photonic devices, metadevices.

In general, for the reconfiguration and tuning of nanoscale structures and metasurfaces, one can employ fast reversible and permanent non-reversible processes, as summarized in Figure 1. Reversible changes in nanostructures are commonly related to a variation of permittivity under laser excitation. For the low laser intensities, the optical Kerr effect and two-photon absorption take place in dielectrics. In metals, where light is absorbed linearly, the major nonlinear mechanism is the heating of free carriers that affects mostly the imaginary part of the permittivity. The effects of free carriers in dielectric structures become important when two- or multiphoton absorption yield higher densities of electron-hole pairs, which increase plasma frequency and linear absorption and lead to an avalanche ionization for larger intensities. With further increase of the excitation fluence, more

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Sergey V. Makarov received a PhD degree in laser physics in 2014 from the Lebedev Physical Institute ofthe Russian Academy of Sciences (Moscow, Russia). He worked as a visiting researcher in the Vienna Technical University (Vienna, Austria) and in 2015 joined ITMO University (St. Petersburg, Russia) as a postdoctoral fellow. Currently he is a senior research fellow. The topics of his research include nonlinear nanophotonics, resonant interaction of light with nanoparticles, nanofabrication, optical nanoanten-nas and advanced optical materials.

AnastasiaS. Zalogina received a Master ofScience degree in condensed-matter physics in 2016from the National Research Tomsk Polytechnic University (Tomsk, Russia). During her MScdegree, she worked on newzincand beryllium metal complexes. Currently, she is a PhD student in ITMO University (St. Petersburg, Russia) and carries out research in the fields of nanofabrication, nanophotonics, single-photon sources and metamaterials.

Mohammad Tajik received his Bachelor degree in materials science and engi-neeringfrom Iran University ofScience and Technology. During his BScdegree, he worked on synthesizing magnetic nanoparticles. He is currently doing his Master degree in the fields of nanophotonics and metamaterials at ITMO University (St. Petersburg, Russia). His research activity is focused on optical properties of GeSbTe alloys for application to optical nanoantennas and metamaterials.

Dmitry A. Zuev received a PhD degree in laserphysics in 2012fromthe Institute of Laser and Information Technologies ofthe Russian Academy ofSciences (Shatura, Moscow Region, Russia). Thetopicof his PhD thesis was related with development of advanced laser technologies for light-emitting devices and solarcells. In 2015 he moved to ITMO University (St. Petersburg, Russia) as a research fellow. The currenttopics of his research include all-dielectric and hybrid nanophotonics, nanofabrication and metamaterials.

Mikhail V. Rybin received a PhD degree in physics in 2009from the Ioffe Institute (St. Petersburg, Russia). In 2010, he joined the Metamaterial Laboratory at ITMO University (St. Petersburg, Russia), where currently he is a senior research fellow. The currenttopics of his research include photonic phase transitions, resonantin-teraction of light with photonic structures, Fano resonances, all-dielectric metamaterials, photonic crystals and nanoantennas.

AleksandrA. Kuchmizhak received a PhD degree in laserphysics in 2012 from the Institute of Automation and Control Processes of FEB RAS (Russia). Currently he is a postdoctoral fellow in Far Eastern Federal University. His research interests cover laser-matter interaction, including structured and vortex beams, and pulsed laser nanofabrication for advanced plasmonicand nanopho-tonic devices.

Saulius Juodkazis received a PhD degree in experimental physics and material science in 1998 jointlyfrom Vilnius University (Lithuania) and Lyon-I University (France). From then on, he held previous faculty positions at the University ofTokushima and Hokkaido University in Japan. From 2009, he is Professor of Nanophotonics and Founding Director ofthe Nanotechnol-ogy FacilityatSwinburne University, Melbourne, Australia. His current research is focused on applying principles of plas-monic light-field enhancement and its spectral control for applications in sensing, solid-state lighting and solar energy conversion. He is a Fellow ofthe Optical Society of America and the International Societyfor Optics and Photonics (SPIE).

YuriS. Kivshar received a PhD degree in theoretical physics in 1984 from the Institute for Low Temperature Physics and Engineering (Kharkov, Ukraine). From 1988 to 1993 he worked at different research centers in USA, France, Spain and Germany, and in 1993 he moved to Australia where he established the Nonlinear Physics Centerat the Australian National University (ANU), being currently Head ofthe Center and ANU Distinguished Professor. His research interests include nonlinear photonics, metamaterials and nanophotonics. He is a Fellow ofthe Australian Academy of Science, the Optical Society of America, the American Physical Society, the Institute of Physics (UK), Deputy Director of theCenterofExcellenceCUDOS (Australia) and Research Directorofthe Metamaterial Laboratory at ITMO University (St. Petersburg, Russia). He received many prestigious national and international awards includingthe Lyle Medal of the Australian Academy ofScience (Australia), the State Prize in Science and Technology (Ukraine), the Lebedev Medal of the Rozhdestvensky Society (Russia) and the Harrie Massey Medal and Prize ofthe Institute of Physics (UK).

ADVANCED

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energy is converted into heat, so that materials undergo strong temperature modulations of their optical properties. In this general picture, higher laser intensities lead to more complex effects, and longer relaxation times should be considered to achieve structural modifications. Finally, at a certain threshold some nonreversible processes become dominant shifting the potential applications from signal processing to data storage. In particular, it is possible to tune nanoantenna optical properties via modification of surrounding media due to phase transitions in materials or via reshaping of the nanoparticle geometry.

The main aim of this review paper is to showcase the rapidly growing fields of light-assisted tuning, reshaping and reconfiguration of nanoparticles, nanostructures and metasurfaces, including all-optical modulation and data storage. Our key idea is to start from low laser intensities that induce only weak reversible changes in nanostructures often used for achieving nanostruc-ture tunability. Then, increasing the laser power, we involve the physics of non-reversible reconfigurations associated with a change of the material structure and melting. We discuss contributions and importance of different physical and chemical processes which can be harnessed for various applications.

2. Background and Platforms

Subwavelength structures in nanophotonics interact with light due to a variety of resonances. Each resonant state of a nanostruc-ture can be characterized by four parameters: amplitude, phase, frequency and decay rate, which depend on size, shape, dielectric permittivity and geometry of the nanostructure. Therefore, tuning of any of these parameters can result in reconfiguration of the optical response of a nanostructure. The strength of lightmatter interaction depends substantially on the decay rate y that is defined by the half-width of the response line in the spectrum normalized to the resonant frequency &>0. This ratio is called the quality factor (or Q-factor): Q = ly/co0.

For large values, the Q-factor is approximated by the number of oscillations required for the energy of an oscillating system to decay to e ~2n (or 0.2%) of its initial energy. On the other hand, the Q-factor shows how many oscillations of the electromagnetic field one needs to form the steady-state regime of an excitation mode, which is important for transient dynamics of nanophotonic structures. One oscillation at wavelength 800 nm takes around 2.7 fs. It means that nanoresonators with Q < 103 are fast enough to potentially beat modern electronic modulators. However, structures with high Q-factors (larger than 103) face the limit dictated by the time of the mode formation rather than some material properties.

In addition, in the data storage applications, information pixels should be as compact as possible (dots-per-inch rate > 105) and cover the whole visible spectral range. Ultra-fast signal modulators in all-optical chips also should be as compact as possible to compete with electronic analogs. However, generally very small structures possess high radiative optical losses, broader resonances and smaller values of the Q-factor. Therefore, to achieve the most effective and compact designs, one should choose the proper type of nanostructures.

In this section, we discuss the properties of typical nanopho-tonic resonators and their Q-factors that can be achieved with

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modern fabrication technologies, keeping in mind their applicability for light-induced tuning and reconfiguration discussed below in this review.

Metallic nanoparticles. When a metal particle is exposed by light, the electromagnetic waves induce collective coherent oscillations of free electrons from the conduction band. Such oscillations are called surface plasmons, and their frequency is determined by a number of factors including electron density, effective electron mass, shape and size of the nanoparticle. Optical properties of metallic nanoparticles with a spherical shape are described by the Mie theory[20] that allows us to classify resonant modes by a series of electric and magnetic multipoles. The lower-order dipolar modes are of most importance since they permit the description of particle response in a low-frequency range where higher-order multipoles can be neglected. Similar multipole classification is employed for the characterization of nanoparticles with non-spherical shape.

The first resonant oscillation of electrons in a metallic nanoparticle is called the dipole plasmon resonance (or "localized surface plasmon resonance"), and it should be distinguished from plasmon excitations that can occur in a bulk metal or at a metal surface. The Q-factor for this mode is usually less than 10, due to high non-radiative ohmic losses in metals even in such popular plasmonic materials as gold, silver and aluminum. Higher modes of plasmon resonances can occur when the size of a nanoparticle becomes larger than ~100 nm: for example, a quadrupole mode where a half of the electron cloud moves parallel to an applied field and the other half moves antiparallel. This mode should possess higher values of the Q-factor due to weaker coupling with light in a surrounding medium. However, high ohmic losses in metals do not allow Q sufficiently exceeding 10. Another limitation for metallic nanoparticles is a difficulty to achieve strong magnetic response in the visible range, requiring the fabrication of specially shaped nanoparticles (such as U- or Q-type particles) to provide circular optical currents interacting with external electromagnetic fields. More details on the optical properties of plasmonic nanoparticles and their applications are exhaustively described in various review papers[1-3] and books.[21,22]

Dielectric nanoparticles. Recently, dielectric nanoparticles with high refractive index have attracted a lot of attention in photonics as an alternative approach to achieve a strong resonant response.[23] By using the term "dielectric", here we assume any non-metallic materials, including high-refractive-index semiconductors such as silicon, gallium arsenide, indium phosphide etc.[24] Dielectrics possess lower non-radiative (ohmic) losses compared with those in plasmonic particles. Also, similar to the modern silicon photonics employed as a major platform for on-chip optical telecommunications, here one can employ the advanced silicon fabrication techniques.

