Оптическая спектроскопия периодических планарных структур на основе материалов с фазовой памятью тема диссертации и автореферата по ВАК РФ 00.00.00, кандидат наук Трофимов Павел Игоревич

• Трофимов Павел Игоревич
• кандидат наук
• 2022

Оглавление диссертации кандидат наук Трофимов Павел Игоревич

Реферат

Synopsis

Введение

Глава 1. Обзор литературы

1.1 Внутренняя структура Ge2Sb2Te5

1.1.1 Кристаллическая фаза GST

1.1.2 Аморфная фаза GST

1.1.3 Феноменологическое описание механизма связывания в GST

1.2 Оптические свойства материалов с фазовой памятью

1.3 Метаповерхности

1.4 Периодические поверхностные структуры индуцироавнные лазером

Глава 2. Численные и экспериментальные методы

2.1 Разработка дизайна кремниевой метаповерхности с тонкими включениями GST и исследование динамики диссипации тепла в этих включениях при облучении их фемтосекундными лазерными импульсами

2.2 Изготовление кремниевой метаповерхности с вкючением материала с фазовой памятью GST

2.3 Спектроскопия задней фокальной плоскости гибридной метаповерхности Si/GST

2.4 Переключение оптических свойств гибридной метаповерхности Si/GST

2.5 Формирование и стирание периодических структур переменной фазы в пленках GST

2.6 Изучение структур на основе GST при помощи электронной микроскопии

2.7 Исследование оптических свойств микролазера на основе кристалла CsPbBr3 интегрированного с нановолноводами из GaP

2.8 Атомно-силовая микроскопия, включая методику переноса на-ночастиц

Глава 3. Селективное управление дипольным оптическим резонансом отражения кремниевой метаповерхности с включением тонкого слоя GST

3.1 Дизайн кремниевой метаповерхности с включением тонкого слоя GST

3.2 Экспериментальное исследование оптических свойств кремниевой метаповерхности с включением тонкого слоя GST

3.3 Выводы по Главе

Глава 4. Реализация промежуточных состояний фазы включения тонкого слоя GST внутри кремниевой метаповерхности

4.1 Многоуровневое переключение кремниевой метаповерхности с включениями тонкого слоя GST

4.2 Выводы по Главе

Глава 5. Формирование периодических структур аморфной и кристаллической фазы в пленках GST под воздействием фемтосекундных лазерных импульсов

5.1 Формирование фазовых решеток в пленках GST под воздействием фемтосекундных лазерных импульсов

5.2 ЛИППС в тонких пленках GST в ближнем инфракрасном диапазоне

5.3 ЛИППС в тонких пленках GST в красной области спектра

5.4 Перезапись фазовых решеток в тонких пленках GST

5.5 Выводы по Главе

Заключение

Благодарности

Список литературы

Приложение Л. Тексты публикаций

Реферат

Общая характеристика диссертации

Введение диссертации (часть автореферата) на тему «Оптическая спектроскопия периодических планарных структур на основе материалов с фазовой памятью»

Актуальность темы исследования

Концепция метаматериалов родилась из идеи создания искусственных сред, способных проявлять оптические свойства, которые не наблюдаются в природе, например, отклонение световых пучков под отрицательным углом [1]. Такие материалы состоят из упорядоченных массивов субволновых элементов и, контролируя их основные параметры, такие как форма, размеры и упорядочение, можно эффективно управлять электромагнитными волнами. Со временем эта идея развилась в концепцию метаповерхно-стей [2-4], плоских аналогов метаматериалов, которые намного лучше сочетаются с развитыми планарными технологиями изготовления. Одним из самых распространенных методов является электронная литография, открывающая непревзойденные возможности по наноразмерному структурированию материалов на больших масштабах. Эта технология, в частности, позволяет создавать фотонные структуры с заданным оптическим откликом, который, однако, фиксируется в момент изготовления выбранным дизайном составляющих элементов. В то же время, для расширения области применения фотонных структур в направлениях квантовых технологий и нейроморфных сетей или в качестве оптических модуляторов необходимо иметь возможность контролировать оптический отклик этих структур уже после их изготовления.

Одним из способов реализации такого функционала является разработка фотонных структур на основе материалов, способных изменять свой показатель преломления под внешним воздействием. На текущий момент активно изучается целый ряд механизмов изменения показателя преломления материалов. Среди них - накачка электрон-дырочной плазмы с помощью облучения фемтосекундными лазерными импульсами [5] химическое допирование [6] и иммерсия фотонных структур в среды на основе жидких кристаллов. В последнем случае, приложение электрического напряжения

к ячейке с жидкими кристаллами приводит к переориентации их молекул и изменению показателя преломления. Этот способ управления оптическим откликом достаточно подробно изучен и хорошо совместим с концепцией ме-таповерхностей, что позволяет реализовывать, например, управление фазой прошедшего света [7]. Еще одним методом может служить интеграция в фотонные структуры диоксида ванадия [8], имеющего два фазовых состояния с заметно отличающимися оптическими свойствами, обратимое переключения между которыми происходит при нагреве материала до критической температуры фазового перехода металл-изолятор. Стоит отметить, что последние два метода (использующие жидкие кристаллы и диоксид ванадия) являются энергозависимыми - то есть требуют постоянного притока энергии для поддержания измененного состояния, что существенно увеличивает энергопотребление перестраиваемых фотонных структур, изготовленных на основе таких материалов.

Халькогенидные материалы с фазовой памятью (МФП), которые уже нашли свое применение в технологиях хранения данных [9-11], лишены таких недостатков. Например, соединение Се28Ь2Те5 [12] уже более 15 лет используется для оптической записи информации, однако в нанофотонике, несмотря на свои уникальные свойства, он до последнего времени применялся достаточно ограниченно. Халькогенидные материалы с фазовой памятью обладают по крайней мере двумя различными фазовыми состояниями - аморфным и кристалическим. При этом, в отличие от жидких кристаллов и У02, в халькогенидных МФП оба состояния являются метастабильными и могут поддерживаться длительное время без внешнего воздействия. Кроме того, в МФП за счет необычного механизма образования химических связей [13] наблюдается сильный контраст физических свойств между этими фазовыми состояниями, в частности, у них сильно различается показатель преломления (Дп=1-2 в ближнем инфракрасном диапазоне). Наконец, при нагреве этих материалов при помощи лазерных или электрических импульсов и последующем их охлаждении возможно реализовывать быстрое (10100 нс) и обратимое переключение между фазами [14]. Таким образом, интеграция планарных фотонных структур с МФП является исключительно

перспективным направлением для создания перестраиваемых оптических устройств. Стоит отметить, что несмотря на расширенные функциональные возможности по управлению светом в таких гибридных структурах по сравнению, например, с диэлектрическими метаповерхностями, оптический отклик гибридных структур будет все равно в значительной степени определяться их геометрическими параметрами, которые остаются неизменными после технологических этапов литографии и травления по сформированной маске.

Преодолеть эти ограничения можно, используя для создания фотонных структур вместо литографии лазерное излучение, при воздействии которого происходит модификация поверхности материала [15]. В последнее время активно исследуется процесс формирования периодических структур на поверхности материалов, находящихся в фокусе линейно поляризованного лазерного излучения [16,17]. Важным преимуществом такой методики является возможность управлять основными геометрическими параметрами создаваемых структур, в том числе их периодом и ориентацией [18]. Общепринятая теория, описывающая образование таких периодических структур, была разработана 1983 году Sipe [19]. Тем не менее, механизмы образования таких структур все еще активно обсуждаются [20-22], так как сильно зависят как от используемого материала и подложки, так и параметров лазерного излучения. Образование периодических структур происходит в результате поглощения в горячих точках электромагнитного поля, образующихся в результате интерференции поля самого импульса со вторичными волнами - рассеянием на дефектах или возбужденными поверхностными плазмонами [23]. При этом, несмотря на то, что наноструктурирование поверхностей образцов под воздействием лазерных импульсов является достаточно дешевым и простым методом управления оптическим откликом фотонных структур, до последнего времени у него было существенное ограничение - необратимость процесса модификации поверхности. В большинстве случаев образование периодических структур сопровождается абляцией материала с поверхности, что не позволяет многократно перестраивать фотонные структуры данным методом. Однако существуют материалы, в которых

при определенных параметрах лазерного излучения вместо абляции наблюдается обратимый фазовый переход, сопровождающийся изменением показателя преломления. Одним из таких материалов является кремний [24,25], но высокая температура фазового перехода, а также сравнительно низкий контраст показателя преломления аморфного и кристаллического кремния в видимом спектральном диапазоне (Дп<0.5 для А>450 нм) существенно ограничивает спектр применений лазерно-индуцированных периодических структур. Халькогенидные материалы с фазовой памятью, например, Ое2БЬ2Те5, обладающие заметно большим контрастом показателя преломления, являются более перспективными кандидатами для формирования подобных фазовых решеток.