In contrast to plasmonic particles, the first resonance of spherical dielectric nanoparticles is a magnetic dipole resonance, and it takes place when the wavelength of light inside a spherical particle equals its diameter X0/ n ^ D.[20] Under this condition, the polarization of the electric field is antiparallel at the opposite boundaries of the nanoparticle, which gives rise to strong coupling to the circulation displacement currents with the magnetic field oscillating in the center.[25] Unlike the magnetic resonances in metallic split-ring resonators, the magnetic dipolar resonances in high-index dielectric nanoresonators are essentially excited by

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Reconfiguration

ultafast processes

fast processes

Temperature

ro ^T o

V^CO C ^

** R

Ablation

Thermo-optical

changing u -y * Reshaping

Photo-thermo chemical

Free carriers

Optical Kerr effect

permanent

Time scale

Figure 1. Schematic presentation of tunable and reconfigurable types of the laser-induced processes at different fluences, defined by the ratio "energy" to "focusing area" of a light beam, expressed in the units of mJ/cm2.

the electric field and not by the magnetic contribution. Low-order magnetic multipoles are useful for the enhanced light-matter interaction due to their higher Q-factors, as compared with the corresponding electric modes, and they enhance a stronger electric field inside the dielectric particle.[26-28]

Interference between electric and magnetic modes results in highly reconfigurable scattering power patterns. Indeed, the electric dipole moment is characterized by a true vector whereas the magnetic dipole moment by a pseudo vector. As a result, the far-field profiles of these two moments have opposite behavior respective to the inversion r r operation. When the resonance frequencies of the electric and magnetic dipoles match and their amplitudes coincide, the so-called Kerker effect[29] can be observed. At some phase relation, these two dipole resonances cancel each other in the backward direction (the first Kerker condition), and at the other condition the radiation vanishes in the forward direction (the second Kerker condition). For example, when the electric and magnetic dipoles obey the first Kerker condition an overall pattern is similar to that of a Huygens' source, i.e. scattered light on such nanoparticle propagates in the forward direction only. Since the Kerker effect depends on a phase relation, it can be tuned by a weak modulation of particle properties.

Another interesting interference effect is the interaction of the electric dipole and toroidal dipole moments, which have the same radiation patterns in the far-field zone. At certain conditions their amplitudes may be in anti-phase that results in cancellation of the radiation from the particle. Fedotov with coauthors[30] reported an observation in microwaves of such radiation suppression they called "anapole modes" with Q-factor exceeding 300. Recently the anapole modes in the visible range were studied experimentally in silicon nanodisks.[31]

Hybrid nanopartides. Hybrid (i.e. metal-dielectric) nanopar-ticles usually consist of a resonant metal element supporting localized plasmons and a dielectric element. Addressing a single nanoparticle, the most popular type of hybrid structures is a core-shell nanoparticle[32] made of two different materials. Core-shell nanoparticles play an important role in modern nanopho-tonics owing to various combinations of materials that can be used, and extended functionalities of the dielectrics (such as luminescence, Raman response, chemical activity, selective absorption etc). From the point of view of resonant properties, the most flexible and potentially reconfigurable designs are based on both resonant plasmonic and resonant dielectric parts. Generally speaking, an optical resonance in complex nanostructures of an arbitrary shape can be understood as a combination of elementary resonances supported by nanostructures of simpler geometries.[33] For instance, one can achieve interference of electric and magnetic optical modes within a single hybrid nanoparticle yielding unique possibilities for an efficient control over the scattering power patterns and near-field at the nanoscale.[34] This property gives additional possibilities for their effective tuning by light.

Nanoparticle dimers and oligomers. Several nanoparticles arranged in certain geometries demonstrate collective effects which are often much richer than those demonstrated by single nanoparticles. The simplest structure is a dimer composed of a pair of (usually identical) nanoparticles. A coupling of two resonant modes (in a majority of the studies, these are the lower-order electric or magnetic dipolar modes) demonstrates a variety of novel effects ranging from the Fano resonance with sharp spectral features, when the coupling between nanoparticles is weak,[35] to the formation of bright and dark modes, when the

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coupling is strong, attributed to the strong field enhancement. The most widely used design is a plasmonic dimer allowing us to enhance a local electric field in a nanoscale gap between the nanoparticles in the range of 10-100 times.[36] In contrast, a dimer composed of two dielectric nanoparticles allows for enhancement of both electric and magnetic fields up to 10-20 times.[37]

Several nanoparticles combined in symmetric clusters are often called oligomers; the term is borrowed from organic chemistry. Metal-based oligomer structures support sharp Fano resonances.[35] Specifically, dielectric oligomeric structures offer novel capabilities of the optically induced resonant magnetic response, which may play an important role in the multifold enhancement of nonlinear optical effects.[38] Hybrid oligomers combine advantages of plasmonic (strong field localization) and dielectric (strong magnetic resonances) structures, but their applications are limited by mismatching of different fabrication steps used for dissimilar types of materials.[39]

Two-dimensional structures and metasurfaces. Depending on the ratio between the lattice spacing and wavelength, two-dimensional (2D) arrays of resonant nanoparticles can serve as 2D photonic crystals supporting Bloch waves providing in addition interesting near-field effects, or metasurfaces for an advanced far-field control.

A promising way to improve the quality of localized plasmon resonances is to utilize the radiative field coupling in regular arrays of nanoparticles. The underlying physics of the proposed resonances is the following. It is known that regular periodic structures can show abrupt changes in reflection, which are referred to as Wood's anomalies.[40] For regular arrays of nanoparticles on a transparent substrate, these anomalies have been explained by Lord Rayleigh as a disappearance of a diffracted beam when it crosses the boundary between an ambient medium and substrate. A transition of a diffraction mode between the ambient medium and substrate is not allowed due to different dispersion relations for light in both the media. As a result, the values of the Q-factor larger than 100 were reported.[41] Significantly higher values (up to Q = 500) were observed in specific arrays of silicon nanoresonators.[42]

Recently, a novel approach of the so-called bound states in the continuum (BIC) was suggested to create photonic structures with larger values of Q. Hsu et al.[43] demonstrated experimentally a photonic crystal membrane with Q-factors exceeding 106. Such extreme values of Q are very attractive for various nonlinear applications. The extremely narrow BIC modes can be observed through lasing, as was reported recently for a finite-extent sample of 8x8 elements.[44] Inspired by high Q values, we anticipate the creation of novel structures by light-induced reconfiguration realizing the BIC effects.

Planar metadevices associated with metasurfaces rely on the scattering properties of ultra-thin subwavelength-scale optical resonators patterned at interfaces to control the polarization, phase, amplitude and dispersion of light.[11,45-49] The design is influenced by the ability to control scattering of light in the resonant scattering regime, for the wavelength scales, or in the non-resonant regime, by using the geometric phase. Plasmonic meta-surfaces were firstly proposed to achieve negative refraction and beam steering, which can be used for dynamical light modulations.

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A combination of the scattering properties of both electric and magnetic resonances in the visible wavelength range is of great interest for the realization of efficient planar metasurfaces with low optical losses. In this case, an effective switching from perfect reflection to near-unit transmission of incident light is possible. One of the main applications of such Huygens' sources is the development of unity-transmission Huygens' metasurfaces that can provide a controllable 2n phase retardation.[50]

Hybrid metasurfaces made of gold and high-index dielectric components attract special attention owing to a more flexible platform for the tuning of resonances. In particular, such structures have been employed for achieving the epsilon-near-zero regime[51] when optical properties of a metasurface are highly sensitive to the modulation of material properties, as discussed below.

3. Reversible Tuning

Searching for reversible and ultra-fast modification of material optical properties by intense laser irradiation is an actively studied area,[52,54] which offers various approaches to achieve strong and fast all-optical signal modulation with speed of information processing more than 10 Gbit/s. A typical design of the all-optical modulators consists of a signal optical line with weak laser pulses, a nonlinear modulator of the signal line and controlling optical line with high-intensity laser pulses. In nonlinear experiments, it is called the "pump-probe" scheme, where the "pump" ultra-fast laser pulse has several orders of magnitude higher energy compared to the "probe" pulse.

The standard metric of optical modulators is power consumption - the energy expended in producing each bit of data, which is also known as the energy per bit, or more colloquially as the "power per bit". Generally, industrial applications can only be justified if their power consumption does not exceed that of present electrical interconnects, and future systems will need to reach power consumption of less than 1 pJ/bit.[55] Another target is to reduce device footprint to the sub-^m2 level, to be comparable with electronic analogs. Therefore, nonlinear optical modulation of optical properties of subwavelength nanostructures is a promising platform for ultra-fast and ultra-compact switching.

There are several nonlinear mechanisms at different time scales, which affect the nanostructure optical response. As shown in Figure 1, fully reversible tuning of material refractive index (An) can be achieved via the optical Kerr effect, photo-excitation of free carriers or optical heating:

An = AnK + Anfc + AnT, (1)

where the first term (AnK) is related to the degenerate four-wave mixing, which is due to the optical Kerr effect, and is described in terms of the Kerr susceptibility x (3)(a, —a, a) of the medium; the second term (Anfc) corresponds to the contribution of free-carrier density; the third term (AnT) is a function of lattice temperature. In order to create nanoscale and ultra-fast optical modulators, it is necessary to make nanostructures consisting of materials with strong nonlinear properties described above. Such nonlinear nanostructures can be separated to purely plasmonic,

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plasmonic with nonlinear dielectrics (hybrid) and all-dielectric refractive-index (or permittivity) variation. In particular, the in-nanostructures. traband term is expressed as a Drude-like response:

3.1. Kerr nonlinearities

n2(Te) = 6 (Te) =

te[te + i YD (Te)]'

(4)

te

Plasmonic nanostructures. Third-order nonlinear response of metals can be interpreted in terms of Kerr-like nonlinearity due to electron heating.[59] During light absorption by a metal, the incident energy is transferred to electrons. The energy deposition is followed by dissipation, which minimizes the total energy through a cascade of relaxation processes. The electron population firstly decays through electron-electron (e-e) interactions, creating a hot electron distribution within a few hundred femtoseconds, followed by a further relaxation via electron-phonon (e-ph) scattering on the time scale of a few picoseconds. In the spectral domain, hot plasmonic electrons induce changes to the plasmonic resonance of the nanostructure by modifying the dielectric constant of the metal.

Generally, the dynamics of the material heating by light can be considered in terms of a two-temperature model. In this model, the material is described by two thermal subsystems corresponding to the electrons with a temperature Te (t) and the lattice with a temperature Tl (t). These two baths are coupled via an electron-phonon constant G and their dynamics is given by the following heat equations for the corresponding temperature and e-e, e-ph relaxation times:

Ce-^ = -G (Te - Ti) + a I, (2)

Cjf = G(Te - Ti), (3)

where Ce and Cl are the heat capacities of electron and lattice subsystems, respectively, I is the laser intensity and a is an absorption coefficient. The electron-phonon coupling coefficient G determines the electron-phonon relaxation time Te-ph. Due to a small mass of electrons as compared with ions (by 103 times), their temperature can be several thousands ofkelvins during light absorption whereas that of ions is lagging due to the characteristic electron-lattice coupling. For simplicity, one can neglect thermal transport terms owing to the localized character of the optical heating of resonant nanostructures.