Диссертационная работа посвящена исследованию планарных фотонных структур с перестраиваемым оптическим откликом на основе материала с фазовой памятью Ое2БЬ2Те5, подвергающихся воздействию фемтосе-кундного лазерного излучения. Рассматриваются различные задачи: изучение оптических свойств кремниевой метаповерхности при фазовом переходе тонкого слоя включения Ое2БЬ2Те5, а также формирование промежуточных состояний фазы такого включения, под воздействием фемтосекундных лазерных импульсов. Кроме того, изучаются процессы формирования и перезаписи структур с периодической модуляцией фазового состояния в тонких пленках Ое2БЬ2Те5 под воздействием фемтосекундных лазерных импульсов.

Основной целью диссертационной работы является исследование оптических свойств периодических планарных фотонных структур на основе халькогенидного материала с фазовой памятью - Ое2БЬ2Те5, облучаемых фемтосекундными лазерными импульсами.

Задачи, решаемые в рамках диссертации:

• Экспериментальная демонстрация селективного управления спектральным положением и амплитудой оптического резонанса отражения кремниевой метаповерхности с включением слоя Ое2БЬ2Те5, соответствующего оптической моде, имеющей максимум распределения электрического поля в слое.

• Экспериментальная демонстрация формирования промежуточных состояний фазы - между полностью кристаллическим и полностью аморфным - слоев Ge2Sb2Te5, расположенных внутри объема кремниевых цилиндров, упорядоченных в квадратную решетку.

• Экспериментальное исследование процессов формирования и перезаписи непрерывных структур с периодической модуляцией фазового состояния в пленках Ge2Sb2Te5.

Научная новизна:

• Впервые экспериментально продемонстрировано, что фазовый переход между кристаллическим и аморфным состояниями слоев Ge2Sb2Te5, расположенных внутри объема кремниевых цилиндров, упорядоченных в квадратную решетку, приводит к селективному спектральному смещению и изменению амплитуды резонанса, наблюдаемого в коэффициенте отражения структуры и соответствующего оптической моде, имеющей максимум распределения электрического поля в слое Ge2Sb2Te5.

• Впервые экспериментально продемонстрировано, что облучение слоев Ge2Sb2Te5, расположенных внутри кремниевых цилиндров, упорядоченных в квадратную решетку, одиночными фемтосекундными лазерными импульсами с возрастающей энергией приводит к формированию промежуточных состояний фазы слоя между полностью кристаллическим и полностью аморфным.

• Впервые экспериментально исследована зависимость параметров структур с периодической модуляцией фазового состояния, формирующихся в пленках Ge2Sb2Te5, при облучении ее фемтосекундными лазерными импульсами, от длины волны и поляризации используемого излучения, а также показана возможность стирания и последующей перезаписи фазовых решеток.

Научная значимость диссертационной работы заключается в исследовании новых явлений, связанных с управлением оптическим откликом планар-ных фотонных структур на основе материала с фазовой памятью Ge2Sb2Te5

при изменении им фазового состояния под воздействием лазерных импульсов.

Практическая значимость. Результаты, представленные в диссертационной работе, могут быть использованы в областях нейроморфных вычислений, квантовых технологий и в голографии для создания нового класса планарных фотонных структур со свойствами, управляемыми оптическим излучением.

Основные положения, выносимые на защиту:

• Фазовый переход между кристаллическим и аморфным состояниями

слоев Ое2БЬ2Те5, расположенных внутри объема кремниевых цилиндров, упорядоченных в квадратную решетку, приводит к спектральному смещению и изменению амплитуды резонанса, наблюдаемого в коэффициенте отражения структуры и соответствующего оптической моде, имеющей максимум распределения электрического поля в слое Ое28Ь2Тев.