The relaxation rate for e-e thermalization is less than 1 ps for gold and silver, whereas e-ph relaxation occurs at the 1-3 ps scale.[56,60] More generally, Te-e ~ tep/ T? and Te-ph = 2Ce/ G, where tep is the plasma frequency. Electron heat capacity is a function of electron temperature: Ce = (n2 NfckB/2TF )Te, where Nfc is the free-carrier density, kB is the Boltzmann constant and TF is the Fermi temperature.[61] Therefore, Te-e decreases with electron temperature, whereas Te- ph increases with electron temperature.[62]

Studies of the differential change in transmission of a solid gold nanoparticle film demonstrated that the plasmon resonance is damped due to increased e-e scattering upon heating the conduction electrons.[63] This is the basic mechanism of ultra-fast

where is the dielectric response of the free electrons at high frequencies, tep is the plasma frequency and yd (T) characterizes the temperature-dependent damping processes experienced by the free electrons. We notice that Te in Eq. (4) depends on the light intensity I, which in turn can be evaluated from Eqs. (2) and (3).

High potential of metallic nanoparticles for optical modulation was revealed when three orders of magnitude enhancement of the optical Kerr effect was observed[59,64] as compared with bulk metals.[65] The derived values were in the range of X(3) ^ 10-7-10-8 cm2/W. It was estimated that one can achieve up to Te ^ 1700 K, Ayd(Te)/yd(Te = 300 K) ^ 4% in a gold or silver nanoparticle under femtosecond laser irradiation, resulting in change of both real and imaginary parts of refractive index around 10-4.[66,67]

Basing on nonlinearity in plasmonic nanostructures, ultra-fast all-optical modulation was achieved by tuning of nanoparticles, different nanoantennas and metasurfaces.[17,68,69] For example, Figure 2a depicts time-dependent measurements of extinction of gold nanorods at different probe wavelengths around the center of their plasmon resonance.[56] There was weak and sub-ps response at the very resonance, whereas, at the off-resonant conditions, stronger and slower (few-ps) modulation was observed and attributed to e-ph relaxation.

The main limitation factor here is small mode volume in metal nano-objects, involving smaller volumes of material to nonlinear processes owing to small skin depth (< 20 nm in visible and near-IR ranges) as compared with dielectrics. On the other hand, the usage of too small nanostructures reduces damage threshold of the optical modulator owing to developing instability driven by surface tension, which is widely used in non-reversible reconfiguration (see corresponding section).

All-dielectric nanostructures. In dielectric materials, one of the fastest observed nonlinear optical responses is the analog of the traditional Kerr electro-optic effect, in which the refractive index of a material changes by an amount that is proportional to the square of the strength of an applied static electric field.[70] Another ultra-fast third-order nonlinearity in dielectrics is two-photon absorption (TPA), resulting in sub-100-fs relaxation time of the relatively small optical modulation and low level of energy per pulse.[70,71] The intensity-dependent complex refractive index is expressed as follows:

AnK = (n + i4- • ^ I, (5)

where I is the light intensity, while n2 (the Kerr coefficient) and P (the TPA coefficient) are interrelated with the real and complex parts of the third-order susceptibility x (3) by the equations n2 = 3Re(x(3))/4cn2e0 and P = 3teIm(x(3))/2c2n2e0. Here c and e0 are the speed of light and the vacuum permittivity, whereas te and n are the light frequency and the material refractive index, respectively. The advantage of using semiconductors in nonlinear

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Figure 2. Examples of Kerr optical nonlinearity in plasmonic, all-dielectric and hybrid nanostructures. (a) Ultra-fast modulation of extinction of plasmonic (gold) nanoparticles in water under intense "pump" femtosecond irradiation (at 0 ps) and weak "probe" pulse with varied delay at different wavelengths of the "probe" laser pulses: +30, +40, 0, -30 and -20 nm relatively to the plasmon resonance at 810 nm. Left upper inset: electron microscopy image of gold nanorods. Right upper inset: schematic of light heating of electrons in gold nanoparticle.[56] (b) Experimentally demonstrated enhanced two-photon absorption in array of the resonant Si nanoparticles as compared with thin amorphous silicon film. Left upper inset: the concept of enhanced two-photon absorption on magnetic dipole mode in a Si nanoparticle. Right upper inset: experimentally measured ultra-fast modulation of the resonant Si nanoparticle.[57] (c) Two-dimensional map of the time evolution of the plasmonic spectral change in a 100 nm nanodisk sample with a 4-nm-thick spacer. Left inset: schematic of gold nanodisks with diameters of 100-150 nm fabricated on top of a 30-nm-thick continuous gold film separated by an oxide spacer layer (a few nanometers thick). Right inset: relative electronic levels of gold and the spacer layer.[58]

all-dielectric nanodevices is their high values of n2 and p, which can be roughly estimated from simple scalings n2 ~ (s E^)-1 and p ~ (s Eg3)-1, where Eg is the band gap of the material and s is the permittivity.[72] For example, the TPA coefficient has the following typical values: p = 80 cm/GW (Ge, X = 2.3 ¡m);[73] p = 23 cm/GW (GaAs, X = 1.06 ^m);[74] p = 22 cm/GW (CdTe, X = 1.06 im);[74] p = 2 cm/GW (Si, X = 1.06 ^m);[75] p = 35 cm/GW (Si, X = 0.6 ¡m).[75]

The Kerr coefficient for semiconductor materials usually is much larger than that for silica. In particular, n2 = 4.5 x 10-5 cm2/GW (Si, X = 1.55 im)[76] versus 1-2x10-6 cm2/GW for optical glasses, X = 1.55 ¡m.[77] Such third-order optical non-linearities enhanced by Mie resonances in high-refractive-index nanoparticles become considerable at intensities above 1-10 GW/cm2 or 0.1-1 mJ/cm2 for ultrashort laser pulses (schematically shown in Figure 1), where one can expect refractive-index variation around AnK & 10-4-10-3.

In particular, the use of resonant dielectric nanoparticles helps to significantly increase the effective value of the TPA coefficient p,[57] giving a major contribution to the total absorption at high laser intensities for such low-loss materials as Si at infrared. An example of TPA enhanced by the Mie resonance is presented in Figure 2b, where transmission of light in the so-called "z-scan scheme" is decreased with placing a sample in the laser focus, when an array of silicon nanodisks is illuminated by a near-resonant focused laser beam. A reference silicon film exhibits two orders of magnitude smaller drop in the transmission spectrum (blue curve in Figure 2b). The Mie resonance induced effective TPA is shown to be peff = 5600 cm/GW, which is almost two orders of magnitude larger than that of a bulk amorphous Si film of the same thickness (as nanodisk height) measured using the same setup, being p & 70 cm/GW. Such enhancement can be employed for high enough optical signal modulation induced by TPA.[57,79]

Hybrid nanostructures. Several studies of hybrid nanostructures aim to combine advantages of plasmonic and dielectric structures by using a variety of approaches. Below, we describe a few most interesting examples.

Despite the relaxation rate being fast enough to overcome the GHz limit for signal processing by electronic transistors, the modulation depth induced by the Kerr-type nonlinearities in metallic nanostructures is less than 1%. At the same time, dielectric resonators are several times larger than their plasmonic counterparts.

In order to enhance the amplitude of modulation, more complicated designs of metal-dielectric nanoantennas were proposed. The first approach is to introduce x(3)-nonlinear dielectric into a dimer plasmonic nanoantenna[80] or to use sharp resonances which are more sensitive to small variation of dielectric permittivity. One of the most promising designs involves the Fano mechanism based on dipole-dipole interaction to enhance nonlinear effects and sensitivity to refractive-index variation.[81,82] In particular, the Fano resonance in hybrid metal-dielectrics can be extremely sharp, whereas the device fingerprint is less than 0.1 ¡m2. Dipole-dipole interaction between plasmonic and dielectric parts in a dimer nanoantenna is another approach for ultra-fast tuning of the scattering pattern of the nanostructure.[83]

The last achievements are related to further acceleration of the ultra-fast response (<1 ps) and increasing the modulation up to a few percent level.[58] In Figure 2c, the design based on a 4 nm titanium oxide layer between a gold nanodisk and a thin film is presented, showing an anomalous fast relaxation time 250 fs) and up to 1.3 % modulation depth of reflection. The intensity of the ultra-fast response correlates with the generation of highly excited surface charges (hot electrons) at the metal surfaces, which strongly depends on the oxide thickness. This large ultra-fast contribution is attributed to the relaxation of energetic plasmonic carriers generated in hot spots. Modulation of the sample

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950 1150 1350

600 1000 1200 950 1150 1350 4.2 4.7 5.2 5.7

Wavelength (nm) Wavelength (nm)

Figure3. Epsilon-near-zero and tunable ultra-fast switches. (a) Schematicdiagram showingthe electron configurations and electronic processes involved in the near-infrared (NIR) intraband pumping. For the conduction band, solid lines and dashed lines are constructed with non-parabolic and parabolic dispersions, respectively.[78] (b) Linear relative permittivity of ITO film measured via spectroscopic ellipsometry (symbols) and estimated by the Drude model (lines). Wavelength dependence of (c) the nonlinear effective refractive index, n2(eff), and (d) the effective nonlinear attenuation constant, Pi(eff). The nonlinear response is enhanced in the epsilon-near-zero region of the spectrum (shaded).[54] (e) SEM image of ITO nanorods. (f) The difference of extinction coefficient spectral maps measured from the ITO nanorods at different angles, as shown in the upper inset.[78]

response is due to a transfer of energetic surface electrons from gold nanoparticles to a TiO2 layer. The plasmon energy of 1 eV is smaller than the band gap of TiO2 (Eg & 3 eV) and therefore cannot directly excite electrons from the valence band to the conduction band. However, the difference between the positions of the conduction bands in TiO2 and the Fermi level of gold relative to the vacuum is smaller than the plasmon energy. This allows the ultra-fast injection of the energetic surface carriers into the TiO2 layer, as shown in Figure 2c.