• Облучение метаповерхности, состоящей из кремниевых дисков с включением кристаллического слоя Се2БЬ2Те5, одиночными фемтосекунд-ными лазерными импульсами с возрастающей энергией приводит к формированию промежуточных состояний фазы слоя между полностью кристаллическим и полностью аморфным. В эксперименте это проявляется в виде плавного спектрального сдвига и увеличения амплитуды резонанса в коэффициенте отражения от метаповерхности, соответствующего оптической моде, имеющей максимум распределения электрического поля в слое Ое2БЬ2Те5.

• Облучение пленки Ое2БЬ2Те5 на сапфире фемтосекундными лазерными импульсами с плотностью потока энергии не ниже порога кристаллизации и одновременное сканирование лазерного пучка вдоль поверхности образца приводит к образованию непрерывной структуры с периодической модуляцией фазового состояния пленки Ое2БЬ2Те5 с периодом и направлением, определяемыми длиной волны и поляризацией лазерного излучения.

• В пленке Ge2Sb2Te5 на сапфире, предварительно кристаллизованной непрерывным лазерным излучением и затем реаморфизованной одиночными фемтосекундными лазерными импульсами, возможно стирание наведенной импульсным лазерным излучением периодической модуляции фазового состояния, а также последующая перезапись фазовых решеток, в том числе с другим периодом или направлением.

Апробация результатов работы. Основные результаты работы были представлены и обсуждались на международных конференциях и школах:

• International Conference on Metamaterials and Nanophotonics METANANO 2021, онлайн

• Аморфные и микрокристаллические полупроводники, 2021, Санкт-Петербург, онлайн

• The 4-nd SchooL on Advanced Light-Emitting and Optical Materials SLALOM, 2021, онлайн

• Saint Petersburg OPEN, 2021, Санкт-Петербург, Россия

• International Conference on Metamaterials and Nanophotonics METANANO 2020, онлайн

• The 2-nd SchooL on Advanced Light-Emitting and Optical Materials SLALOM, 2019, Санкт-Петербург, Россия

• International Conference on Metamaterials and Nanophotonics METANANO, 2019, Санкт-Петербург, Россия

• 7th International Topical Meeting on Nanophotonics and Metamaterials Nanometa, 2019, Tirol, Австрия

• Saint-Petersburg OPEN 2019, Санкт-Петербург, Россия

Также результаты, полученные в ходе подготовки диссертации, были представлены на научных семинарах в Университете ИТМО и на форуме Наука будущего.

Публикации. Основные результаты диссертации отражены в 7 научных работах, среди них 3 статьи в научных журналах, индексируемых научными базами данных Scopus и Web of Science, и 3 в рецензируемых конферен-ционных журналах, индексируемых научными базами данных Scopus.

Достоверность проведенных исследований обеспечивается воспроизводимостью результатов измерений и разнообразием используемых передовых методов исследования, хорошим согласованием полученных экспериментальных результатов с данными численных расчетов других авторов и их непротиворечивостью данным из более ранних литературных источников, апробацией полученных результатов на международных научных конференциях, а также публикацией статей по результатам диссертации в ведущих рецензируемых международных журналах.

Личный вклад соискателя в полученные результаты данной диссертационной работы состоит в разработке методики многоуровневой перестройки оптических свойств метаповерхности с включением слоя Се2БЬ2Те5, а также методики формирования и перезаписи структур с периодической модуляцией фазового состояния в пленках Ое2БЬ2Те5. Соискатель участвовал в разработке экспериментальной установки по спектроскопии с угловым разрешением в инфракрасном спектральном диапазоне; непосредственно проводил все измерения оптических свойств периодических фотонных структур, результаты которых представлены в диссертации, а также осуществлял при помощи фемтосекундных лазерных импульсов переключение оптического отклика структур и исследовал топографию их поверхности методами атомно-силовой микроскопии; участвовал в формулировках целей и задач исследований и в написании научных статей.

Структура и объем диссертации Диссертационная работа состоит из введения, пяти глав и заключения. Работа изложена на 88 страницах, содержит 40 рисунков. Список литературы содержит 188 наименований.

ОСНОВНОЕ СОДЕРЖАНИЕ РАБОТЫ

Во введении сформулированы цели и задачи диссертационной работы, обоснована их актуальность и научная новизная, изложены защищаемые положения, а также сформулирована практическая значимость работы.

В первой главе представлен обзор литературы, посвященный управлению оптическими свойствами планарных фотонных структур. Первая часть главы посвящена материалу Ge2Sb2Te5 (GST) - одному из представителей халькогенидных материалов с фазовой памятью, описана структура его решетки в трех различных фазовых состояниях и оптические свойства, обсуждены различные подходы к описанию процессов фазового перехода в GST, а также феноменологическое описание механизма связывания в GST. Вторая часть данной главы посвящена описанию оптических свойств халь-когенидных материалов с фазовой памятью. В третьей части главы кратко описано научное направление диэлектрических метаповерхностей, а также способы реализации в таких структурах перестраиваемого оптического отклика. Заключительная часть главы посвящена формированию периодических структур на поверхности различных материалов под воздействием коротких лазерных импульсов. Описываются возможные механизмы формирования таких структур, а также обсуждается потенциальная обратимость этого процесса.

Во второй главе диссертации описаны основные методики, использованные для изготовления образцов, исследованных в диссертационной работе, а также численного моделирования и экспериментального исследования их свойств. Основное внимание уделено разработанным соискателем методикам многоуровневой перестройки фазового состояния включений GST в кремниевой метаповерхности, а также методикам записи и стирания под воздействием фемтосекундных лазерных импульсов структур с периодической модуляцией фазового состояния в тонких пленках GST. Кроме того, описаны методики изготовления планарных структур на основе GST и исследования их оптических свойств на установке спектроскопии задней фокальной плоскости, реализованной автором, а также методика характери-зации фазового распределения GST в исследуемых структурах при помощи просвечивающей электронной микроскопии. Наконец, в главе описаны исследования оптических свойств компактных источников на основе галоге-нидных перовскитов, перестраиваемых химически, проведенные на той же

установке конфокальной спектроскопии, использованной для исследования перестраиваемых фотонных структур на основе GST.

Третья глава диссертации посвящена экспериментальному исследованию оптических свойств метаповерхности, состоящей из кремниевых цилиндров, содержащих включения слоев материала с фазовой памятью GST. В главе описана экспериментально подтвержденная концепция селективного управления спектральным положением и амплитудой резонанса отражения такой метаповерхности за счет переключения фазы включения GST из аморфной в кристаллическую (рисунок 1(a,c)). Численный расчет оптических свойств метаповерхности, проведенный в Университете Эксетера (Англия), показал, что в исследуемой структуре наблюдаются оптические резонансы отражения, формирующиеся из электро- и магнитодипольных мод отдельных дисков. На рисунке 1(d-e) представлены карты распределения ближнего поля внутри цилиндров метаповерхности для обоих фазовых состояний включения GST. Поскольку тонкий слой GST располагается в максимумах распределения электрического поля электродипольной моды, при изменении фазы включения GST из аморфной в кристаллическую наблюдается красный сдвиг и уменьшение амплитуды соответствующего (длинноволнового) резонаса отражения за счет увеличения мнимой и действительной части показателя преломления GST (рисунок 1(b)). При этом магнитодипольный (коротковолновый) резонанс, распределение поля которого не имеет ярко выраженных максимумов в области слоя GST, практически не модифицируется. Для экспериментального исследования оптических свойств гибридной метаповерхности она была изготовлена по дизайну с рисунка 2(a) при помощи магнетронного напыления и последующей литографии и реактивного ионного травления.

.у X

Рисунок 1 — (а) Схематичное изображение структуры предлагаемой гибридной метаповерхности Si/GST, состоящей из массивов нанодисков Si/GST на подложке SiO2. (b) Показатель преломления (слева) и

коэффициент поглощения (справа) аморфного GST (a-GST), кристаллического GST (c-GST) и (аморфного) кремния. Рабочий спектральный диапазон гибридной структуры выделен желтым цветом: в этой области пик a-GST и кремния хорошо совпадают. (с) Принципиальная

схема работы устройства: гибридные цилиндры Si/GST эффективно ведут себя как будто целиком состоят из кремния, когда GST является аморфным,

а резонансные моды, поддерживаемые структурой (и соответственно, ее оптический отклик), могут быть изменены при внешнем воздействии путем переключения слоя GST между его аморфным и кристаллическим состояниями. (d) и (e) Распределение электромагнитного поля электрического (d) и магнитного (e) резонансов гибридных циллиндров для аморфной (слева) и кристаллической(справа) фаз включения GST.