Another approach is to use low-loss materials in which the real part of the dielectric permittivity lies between negative and positive ranges, i.e. between metal and dielectric states. Simple calculus shows that, for a given change (As) in the permittivity s, the resulting change in the refractive index is given for a lossless approximation by An = As/(2^/s). This change becomes large as the permittivity becomes small, suggesting that the epsilon-near-zero (ENZ) frequencies of a material system should give rise to strong nonlinear optical properties. Materials possessing free charges, such as metals and highly doped semiconductors, have zero real permittivity at the bulk plasmon wavelength.[84] A number of authors have reported on the unusual properties of matter under ENZ conditions and on their promise for applications in nonlinear optics.[85-88]

An example is commercially available indium tin oxide (ITO), which is a CMOS-compatible degenerate semiconductor with non-parabolic bands (Figure 3a),[78] serving as the ENZ medium (Figure 3b). The zero-permittivity wavelength of ITO occurs at near-infrared wavelengths and can be tuned by controlling the doping density or by applying a static electric field. Recent results

indicate that ITO exhibits positive n2(eff and negative peff, corresponding to self-focusing and saturable absorption, respectively. In the ENZ range, A = 1240 nm, the n2(f and Peff values for an incident angle of 60° are approximately 43 and 45 times larger than for normal incidence, respectively, whereas they are three orders of magnitude larger as compared with those at A = 970 nm, i.e. in the "dielectric" range; see Figure 3c and d[54] As a result, the magnitude of the optically induced ultra-fast relative change of 170% in comparison to the linear value can be extremely high, whereas the typical relaxation rate is less than 400 fs.[54]

Nonlinear response of nanorods made from ITO can be stronger if they possess resonant properties.[78,93] Moreover, ultra-fast switching can be achieved in a broad range of wavelengths (from near- to mid-infrared) by changing the angle of incidence (see the inset below Figure 3e). Figure 3f demonstrates sub-400-fs relaxation rate with changing of optical density up to 1.

The much faster and stronger response of the ITO deviates from that of the metals (where e-ph relaxation dominates) because of two major reasons: (1) the free-electron density in ITO is two orders of magnitude smaller than that of noble metals such as gold, resulting in much smaller electron heat capacity and larger change in the electron temperature if all other parameters are held constant; (2) owing to the non-parabolicity of the conduction band, the time-dependent meff of the photoexcited electrons is strongly dependent on the Fermi distribution function (i.e. electron temperature)[53]; and (3) owing to a relatively smaller free-electron density, the Fermi level is quite low in the conduction band (~1 eV for ITO). Due to the last property, infrared radiation at ENZ wavelengths can excite even the electrons

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Figure 4. Free-carrier generation in plasmonic, all-dielectric and hybrid nanostructures. (a) Transient extinction map measured for the arrays of gold nanorods forTM-polarized probe light at an angle of incidence of 20o. Pump wavelength is 465 nm; fluence is 0.7 mJ/cm2. Left upper inset: image of the nanorod array. Right upper inset: sketch of all-optical modulation of plasmonic nanorods.[89] (b) Experimentally measured dependence of normalized differential reflection of a Si nanoparticle on laser fluence. Lower inset: calculated reconfiguration of scattering power pattern of silicon nanoantenna at different electron-hole plasma frequencies.[90] (c) Schematic illustration of reconfigurable hybrid Au/Si nanoantenna at low and high photo-excitation of the Si part, corresponding "OFF" and "ON" states.[91] (d) Left: all-optical tuning an ITO surface via local field enhancement near a gold dipole nanoantenna. Right: measured extinction spectra at 0 ps and 5 ps delay between "pump" and "probe" laser pulses.[92]

of the lowest-energy conduction band (in contrast, the Fermi level of gold is 6.42 eV and infrared light excites only those electrons that sit near the Fermi level).[54]

3.2. All-optical generation of free carriers

According to Eq. 4, one can tune the refractive index of even dielectric materials by varying the plasma frequency with ultra-fast "photo-injection" of free carriers (œ2p ~ Nfc) under laser irradiation with higher intensities compared with those for Kerr-type nonlinearities, as shown in Figure 1. The equation governing free-carrier generation and relaxation in nanostructures (i.e. with neglected diffusion) is called the rate equation and takes the form

dNf

Nfc Wi W2 dt T hœ 2hœ

fc

(6)

Here Wu are the volume-averaged dissipation rates due to one- and two-photon absorption, respectively, and t is the phenomenological free-carrier relaxation time. The absorption rates are written as Wi = & (1 > Im(s) and W2 = & (I2) Im(x(3)), where (...> denotes averaging of incident electric field intensity I (or 12) over the nanostructure volume and & is the radial frequency of incident light. It should be noted that multiphoton (three or more photons) generation of free carriers is observed at very high intensities in wide-gap dielectrics, which usually possess relatively low refractive index.[94] Further we describe the relaxation time of electrons in metals and dielectrics in corresponding sections.

Plasmonic nanostructures. Nonlinearity of metals related to the interband excitation of electrons results in relatively slow recovery times owing to a longer relaxation process as compared with thermalization of hot electrons after intraband excitation. For example, modulation of surface plasmon-polaritons in aluminum gives 60 ps modulation recovery speed at photon energies around the interband absorption peak (about 1.5 eV or 825 nm).[68] In gold nanorods,[89] transitions between the d and the sp bands (larger than 2.4 eV or less then 515 nm) result in a much slower relaxation rate as compared with the dynamics around 1.6 eV (775 nm), i.e. in the intraband spectral range, as shown in Figure 4a. High intrinsic (ohmic) losses in metals and initial free-carrier concentration, as well as relatively slow response, motivated researchers to find other materials supporting interband transitions with ultra-fast relaxation.

All-dielectric nanostructures. At intensities 101-102 GW/cm2, nonlinear light absorption in dielectrics leads to significant permittivity modulation (As ~ 10-1-100[95]) via variation of free-carrier density over several orders of magnitude. Generally, the dependence of the refractive index on the plasma frequency can be described by the set of equations (4) and (6). The Drude contribution described by the second term in Eq. (4) strongly dominates the others in the infrared range and at relatively low intensities.[96] The band filling effect is included in the first term of Eq. (4), being important when the free-carrier density becomes comparable with the capacity of the conduction band (Nfc > 1020 cm-3). The band gap renormalization also contributes to the first term of Eq. (4) and plays a significant role at wavelengths where the dispersion of the dielectric permittivity d s/d & is considerable (for example, silicon at lower wavelengths than 800 nm).[90]

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Typical relaxation time of dense free-electron gas (> 1020 cm-3) via the Auger-recombination process is strongly dependent on laser intensity and typically lies in the range of 1-100 ps.[95,97] Generally, the total free-carrier relaxation rate (1/t) can be represented as a polynomial function of free-carrier density: 1/t = 1/ttr(Nfc) + 1/tbm(N}c) + 1/ta(N2fc), where each term corresponds to trapping (1/ttr), bi-molecular (1/tbm) and Auger (1/ta) mechanisms.[98]

Despite the relaxation time is longer than that of Kerr-type non-linearities, the achievable modulation amplitude is considerably larger for most of materials.[70] Previously, this effect was employed for relatively large dielectric-based optical switches (photonic crystals, waveguides etc) with a narrow pass band and high Q-factor.[96] Reconfiguration of the scattering diagram of an all-dielectric nanoantenna (Figure 4b)[90,99] or metasurface[79,100] in the vicinity of the Kerker conditions[29] via free-carrier photoexcitation was shown experimentally. A number of theoretical designs were proposed for highly effective carrier-induced tuning of metasurfaces[101] and beam steering on tunable silicon dimers.[102] For such kinds of nanodevices, typical values of reflection modulation are up to a few tens of percent (A R/ R ~ 10-210-1), and it is much faster than those based on generation of free electrons in the conduction band by applying voltage.[103]

Hybrid nanostructures. The first hybrid all-optical modulator based on free-carriers generation was demonstrated with surface plasmon-polaritons propagating on metal covered by a layer of semiconductor quantum dots.[104] This design has a high modulation depth, but relatively slow (ns-scale) speed. To go down to sub-ps modulation speed, a hybrid metasurface was proposed,[105] demonstrating 20% transmittance variation with decay time 600 fs in the visible range. This metasurface is a three-layer (Au/a-Si/Au) slab with periodic array of nanoholes, where the nonlinearity is driven by free-carrier generation in the amorphous silicon layer.

A more compact hybrid nanostructure was developed by filling the antenna gap with amorphous silicon.[91] Progressive antenna-gap loading with transition from capacitive to conductive coupling regimes between two closely spaced metal nanorods was achieved due to variations in the free-carrier density in the semiconductor (see Figure 4c). A strong modification of the antenna response was theoretically predicted both in the far-field response and in the local near-field intensity.

As shown in Figure 4d, by exploiting the large free-carrier nonlinearity of ITO around the bulk plasmon frequency, the hot-electron injection provides a larger modulation of the antenna dipole resonance wavelength.[92] Such hybrid nanoantenna demonstrates picosecond nonlinear response involving fast hot-electron injection from the gold antenna, followed by picosecond thermalization and a local reduction of the ITO free-carrier density. Light reflection variation with such nanoantennas was around AR/R~ 10-2.

3.3. Thermo-optical nonlinearity

One of the slowest optical nonlinear effects is thermo-optical modulation of permittivity, which is usually increased with growth of temperature.[106] This effect can be correlated to the de-

laser

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formation of the band structure at some critical points of the combined density of states.[107] Similarly to the above-mentioned Kerr and plasma-driven mechanisms of nonlinearity, the thermal-induced refractive-index change has real and imaginary parts being linearly dependent on temperature. Typical dependence of the dielectric permittivity on temperature is given by the following expression:

Anj = VAT, (7)

where V is the complex thermo-optical coefficient. Silicon has relatively high V, the real part of V is equal to 4.5 x 10-4 K-1 and the imaginary part is equal to 0.1 x 10-4 K-1,[108] whereas GaAs has four times smaller corresponding values. Density of intrinsic carriers of semiconductors is also a function of temperature and can be tuned over several orders of magnitude.[109] For instance, pure crystalline silicon has the following dependence: Nfc = 3.1 x 1016 • T3/2 • exp(-1.206/kBT), where кв is the Boltzmann constant, T is the material temperature, and the density is expressed in cm-3. This equation predicts growth from 109 cm-3 at 273 K up to 1017 cm-3 at 800 K. Such increase of free-carrier density affects absorption of the material.

In the work,[110] optical properties of microspheres from Ge (V ^ 4 x 10-4 K-1) and PbTe (V ^ -15 x 10-4 K-1) materials with Mie resonances were tuned in the infrared range by varying the temperature in the range of 80-573 K. This approach allowed one to reversibly reconfigure all-dielectric nanoantenna resonances over their full width, providing strong modulation of the scattered/transmitted optical signal. On the other hand, resonant dielectric nanoparticles can be effectively heated by incident light,[111] paving the way to all-dielectric nonlinear thermophoton-ics.

Typical rising time of the thermal nonlinearity is governed by an electron-phonon scattering time scale lying in the interval of 1-10 ps.[112] The relaxation time in this case is much slower than for optically induced nonlinearities described above, being usually about 1-100 ns and governed by phonon-phonon scattering and strongly dependent on the thermal conductivity of the surrounding medium.