Оптические свойства изготовленного образца были исследованы на

установке спектроскопии задней фокальной плоскости в спектральном диапазоне 1100-1700 нм, интегрированной с широкополосным когерентным источником суперконтиниума, которая позволила получать спектры отражения метаповерхности с разрешением по углу падения в 1 градус. Были экспериментально получены как спектры отражения при нормальном падении для различных фаз включения GST, так и карты спектров отражения в зависимости от угла падения для двух направлений линейной поляризации света: TM(p-) и TE(s-). Для переключения GST из аморфного состояния в кристаллическое образец метаповерхности нагревался при помощи плитки до температуры 250 градусов в течение 15 минут, что приводило к полной кристаллизации слоя. При таком переключении фазы включения GST в гибридной метаповерхности наблюдалась модуляция амплитуды отражения 72% на длине волны 1550 нм (рисунок 2(b)).

Экспериментально измереные карты спектров отражения в зависимости от угла падения света на гибридную структуру для обеих состояний фазы включения GST показаны на рисуноке 2(c-f). Они показывают, что для такого дизайна гибридной метаповерхности наблюдается сильная зависимость спектров отражения от поляризации падающего излучения при ненулевом угле падения. На экспериментально полученных спектрах отражения в зависимости от угла видно, что для TM-поляризации падающего света наблюдается спектральный сдвиг и расщепление коротковолнового магнито-дипольного резонанса, в том время как для TE-поляризации данный эффект отсутствует. Такое поведение объясняется более сильным взаимодействием магнито-дипольных мод цилиндров для TM-поляризованного света, что приводит к появлению ненулевой групповой скорости у возбуждаемых мод структуры.

1250 1300 1350 1400 1450 1500 1550 1500 Л, нм

(с) a-GST, TM (d) c-GST, TM (e) a-GST, ТЕ (0 c-GST, ТЕ

1200 1300 1400 1500 1600 1200 1300 1400 1500 1600 1200 1300 1400 1500 1600 1200 1300 1400 1500 1600 А, нм Л, нм Л, нм А, нм

Рисунок 2 — Оптический отклик гибридных метаповерхностей, изготовленных на основе кремния и GST. (a) СЭМ-изображение участка изготовленной метаповерхности, показаны шесть элементарных ячеек. (b) Экспериментально измеренные спектры отражения при нормальном падении для образца гибридной метаповерхности со слоем GST как в аморфном (зеленый), так и в кристаллическом (красный) состояниях. (c)—(f) Полученные экспериментально (сверху) и численно (снизу) спектры отражения с угловым разрешением: (с) и (d) при TM-возбуждении, когда

GST является (с) аморфным и (d) кристаллическим; (e) и (f) при TE возбуждении, когда GST является (e) аморфным и (f) кристаллическим. Экспериментальные спектры отражения показывают хорошее согласие с

численными расчетами и демонстрируют сильную поляризационную зависимость спектрального положения оптического резонанса отражения, соотвествующего магнитодипольной моде гибридной метаповерхности, при

ненулевых углах падения.

В четвертой главе диссертации описываются результаты экспериментального исследования многоуровневого переключения фазового состояния тонких слоев GST в резонансной кремниевой метаповерхности. Такие промежуточные состояния в эксперименте проявляются в плавном спектральном сдвиге и изменении амплитуды резонанса отражения гибридной структуры. Поскольку плавный сдвиг резонанса отражения, как правило, сопровождается плавным изменением фазы прошедшего света, то экспериментальная реализация плавного управления фазовым состоянием включения GST является важным шагом для создания фотонных устройств, позволяющих отклонять пучки света на заданные углы за счет пространственного градиента фазы прошедшего или отраженного излучения [26]. Исследования, описанные в данной главе, были проведены на той же структуре, на которой были получены результаты, описанные в главе 3.