In case of pulsed laser irradiation, it is possible to mechanically tune optical properties of nanostructures via its ultra-fast heating and thermal expansion. A pump pulse triggers acoustical vibrations, which lead to a periodic variation of the nanoantenna size[118] or geometrical parameters of a metamaterial.[119] As a result, periodic mechanical vibrations and optical modulation can be achieved with frequencies from Hz up to sub-THz values.

3.4. Phase-changing and GST materials

A majority of the effects described above allow for relatively weak modification of dielectric properties of nanoantennas and meta-surfaces and tune them continuously (i.e. without threshold) with increasing light intensity, as described by Eqs. (2)-(7). Further increase of the excitation light intensity leads to inherent phase transitions in the materials associated with a change of material parameters, when the local temperature of a sample exceeds a threshold value Tc. Most of such phase transitions are

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Figure 5. Volatile and non-volatile optically induced phase transitions. Schematics of optically induced phase transitions: (a) volatile insulator-to-metal transition in VO2 ^ V2O5 ^ VO2; (b) non-volatile crystalline-to-amorphous transition; (c) non-volatile amorphous-to-crystalline transition. Real (d) and imaginary (e) parts of dielectric permittivity for VO2 (s± ),[113] Si,[114-116] Ge2Sb2Te5.[117]

volatile so that a material returns to its initial phase upon cooling. A typical insulator-to-metal transition in vanadium dioxide is widely used in photonic applications. This transition can be driven by an ultra-fast laser pulse that heats a nanoantenna above Tc = 680 C (see Figure 5a). A hybrid VO2 metasurface switched by a sub-100-fs pulse was reported in Ref. [120]. A metasurface operating at the near-IR range consists of an array of crossed gold nanoantennas fabricated on top of a VO2 film. Several picojoules per antenna is sufficient for the device switching due to an abrupt change in dielectric properties of the VO2 film (see Figure 5d and e). Another degree of freedom in VO2-based structures is in a modification of the transition temperature Tc. By using ion-beam irradiation, the critical temperature Tc can be reduced to 300C for a 500x500 nm2 square defined by a polymethyl methacry-late mask.[121] Such defect engineering allows for control of the switching behavior.

Another class of phase transitions, namely transitions between crystalline and amorphous phases, paves the way for designing a non-volatile photonic nanostructure made of materials with a glass-transition effect. The amorphous phase generally has higher configuration entropy than the lowest free-energy state in the crystalline phase. At the same time, below the melting temperature Tm viscosity exhibiting strong increase with the decreasing of the temperature suppresses atomic diffusion at the glasstransition temperature Tg. Thus, a rapid quenching of melted material (the cooling rate higher than the crystalline rate) prohibits atoms from forming an ordered lattice, resulting in formation of a quasi-stable amorphous phase. In the case of nanopho-tonic structures, a femtosecond optical pulse that delivers energy to material for heating it above Tm succeeded by quenching the sample below the glass-transition temperature Tg leads to freezing a disorder making the transition non-volatile (see Figure 5b).

However, this process can be reversed. A train of pulses, each of them heating the antenna to the temperature above Tg but below Tm, allows transforming the material back into a crystalline phase through a sequence of amorphous phases with decreasing disorder (see Figure 5c).

Non-volatile transitions were demonstrated for silicon nanospheres fabricated by a laser ablation method (femtosecond laser printing).[122] Amorphous (a-Si) to crystalline (c-Si) transitions in silicon lead to a decrease of dielectric permittivity in the spectral range from 500 to 900 nm (see Figure 5d). The physical origin of this behavior is the following. In c-Si, electron transitions in the interval from 365 to 1130 nm are indirect, that is, they take place between states of different wave vectors with the simultaneous absorption or emission of a phonon. At the same time the lack of wave-vector conservation in a-Si makes these transitions quasi-allowed resulting in stronger optical response (Figure 5d). Also, there are differences in the density of valence states, which also account for the longer wavelength shift in the maximum of the imaginary part of £.[123] This moderate change of the permittivity (from sa-Si = 16.5toec-Si = 14 at wavelength X & 700 nm) can shift a position of the Mie resonance up to 70 nm.[122] By using the femtosecond laser printing, Zywietz et al.[122] fabricated a square lattice (lattice spacing 5 ¡m) of similar a-Si nanoparticles that have the magnetic dipole Mie resonance at X & 720 nm. Next, the laser-induced crystallization in the array of these a-Si nanoparticles allowed for selective change of the properties of single nanoparticles. After the crystallization, the Mie resonance is shifted to X & 650 nm since the silicon permittivity is decreased due to the laser-induced phase transition.

By using another important compound based on germanium (Ge)-antimony (Sb)-tellurium (Te) alloys, and often referred to

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as GST, one can also achieve non-volatile transitions. Recently, such GST alloys catch a lot of attention in photonics since their dielectric properties demonstrate a very strong modulation (see Figure 5d). In contrast to the moderate change in optical and electrical properties of amorphous and crystalline phases of sp3-bonded semiconductors such as silicon, GST alloys possess un-saturated covalent bonds leading to a resonant bonding to exist in the crystalline phase.[124] The resulting ground state can be explained as a superposition of symmetrically equivalent states with saturated-bond configurations. Thus, electrons are effectively de-localized resulting in high dielectric permittivity values (see Figure 5d). On the other hand, the resonant bonding requires the long-range ordering and in the amorphous phase this ordering is not possible causing a strong contrast in optical properties of a-GST and c-GST alloys.[125] Previously, GST was employed as a material for optical disks,[126] memory-stick devices (with a fast speed,[127] over 105 read/write cycles and long data retention time of more than 10 years[128]) and solid-state displays.[129] Due to low losses for A > 1.5 im (see Figure 5e) the GST-based photonic devices may operate in the infrared range. For example, the atmospheric transmission windows in the spectral intervals of3-5 im and 8-12 im allow a variety of applications from security to astronomy. We notice that, depending on its composition, GST may refer to Ge2Sb2Te5, Ge3Sb2Te6 and other compounds. Also, there exist other composites, such as InSb or GST-related alloys, with atomic substitutions where Ge is replaced by Si or Sn or Sb is replaced by As or Bi, and even four-component alloys such as Ag6.0In4 4Sn61.0Te28 6 (AIST); they demonstrate similar changes in dielectric properties. However, most of these studies have been made for memory applications such as Ge2Sb2Te5 alloy, because of its data retention capabilities and high state discrimination down to the nanoscale.[130]

Since the dielectric permittivity of GST can be changed dramatically (Figure 5d and e), the simplest idea was to use it as a substrate that affects properties of plasmonic nanoantennas. A metasurface consisting of a gold split-ring resonator on a GST surface was theorized in Ref. [135]. The structure with a GST film in amorphous state was designed to behave as a birefringent surface operating at the A = 8.54-12 im band. When the phase transition occurred, the metasurface starts to work as a metallic mirror. Another theoretical proposal was to hybrid a GST layer into a one-side-patterned metal-insulator-metal structure for a tunable plasmonic absorber.[136] The perfect absorber based on a GST film sandwiched between the gold disk array and the SiO2 insulating layer on a gold mirror demonstrates absorption peak tuning in a large range from A = 2000 nm (GST in amorphous phase) to A = 2650 nm (crystalline phase) with the absorption kept above 0.96.

As a step toward an experimental realization of metasurfaces based on the phase-changing material switched by laser pulses, samples can be heated above the glass transition temperature Tg (about 160°C for GST) to simulate optically induced amorphous-to-crystalline phase transitions (see Figure 5c). Tuning of resonances in aluminum nanoantennas was experimentally demonstrated with amorphous phase of a GST layer changing into crystalline phase.[131] After transition, the reflection spectra shown in Figure 6a were stable at room temperature. Similar temperatures were used for the experimental study of switching in a perfect absorber based on a metal-insulator-metal structure.[137]

In addition, heating was used to prove the concept for plasmonic nanoantennas with a GST layer[138] and a chiral metasurface with tunability of the circular dichroism response from A = 4150 nm to A = 4900 nm.[139]

Metasurfaces with optically induced tuning were reported for an array of asymmetric split-ring slots in a gold film[132] and an array of aluminum nanobar antennas.[140] The antennas were in the neighborhood of the GST layer. A metadevice with split-ring slots with a unit-cell size of 400 nm was designed to operate in the near-infrared range. Phase transitions were initiated homogeneously across large (about 50 im diameter) areas of the GST film in the metasurface device by a single-pulse laser excitation at a wavelength of 660 nm (selected for its strong absorption in GST). The crystalline-to-amorphous phase transition was achieved with 50 ns pulses at the peak intensity of order 0.25 mW/im2 (i.e. fluence is around 1.25 mJ/cm2). The pulse parameters for the reverse transition were 100 ns and 0.1 mW/im2 (i.e. fluence is around 1 mJ/cm2), respectively. The increase of the real part of the GST permittivity (Figure 5d) leads to a 200 nm shift of the resonance while the increase of the imaginary part reduces its quality factor (Figure 6b). As a result, the reflection is modulated at A = 1600 nm as much as by a factor of four that makes it possible to use this effect for optical interconnect applications in data processing architectures.[132] The key component of optically induced bidirectional switching is an effective heat removal for quenching the material and freezing the disorder in the amorphous phase. We notice that the experimental metasurface with an array of asymmetric split-ring slots has a thickness of only 175 nm.[132] Another switching scheme was reported for the metasurface with aluminum bars.[140] The authors used a 50 fs pulse with a fluence of 51 mJ/cm2 for the crystalline-to-amorphous phase transition. For the recrystallization process they illuminated the sample for 1 s by a laser in a repetitive mode (960 Hz) with a fluence of about 31 mJ/cm2 per 50 fs pulse.

The more challenging problem is to fabricate nanoantennas from GST material. Starting out from the GST film placed into the one-side-patterned metal-insulator-metal structure it is instructive to study scattering on a single-structure element (meta-molecule). Such tunable nanoantenna[133] composed of two aluminum nanopatches with different lateral dimensions separated by a GST layer (Figure 6c) possesses both electric and magnetic dipole resonances related to symmetric and asymmetric plas-monic modes. By adjusting geometric parameters it is possible to design an optical antenna that operates in two regimes: (i) the antenna has a predominant electric dipole mode with omnidirectional radiation pattern (when GST is in the crystalline phase); (ii) it supports balanced amplitudes of electric and magnetic dipoles to satisfy the so-called Kerker condition[29] resulting in a directional radiation pattern with zero backscattering (for the amorphous GST phase). An optical response of a square array of such elements varies dramatically in different GST phases. The meta-surface behaves as a reflector in the crystalline phase while it is a nearly perfect absorber in the amorphous phase at A & 3.5 im (Figure 6c). However, this study[133] concerning a nanoantenna with a GST layer only employs it as the dielectric spacer between metallic nanopatches to tune the plasmonic resonances. Another theoretical paper[134] considered a GST rod as the building block of a dolmen metamolecule (Figure 6d) sitting on a free-standing

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Figure 6. Tuning of optical properties by phase transitions in GST. (a) Top: SEM image of metasurface with array of Al rod nanoantennas on top of GST film; bottom: experimentally measured reflectance of the metasurface.[131] Amorphous GST phase (violet dashed curve) and switched to crystalline GST phase (red solid curve). (b) Top: SEM image of metasurface with array of asymmetric split-ring slots in a gold film placed on top of a GST film; bottom: experimentally measured reflectance of the metasurface.[132] Curve styles are same as in (a). (c) Top: unit cell of metasurface with an array of metamolecules consists of a GST spacer between two Al nanopatches;[133] bottom: calculated reflection spectra of metasurface being in state with amorphous GST phase (violet dashed curve) and switched to crystalline GST phase (red solid curve). (d) Top: unit cell of metasurface with an array of GST dolmen metamolecules;[134] bottom: calculated transmission spectra of the metasurface with different states of GST dolmen molecule: no crystalline bars (violet dashed curve), the top bar in crystalline GST phase (magenta solid curve), a side bar in crystalline GST phase (green dotted curve).

silicon nitride substrate. The nanostructure is composed of a horizontal rod working as dipole antenna and a pair of vertical rods as quadrupole antenna. It is known that metasurfaces composed of plasmonic nanoparticles may demonstrate a window of the electromagnetically induced transparency (EIT).[141,142] By adjusting geometric parameters of amorphous GST bars a transmission spectrum demonstrates an EIT profile in the X = 1400-1600 nm range with the window at around 1483 nm (violet dashed curve in Figure 6d). When a vertical rod is selectively switched into the crystalline GST phase the EIT profile becomes broader (green dotted curve in Figure 6d) due to the increasing imaginary part of the permittivity of GST (Figure 5e) and, if the horizontal bar becomes crystalline, the EIT-like resonance disappears (magenta solid curve in Figure 6d) because of the large difference in the resonant wavelengths between the dipole and quadrupole modes.[134]

Recently, Wang et al.[143] reported on a dipolar metasurface operating around a wavelength of X = 2/m, where absorption in GST is low enough (see Figure 5e) to achieve resonances. They designed the metasurface comprising a two-dimensional array of rectangular crystalline inclusions in the amorphous GST film. Both transmission and reflection spectra show the resonant feature at X = 2 /m for light polarized along the inclusion and no dips or peaks for the orthogonal polarization. For wavelengths X > 1.78 /xm the structure does not scatter light in the non-zero diffraction orders,[144] demonstrating a true metamaterial nature.[143]

Another promising mechanism for reconfigurating phase-changing nanophotonic structures is the glass relaxation and re-flow under high-temperature annealing driven by stress generation around the laser-structured regions.[147,148] Similar effects are anticipated in polymers with rubber-glass transitions where the strain defines the material response in an elastic or inelastic manner allowing for visco-elastic flows to occur at high strains.

Hence, by controlling the local strain induced by laser structuring and thermal conditions, the material reflow and corresponding change of the refractive index can be engineered and/or reconfigured. Interestingly, optically induced defects, color centers, interstitials and vacancies are annealed during regeneration, and the actual structural damage of the host glass and local strain is required for the regeneration. The defects recorded inside a dielectric host with fs-laser pulses can reach very high densities of 1019-1020 cm-3, as was measured recently.[149] Photo-excitation of electrons at such densities would create plasmas corresponding to the plasmon frequencies at far-IR and THz wavelengths. By recording nanophotonic structures in the host materials with high transmissivity at IR and THz spectral ranges, e.g., KBr,[149] diamond and sapphire, it would be possible to create optical elements which can be tuned by irradiation in the optical absorption band, usually operating in the UV spectral range. Such optically reconfigurable optical elements could enrich a toolbox of optical elements at the IR and THz spectral ranges and foster the development of chemical sensors at the molecular fingerprinting spectral range.

4. Non-Reversible Reconfigurations

The reversible tuning of optical properties of nanostructures is essential for light switching and modulation. Equally appealing is a possibility to modify an optical response of nanostructures permanently (or on a certain period of time) that might be important for a wide range of applications: from dense data storage to optical sensors and lab-on-a-chip systems. From this point of view, both the reversible long-term modifications of optical properties of nanostructures (e.g., via light-induced assembly of nanoparticles,[152] spatial alignment of individual nanoantennas[153] etc) as well as the concept of precise permanent

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Figure 7. Photothermochemical non-reversible tuning of nanostructures (a) Sketch of laser-induced chemical vapor deposition triggering of the Ge nanowire growth on the illuminated Au nanoparticle as a demonstration of controlled optical tuning of Au/Ge nanoantenna: resonant light absorption and heating of Au nanoparticle lead to Ge supersaturation of Au nanoparticle, nucleation and nanowire growth. (b) High-angle backscattered SEM image of fabricated nanowire. (c) Finite-difference time-domain (FDTD) simulation of scattering spectra for Ge cylinder with increasing length of 180 nm diameter.[145] (d) SEM images of a pair of gold nanoblocks: (left) before irradiation by femtosecond laser beam, (center) exposed by the laser beam polarized linearly, (right) the laser beam is polarized transversely to the long axis of the pair. Plasmon modes localize in the nanogaps (center) or on the left and right tips of the nanoblocks (right) depending on the laser beam polarization and initiate local photopolymerization triggering after a short exposure; (e) theoretically calculated near-field patterns at selected planes for the irradiated samples.[146] The field intensity represents the intensity enhancement factor.

recording ofdesired optical response in a single nanoantenna are promising for data storage devices and highly desirable.

In this section, we consider non-reversible tuning of optical properties of nanostructures taking into account a range of processes that lead to the permanent modification of optical response: photothermochemical, annealing and melting, and boiling (see Figure 1).

It should be noted that spectral selectivity and precision of modification of optical properties in nanostructures is vital for an optimal utilization of the operational spectral range of nanoantennas. From this point of view, the applications of laser radiation are very attractive because laser-assisted processes open wide perspectives for the manipulation of physical properties of nanostructures in terms of high spatial localization and spectral selectivity, thus providing fine tuning of nanoantennas.

Importantly, non-reversible reconfigurations require higher intensities of light, thus imposing limitations on the use of materials. However, gold is one of the best platforms for reconfigurable nanostructures owing to its low oxidation rate at high temperatures, good plasticity, effective light-to-heat conversion efficiency and excellent resonant optical properties.

4.1. Near-field-induced chemical processes

The utilization of resonant properties of plasmonic nanoparti-cles for localized chemical growth can be applied during the fabrication processes.[154-160] Nanostructures are tuned to a specific wavelength by laser-assisted growth processes via localized heating and control of chemical reaction kinetics. A selective ab-

sorption in nanostructures can be localized, effectively activating chemical reactions in a specially selected way via (photo)catalytic reaction, or provide a nanoscale control for the growth processes making it possible to fabricate complicated nanostructures via a self-assembly approach. In this case, a modification of initial nanostructures involved in the growth process does not occur, because activation of chemical reactions is carried out at hot spots engineered in a special way in the initial nanostructure.

Laser-assisted growth. A good example of fine tuning of complicated nanostructures via the laser-assisted growth process can be found in Ref. [145], where a selective growth of gold/germanium nanowires is achieved via strongly enhanced light absorption (see Figure 7). At the initial stage, the electric resonance ofa gold nanoparticle provides localized heating at the nanoscale and initiates a chemical reaction with the Ge precursor. Then, with the growth of Ge nanocrystal, the inherent magnetic and electric resonant modes of the nanostructure are changed by modifying absorption. Therefore, the nanowire growth is controlled in real time making it possible to tune optical properties of individual nanowires by changing the laser power.

Scattering properties of single gold nanoparticles can be altered[161] via chemical reactions taking place under illumination by a cw-laser beam, thus providing a controllable growth of the nanoparticles as well as modification of their optical anisotropy via shape changes, which brings sensitivity to the light polarization.

Photopolymerization. Near-field enhancement in plasmonic nanostructures can also be applied for increasing the photopolymerization rates via both coherent[162,163] and incoherent light sources.[146] Gold nanoblock structures separated by

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Figure 8. Light-induced assembling and nanowelding of resonant nanoantennas. (a) Schematic of plasmon resonance tuning by assembling and welding of nanoparticles. Left to right: charge-stabilized 50 nm gold nanospheres exhibiting single plasmon (SP) resonance at 532 nm; adding of cucurbituril (CB) molecules to assemble nanoparticles into chains possessing capacitive chain plasmon (CCP) resonance at 745 nm; illumination with femtosecond laser pulses connects chains by metal thread into strings, giving threaded chain plasmon (TCP) resonance at 1100 nm.[150] (b) SEM images and (c) scattering spectra of gold dimers before and after laser nanowelding.[150] (d) TEM micrographs of the gold nanorods and (e) their extinction spectra changes due to a tip-to-tip assembly.[151]

nanometer-sized gaps are employed to increase the photopoly-merization rates of photoresist by orders of magnitude at low laser fluence.[162] Figure 7d demonstrates nonlinear absorption and photopolymerization in a nanogap of a gold nan-odimer with the sensitivity to polarization of the incident radiation. Light-field enhancement[164] and heat localization at the nanoscale[162] are both playing an important role in polymerization (see Figure 7e). Importantly, the melting temperatures of metallic nano-objects are usually higher than the damage temperature of polymers, which can result in local laser ablation of surrounding soft media.[165] Another approach based on the use of plasmonic-enhanced synthesis of polymer nanostructures by laser-trapped nanoparticles and nanowires was demonstrated in polydimethylsiloxane, and it could provide a subdiffraction-limited resolution.[163]

Hybrid (semiconductor-metal) nanoparticles were shown to be another promising approach for localized photopolymeriza-tion. Unlike conventional photo-initiators that are consumed upon irradiation, these particles form radicals through a photo-catalytic process and possess giant two-photon absorption cross section. Namely, enhanced light absorption by the semiconductor nanorod is followed by charge separation and electron transfer to the attached metallic nanoparticle, enabling redox reactions to form radicals.[166]

4.2. Reshaping via local heating and melting

The laser fluences applied in the near-field-induced photochemical processes employ the resonant properties of nanoparticles but they do not change their shapes. For a change of shapes, the fluence should be higher to cause heating, which may lead to nanoparticle bonding or controllable change of the geometry and as a result to modification of the nanostructure optical response.