Для того, чтобы экспериментально показать возможность плавного изменения оптического отклика структуры, в рамках диссертации была разработана методика многоуровневого обратимого переключения метапо-верхности одиночными фемтосекундными лазерными импульсами с возрастающей плотностью потока энергии. Основной сложностью для реализации обратимого переключения является необходимость обеспечить в процессе аморфизации высокую скорость охлаждения GST (1-20 °С/нс после его нагрева до критической температуры [27-30]). Такая высокая скорость достигается за счет трех основных факторов. Во-первых в структуре используются только слои GST с малой толщиной (в данном случае примерно 20 нм), позволяющей быстро отводить поглощенную энергию к поверхности слоя. Во-вторых, окружающий GST материал - кремний, имеет на два порядка большую теплопроводность по сравнению с GST, и существенно больший объем, что позволяет эффективно отводить тепло от поверхности GST к подложке. Наконец, во время реаморфизации используются короткие (150 фс) импульсы на длине волны 1050 нм с низкой частотой следования (20 Гц), практически не поглощающейся в кремнии. В результате во время процесса переключения создается большой градиент температуры между GST и его окружением, что обеспечивает быстрое охлаждение GST и, соот-

Список литературы диссертационного исследования кандидат наук Трофимов Павел Игоревич, 2022 год

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Vol. 7, No. 5 / May 2020 / Optica

Research Article

optica

Reconfigurable multilevel control of hybrid all-dielectric phase-change metasurfaces

Carlota Ruiz de Galarreta,1 Ivan Sinev,2 Arseny M. Alexeev,1 Pavel Trofimov,2 Konstantin Ladutenko,2 Santiago Garcia-Cuevas Carrillo,1 Emanuele Gemo,1 Anna Baldycheva,1 Jacopo Bertolotti,1 and C. David Wright1*©

1 College of Engineering Mathematics and Physical Sciences, University of Exeter, Exeter EX4 4QF, UK 2ITMO University, 197101 St. Petersburg, Russia *Corresponding author: [e-mail]

Received 25 November 2019; revised 18 March 2020; accepted 5 April 2020 (Doc. ID 384138); published 7 May 2020

All-dielectric metasurfaces comprising arrays of nanostructured high-refractive-index materials are re-imagining what is achievable in terms of the manipulation of light. However, the functionality of conventional dielectric-based metasurfaces is fixed by design; thus, their optical response is locked in at the fabrication stage. A far wider range of applications could be addressed if dynamic and reconfigurable control were possible. We demonstrate this here via the novel concept of hybrid metasurfaces, in which reconfigurability is achieved by embedding sub-wavelength inclusions of chalcogenide phase-change materials within the body of silicon nanoresonators. By strategic placement of an ultra-thin Ge2Sb2Te5 layer and reversible switching of its phase-state, we show individual, multilevel, and dynamic control of meta-surface resonances. We showcase our concept via the design, fabrication, and characterization of metadevices capable of dynamically filtering and modulating light in the near infrared (O and C telecom bands), with modulation depths as high as 70% and multilevel tunability. Finally, we show numerically how the same approach can be re-scaled to shorter wavelengths via appropriate material selection, paving the way to additional applications, such as high-efficiency vivid structural color generators in the visible spectrum. We believe that the concept of hybrid all-dielectric/phase-change metasurfaces presented in this work could pave the way for a wide range of design possibilities in terms of multilevel, reconfigurable, and high-efficiency light manipulation.

Published by The Optical Society under the terms of the Creative Commons Attribution 4.0 License. Further distribution of this work must maintain attribution to the author(s) and the published article's title, journal citation, and DOI.

https://doi.org/10.1364/OPTICA.384138

1. INTRODUCTION

Optical metasurfaces offer a technologically important route toward the realization of lightweight and compact photonic devices with novel functionalities [1—5]. Ever since the concept of optical metasurfaces emerged, a number of novel photonic devices with exciting properties have been reported, including frequency selective surfaces and/or absorbers [6,7], flat lenses [8], polarizers [9], beam steerers [10], holograms, and more [1—3]. In such devices, subwavelength building blocks (often termed meta-atoms) supporting electric and/or magnetic resonances can be used as a designer interface for local engineering of the phase, amplitude, and polarization of light. The most promising platform for obtaining a strong magnetic response at optical frequencies is currently based on all-dielectric nanophotonics, in which the meta-atoms are made of high-refractive-index materials, such as silicon, germanium, or gallium phosphide [11—18]. Isolated nanoantennas (e.g., spheres or disks) of such materials support a series of scattering resonances (usually termed Mie resonances) of both electric and magnetic types [11]. The effective magnetic

response in high-index all-dielectric nanoantennas is driven by displacement currents, rather than, as in the case of more conventional metal plasmonic metasurfaces, by conduction currents [11,15]. As a result, all-dielectric nanoantennas are also practically free from ohmic losses, leading to much higher efficiencies of operation when compared to plasmonic-based designs. In addition, the interaction of equally strong magnetic and electric dipole resonances, enabled by the use of dielectric meta-atoms, brings about a huge range of opportunities for the manipulation of light and vast degrees of freedom in terms of design [13,18—20].