Assembling and nanowelding. As was demonstrated in many works, the laser radiation is a versatile tool for tuning optical properties of single nanoantennas by employing their resonant properties during the reconfiguration process. Assembling of separate nanoantennas via hot spots takes place when resonant nanoantennas are connected together.[168-171] Light-induced laser joining is an effective tool for fine tuning of optical properties of complicated assembled nanosystems. The induced modifications can be observed and controlled in real time by monitoring optical properties of nanoantennas undergoing an assembly.

A typical approach for the light-induced assembling of resonant nanoparticles is illustrated in Figure 8a. Initially, single nanoparticles are diluted in a solution. Their bonding can be monitored optically by measurement of extinction spectra. In particular, the single-plasmon resonance of 50 nm diameter

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Figure 9. Surface coloring via non-reversible tuning of single nanoparticle optical properties (a) Imprinted nanostructure undergoes local laser heating and reshaping in the dependence on laser energy density, which in turn provides a variety of surface structure shapes with different spectral positions of plasmonic resonances and colors of the structures. Left: SEM images of laser-reshaped nanostructures at pulse energies ranging from (i)-(v) 0 to 535 nJ, respectively. Scale bar is 200 nm. Right: schematic of the structure tuned during the laser-induced modification. (b) Simulated spectra of the structures describing the morphology transformation: t - thickness of the initial round-cornered disks, r - radius of the sphere, D - diameter of holes created after ablation of spheres from the polymer interface with laser energy rise; (c) Specific created color design in a blue tone (top); resolution is 127k DPI, scale bar is 10 /m. Bottom: color design for different colors; scale bar is 5 /m (adopted from Ref. [167]).

gold nanospheres is situated at 532 nm.[150] The second step is to connect nanoparticles together by interparticle molecular linkers that produce large aggregates. The authors of Ref. [150] exploited cucurbituril (CB) molecules, which glue nanoparticles with rigid 0.9 nm gaps. As a result large aggregates are produced (Figure 8a). Optical properties of chains formed by the gold nanoparticles mediated by CBs are related to capacitive chain plasmon (CCP) resonances at around 745 nm. The last step is processing of light-induced large-scale threading. The threads are formed by using unfocused 200 fs pulses of 90 MW/cm2 intensity generated by a 805 nm laser. A threaded chain plasmon (TCP) is shifted to near-infrared wavelengths up to 1100 nm. It should be noted that the TCP resonance wavelength depends on the thickness of bridges between nanoparticles, which was clearly demonstrated for tuning of dimer plasmon resonance[150] shown in Figure 8b and c.

A similar approach was suggested in Ref. [151], providing a plasmonic response at the near IR via the tip-to-tip welding of gold nanorods (Figure 8d and e). Other methods of nanowire welding have also been developed including the use of cw lasers,[172,173] femtosecond pulses[174] and even non-coherent sources of light.[175,176]

Reshaping of individual nanoparticles. Various reshaping approaches and fabrication techniques were suggested for nanoan-tennas. A dewetting mechanism related to the minimization of the total energy of thin-film surfaces occurring under heating is one of such commonly used examples.[178-180] The laser-matter interaction responsible for reshaping is well understood[61,181]

as described in the recent review on tunability of plasmonic nanoantennas.[182] Here we highlight a few typical applications of non-reversible modification of nanoscale structures.

A promising application of nanoparticle reshaping is in data storage, color printing and other issues. Color printing of plasmonic metasurfaces with laser writing is demonstrated in Figure 9.[167] A hybrid structure consisting of metal disks placed on top of dielectric pillars underwent a modification induced by laser irradiation and heating at a selected location. Laser heating of separate metal disks by laser pulses at the fluence high enough to melt and reshape patterned nanostructures produces irreversible changes owing to surface tension.[183] A surface plas-mon resonance leads to the electric field confinement and a change of morphology of nanoantennas at the patterned meta-surface in accord with the pattern of the exposed light beam adjusting reflection properties (see Figure 9b). Thus, at a specific wavelength, the transformation of a thin disk into a thicker disk or sphere occurs (see Figure 9a). Specific color patterns in a blue tone were obtained with a five-color printed image (see Figure 9c). This approach looks suitable for data storage provided high speed and resolution at low powers is realized.

In the data storage, the present memory limit due to the current density has to be surpassed for the substantial growth of data traffic and archive applications. Plasmonic multidimensional data storage is another application with a strong practical focus. Zijlstra et al.[177] demonstrated plasmonic data storage based on plasmonic nanoantennas; they used five-dimensional resolution, namely x and y in-plane dimensions plus wavelength

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Figure 10. Laser-induced selective reshaping for multilevel information recording. (a) A schematic illustration of the patterning process. Top: the s-polarized laser beam reshapes in the focal volume gold nanorods with the corresponding aspect ratio and orientation along the polarization vector. Bottom: reshaping of nanorods with other aspect ratios and the orthogonal orientation. (b) Examples of normalized extinction spectra of the as-prepared gold nanorod solutions. Insets in the top show transmission electron micrographs of the corresponding structures. (c) Five-dimensional patterning and readout.[177]

and polarization sensitivity, whereas the distinct energy threshold required for the photothermal recording mechanism provides the axial selectivity. The demonstrated method employs interaction of polarized light and hybrid nanostructured gold nanorods embedded in polyvinyl alcohol layers of 1 ^m being separated by transparent spacers of 10 ^m thickness. Initially the gold nanorods are oriented in a random manner. For sufficiently high s-polarized laser pulse energy, the selected nanorods oriented along the vector of polarization heat up to above the threshold melting temperature, and transform their shape into shorter rods or spherical particles. This results in a depleted population of nanorods with a certain aspect ratio and orientation (Figure 10a, top) and hence a polarization-dependent bleaching occurs in the extinction profile in the reading procedure. For recording an information bit the authors of Ref. [177] used a single femtosecond laser pulse (at 840 nm and energy of 0.28 nJ in the focal plane of the objective). Similarly, a p-polarized beam selectively reshapes nanorods with orthogonal polarization (Figure 10a, bottom). In that way, recording was utilized by reshaping of nanoan-tennas. The optical response (Figures 10b and 10c) depends on the light polarization, length and orientation of gold nanorods, and it could be described by employing the Gans theory via a surface integral technique where nanorod polarizability is directly related to the particle length along the x, y and z axes and inversely scales with the depolarization factor along the coordinate axes.[184]

Another approach for a fine tuning of the scattering properties of more complicated hybrid nanoantennas (consisting of plasmonic and high-index nanoparticles) via selective reshaping of the plasmonic component was proposed by Zuev et al.[39] A

gold nanodisk placed on top of a truncated silicon nanocone was reshaped by high-intensity fs laser pulses (see Figure 11a). Absorbed energy and diameter-to-thickness ratio of nanodisks are the key features for this reshaping, and they provide a control over the hybrid nanostructures and their resonances. After the modification, the spectral position of the lower-order resonance in the nanostructure shifts from the electrical dipole resonance of a gold nanoparticle relatively to the electrical and magnetic Mie resonances of a silicon nanoparticle. This way of the fine tuning provides a precise spectral modification of the hybrid nanoan-tennas manifested in their scattering spectra (see Figure 11b and c).

In Figure 11c the evolution of a single hybrid nanoantenna with increase of laser fluence is shown, revealing two distinguished processes: reshaping of metal disk and, then, dielectric nanocone. The latter process also leads to spectral reconfiguration of optical properties due to strong dependence of Mie-type resonances on shape of the dielectric nanoparticle as discussed in Section 2. This concept was successfully realized for laser-postprocess Ge color metasurfaces with morphology-dependent resonances.[185] Compared to plasmonic analogs, color surfaces with high-index dielectrics, such as germanium (Ge), have a lower reflectance, yielding a superior color contrast.

Several groups demonstrated the shape transformation of gold nanorods into spheres with fs-laser melting.[177,186-188] From the microscopical point of view, this process starts at the interior of a rod by creating point and line defects, which eventually leads to the formation of planar stacking defects and twinning. Laser-induced melting of single nanorods is followed by the surface diffusion of gold atoms from the tips to the center of the rod. It gives an opportunity of optical near-field manipulation on the subdiffraction length scales with an fs laser on gold nanorods. Figure 12a illustrates that a bending of gold nanorods is rendered by longer laser pulses than that required for producing spheres.[189] A gradual growth of the laser power leads to an increase of the gold nanorod bending (starting from the intensity greater than 0.45 MW/cm2). The amount of heat, force and polarization oflaser pulses defines the bending degree and alignment on a substrate (see Figure 12b), whereas the opening angle and length of the gold nanorod are determined by the plasmonic resonances (see Figure 12c). This approach can be applied for creating novel types of optical resonators with spatially varying phase response and subwavelength separation.[190] At very low laser powers, surface diffusion of gold atoms was shown to define reshaping of nanorods with activation energy dependent on the surface curvature.[191]

4.3. Preablative reshaping

At the highest laser intensities, it is possible to achieve a regime of laser ablation (meaning "material removal"), as shown schematically in Figure 1. Below an ablation threshold, a series of preablative processes, including local melting, high-pressure generation, stress-induced deformation and nanoscale hydro-dynamic flows with possible instabilities, occur at different time scales. According to Eqs. (2), (3) and (6), all these thermal processes are longer than picosecond times (>10 ps). Simple

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600 700

Wavelength (nm)

28 mj/cm* 48 mj/cm 70 mj/cnr 80 mj/cm2 105 mj/cm'

unmodified

Au nanoparticle reshaping

Si nanoparticle melting

Figure 11. Hybrid metal-dielectric nanoantennas with non-reversibly tunable optical properties (a) Schematic of the fabrication process. The hybrid nanoantenna is fabricated by the set of lithography procedures: e-beam lithography, metal evaporation, lift-off procedure and gas-phase chemical etching. Then femtosecond laser radiation is applied for modification ofthe metal component shape, changing the optical properties ofthe nanoantenna. (b) Dependence of scattering optical properties of nanoantenna on the geometry ofthe gold component: before - nanodisk (violet) and after - nanosphere (green). (c) Top: modification of hybrid nanoantenna shape with the rise of laser energy density. Bottom: molecular dynamic simulation of gold component reshaping process. The materials ofthe nanoantenna are represented in colors: Si (gray), Cr (blue) and Au (yellow) (adopted from Ref. [39]).

estimation of thermal transport in solids under the pulsed heating gives the following characteristic scale of the heat-affected zone: Lth ^ yjDthTp, where Dth and Tp are thermal diffusivity and pulse duration, respectively.[61] Therefore, by using ultrashort (fs or few-ps) pulses, one can localize thermal transport inside the irradiated material or nanostructure.[61,162] Remarkably, due to fast cooling rates for the heated nanoscale objects (^0.01-100 K/ps[192,193]), this property allows for "freezing" of some preablative states (e.g., ascending flow of a molten material) and can provide more complicated reshaping of nanostructures.

Three-dimensional reshaping of nanostructures. The light-induced shape control in three dimensions opens a new degree of freedom for variation of the optical properties of the reshaped nanostructures, where nanobumps and nanojets are ejected from hot spots of metal nanoprisms[194] and G-shape nanostructures[195] upon the laser pulses with high intensity. Indeed, during the fast reshaping induced by melting, the center

of mass is moved upward, which could lead to material ejection from the surface due to inertia.[196] The precise spatial control of the reshaping can be achieved via variation of polarization of the heating light.[195] Generally, upon increasing the laser fluence, the nanobumps become larger and, at the values >50 mJ/cm2, they transform into nanojets. For higher laser fluences of >80 mJ/cm2, a spherical nanodroplet of the radius ~50 nm can be ejected from the hot spot.[195]

Reshaping of thin films. In addition to the non-reversible reconfiguration of the preliminary fabricated nanostructures, a very strong (>100 mJ/cm2) and ultra-fast laser pulse spatially localized onto a diffraction-limited focused spot on a metal film can be used to change permanently the local optical response of a small area of the irradiated film. Under spatially confined ultra-short excitation, irradiated material undergoes solid-liquid-solid phase transitions, typically on the nanosecond time scales, resulting in a local reshaping and formation of

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600 700 Wavelength (nm)

Figure 12. Control of optical properties of metallic nanorods by laser-induced bending. (a) Schematic illustration of optical gold nanorod bending. Laser beam is focused in solution with Au nanorods; some of them (which are in the focus) align horizontally. Then they are heated by the laser beam; during this process optical forces direct the nanoparticles toward the substrate. The bending angle depends on the laser energy. (b) Printed gold nanorods with different bending angles; scale bar is 100 nm. (c) Numerical scattering spectra of a bent gold nanorod with a 105° opening angle (adopted from Ref. [189]).

different nanoscale surface features exhibiting properties of optical nanoantennas. Under femtosecond-pulse irradiation, the local molten part of the noble-metal thin film, characterized by weaker adhesion to the supporting substrate, detaches through ultra-fast optical heating and local high-pressure generation,[198] producing a parabola-shaped hollow bump or nanovoid (I-IV in Figure 13a) whose geometric parameters can be tuned precisely by applied fluence.[199,200] Such nanostructures were produced on surfaces of Au and Ag films, and they demonstrate tunable size-dependent resonant light scattering in the visible spectral range,[197] supporting excitation and interference of axial and transverse surface plasmon modes in nanovoid shells (see Figure 13b and c). The spectral position of the resonant peak was shown to follow a simple standing-wave model suggesting a certain integer number of plasmon wavelengths mXres to fit the effective circumference neff L of the outer nanovoid shell (see Figure 13d,e), where neff is the effective refractive index of the plasmon mode supported by the air-metal interface.[201,202] It is worth noting that such breaking-symmetry plasmonic nanoantennas formed after the reshaping of a metal film can support magnetic optical response in the visible range.[203,204]

Ultra-short laser irradiation of metal films with higher flu-ences initiates accumulation of a molten material at the center of the nanobump, making thinner the peripheral part of the shell

Figure 13. Light-induced reshaping of a metal film into resonant bumps. (a) Series of energy-resolved side-view SEM images (at 450 angle) of the structures produced under irradiation of a 60-nm-thick gold film by a single fs-laser pulse at different pulse energies (adopted from Ref. [197]). The shapes are supported with the corresponding dark-field optical images (b) showing pronounced changes of the scattering colors in the visible spectral range. The applied/absorbed pulse energy determines the characteristic dimensions and shape of fabricated nanostructures. (c) Experimentally measured (solid) and calculated (colored areas) normalized backscatter-ing spectra from the individual Ag nanovoids (marked as I-IV). All spectra were normalized and vertically offset by 10 a.u. for clarity. (d) Sketch schematically illustrating the origin of the resonant single-color scattering and geometry simulated using the FDTD calculations. (e) Experimentally measured (filled circles) and numerically calculated (solid lines) maximum scattering wavelength versus circumference L for Au and Ag nanovoids.

and forming the nanoneedle (or "nanojet"[200]) on top (see the image I in Figure 14a). A fine adjustment of the applied fluence provides hydrodynamic-assisted tuning of the height-to-width ratio of the nanojet, thus shifting its plasmonic resonance to a broad spectral range. Using this approach,[205] it was demonstrated that plasmonic modes in near- and mid-infrared spectral ranges can be excited by employing such reconfiguration (see Figure 14). Additionally, it was demonstrated that the resonant condition for such dipolar-like nanoantenna follows the simple linear relation L = X0/4, connecting the resonant wavelength A0 with twice longer height of the nanojet L given by the actual height and its mirror image inside the metal film (see Figure 14b). Considering possibilities of relatively fast and cheap fabrication of such high-aspect-ratio nanoantennas via direct ablative reshaping of noble-metal films with multiplexed laser beams,[206] this approach can be employed for laser printing of substrates supporting surface-enhanced Raman scattering (SERS)[207] and enhanced electron

emission.[208,209]

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Figure 14. Reshaping of a metal film into resonant nanorods. (a) Fourier transform infrared (FTIR) spectra of gold fs-laser-radiation-induced nanoantennas. The shift of the central resonance wavelength is achieved by systematic variation ofthe pulse energy applied to control the characteristic length L ofthe corresponding nanoantenna. Series of energy-resolved SEM images demonstrating the change of the nanoantenna length L are given as insets. (b) Nanoantenna length L versus resonant FTIR wavelength (orange circles) in comparison to the linear fit with X0/4 model (red line). The inset explains the X0/4 model with the nanoantenna and its mirror image reflected inside the substrate.[205]

For a certain critical fluence, a huge amount of molten metal material is ejected via the nanojet formation mechanism[198] leaving a nano- or micro-sized through hole in the irradiated part of the metal film.[210] This regime of light-matter interaction corresponds to laser ablation and, thus, further increase of laser flu-ences is not applicable for precise tuning and reconfiguration of advanced photonic nanostructures.

5. Conclusion and Outlook

We have outlined the recent developments and arising trends in the physics of advanced reconfigurable nanostructures. In particular, we have compared reversible tuning of nanostructures based on high-index dielectrics and phase-changing materials with plasmonic and hybrid nanostructures. We have shown that the use of dielectric and phase-changing materials, as well as epsilon-near-zero materials, allow for low-loss and deep reconfiguration of various types of photonic nanostructures. Among advantages of plasmonic nanostructures we notice their controllable hydrodynamic properties at the nanoscale,

chemical stability against oxidization (especially for gold) and ultra-compact designs.

In Table 1, we present some examples of the materials suitable for light-induced tuning and reconfiguration. Gold, as one of the most popular materials, demonstrates the best properties in welding/reshaping only, whereas ITO, some semiconductors, polymers or organic-inorganic perovskites are faster in reconfigurable tuning and provide stronger modulation depths. However, fabrication of nanostructures from these materials is still challenging, except polymers and perovskites that are wet-chemistry processing materials and can be nanopatterned via cost-effective nano-imprint technology.[211] Based on this comparison, we anticipate further merging of nanophotonic designs with organic-inorganic materials (e.g., perovskites,[211,212] metal-organic frameworks[213] and dyes[214]), two-dimensional materials[215] such as graphene,[52,216] black phosphorus[217] and other types of van-der-Waals materials[218]) as well as carbon nanotubes.[219] These materials can exhibit various effects: high carrier mobility, flexibility, Kerr nonlinearity as well as strong and easily reversible phase transitions.

In addition to novel breakthrough concepts in material physics, we anticipate the development of novel nanophotonic designs. The important target of the recent developments is to combine the physics of metamaterials, plasmonics and resonant photonic structures with nonlinear optics to facilitate new discoveries. For example, bound states in the continuum, anapole and Fano resonances, epsilon-near-zero materials and Huygens' sources are novel rising directions in modern photonics that extend the horizons of nonlinear nanophotonics. Development of these concepts can further increase Q-factor of resonances, enhance nonlinear response as well as reduce the ratio between the applied pulse energy and the modulation depth.

Taking into account the recent achievements in material science and nanophotonics, one can expect the appearance of commercially available tunable and reconfigurable metadevices. Ultra-compact all-optical modulators will work with the speed in Tbit/s beating all electronic components, according to subpi-cosecond times of nonlinearity for a number of materials (see Table 1). Other promising applications of metadevices are color technologies rapidly approaching the scales of surface decoration, durable optical data storage and advanced optical security devices.

Acknowledgements

The authors thank Prof. B. Lukyanchuk for useful discussions and Dr. A. Zhizhchenko for help with graphics. S.V.M. acknowledges the ITMO Fellowship Program. The work was partially supported by the Ministry of Education and Science of the Russian Federation (Project Nos. 16.8939.2017/8.9, 3.1500.2017/4.6 and 2.2267.2017/4.6), the Russian Foundation for Basic Research (Grant Nos. 17-03-00621, 17-02-00538, 1602-00461, 16-37-60101, 16-29-05317 and 17-02-00571) and the Australian Research Council.

Conflict of Interest

The authors have declared no conflict of interest.

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Types of materials Examples Melting (Tm), degradaton (Td) temperatures Ultrafast optical modulalon, 112 Recovery time of photoexcited carriers Phase or structure changing Welding, reshaping

Kelvins (K) cm2/W picoseconds (ps) efficiency efficiency

metals Au 7m=1337 10"14-10^ r=l-3 low high

Au NPs in dielectric matrix 7m=500-1000 10-9-10-10 r=l-3 low moderate

Si 7m=l687 10 13 - 10 14 c-Si: r > 10; nc-Si; r> 1 moderate moderate

V) Ge 7m=900 10-13 r > 50 moderate moderate

O ■M o GaAs 7"m=1337 10-12 _ 10-13 r > 1 moderate moderate

3 xs GexSbyTez Tm=900 10-12 _ 10-14 r > 1 high moderate

C o o E Q to GaAs/GaAlAs QWs Td=1100 10-9-10-1° r > 2 low low

ITO Tm= 1800-2200 lO'iO-icr11 r > 0.4 low low

organics Polymers 7^=400-1000 lO-io r > 1 high high

Dyes 7^=200-500 10-7-10'14 r > 100 moderate low

Perovskites 7^=300-700 lO-io _ 10-12 r > 1 moderate moderate

Organic-inorgani< material Metal- organics frameworks 7^=600-800 10-9-10-n r > 7 high low