However, the functionality of all-dielectric metamaterials and metasurfaces is generally fixed by design, i.e., the optical response is determined by the size, shape, spatial arrangement, and constituent material properties of the high-index dielectric nanoantennas used. A far wider array of potential applications could be addressed if dynamic and reconfigurable (i.e., fast, multilevel, and reversible) control of the dielectric metasurface properties could be achieved. However, the dynamic control of all-dielectric metasurfaces is a very under-explored topic (though some interesting approaches have been made, for example by embedding structures into a liquid

2334-2536/20/050476-09 Journal ©2020 Optical Society of America

crystal matrix [21,22], or the tuning of silicon refractive index through ultrafast photoexcitation [23]). We here address this key omission by developing, to the best of our knowledge, novel hybrid all-dielectric metasurfaces, in which reconfigurable and multilevel control is achieved by embedding subwavelength inclusions of a switchable and tunable chalcogenide phase-change layer within the body of high-index all-dielectric nanoantennas. Chalcogenide phase-change materials (PCMs), such as the GeSbTe-based alloys, can be switched quickly (nanoseconds or less) [24] and repeatedly (up to 1015 cycles) [25] between amorphous and crystalline states (and even between intermediate phases) by appropriate thermal, optical, or electrical stimuli (each of those ultimately leading to thermally driven change of the structural phase) [26,27]. Such phase-state switching is non-volatile in nature (as opposed to the alternative phase-change material VO2, which is volatile) [28,29] and results in a huge contrast in the complex refractive index, making chalcogenide PCMs very attractive for the creation of fast, energy-efficient (low-power consumption), dynamically reconfigurable optical devices and metasurfaces [30]. Indeed, a number of plasmonic-based metasurfaces incorporating PCMs have been reported [31-38], along with the direct structuring of PCMs to yield dielectric metasurfaces [39,40]. In the latter approach, however, the dielectric antennas are made entirely of chalcogenide material, necessitating the use of large PCM volumes that can degrade significantly the optical performance (i.e., lead to low efficiencies due to dielectric losses) of the device. Perhaps even more importantly, large PCM volumes also preclude the achievement of the high cooling rates (typically tens of degrees per nanosecond [27]) required for re-amorphization of the PCM (due to the characteristic low thermal conductivity of PCMs) [26,31].

In this paper, therefore, we propose and experimentally demonstrate a new class of dynamically reconfigurable and multilevel all-dielectric metasurfaces based on a hybrid of high-index nanoresonators, combined with deeply subwavelength-sized (down to /100) inclusions of chalcogenide-based phase-change materials. We illustrate our concept by designing, fabricating, and experimentally characterizing hybrid Si

nanocylinder/Ge2Sb2Te5 devices that work at telecoms wavelengths as reconfigurable dual-band (O and C band) to mono-band (C band only) spectral filters/modulators. Unlike previous designs that were involving optically resonant arrays on top of PCM films [33], here we incorporate the PCM directly in the volume of the resonator. As we demonstrate below, this not only provides extremely efficient switching by enabling the use of very thin PCM layers, but it also allows us to selectively address/control metasurface resonances (specifically the electric dipole mode) by strategically positioning the PCM inclusions at the electric field anti-nodes. Moreover, laser-induced multilevel switching of the PCM between states results in independent, non-volatile, multilevel, and dynamically reconfigurable control of the resonant modes. This opens up the possibility of new design degrees of freedom and functionalities not achievable otherwise (e.g., with resonators fully made entirely of PCMs [39]).

Finally, we show scalability of our approach toward other spectral ranges and device functionalities via numerical simulations. In particular, by judicious choice of materials and structure geometry, we show reconfigurable color generation in the visible spectrum via hybrid metasurfaces that combine the high-index dielectric TiO2 (rutile) with the novel, low-loss PCM Sb2S3 [41]. We believe hybrid dielectric/phase-change metasurfaces such as the ones proposed in this work, could open up a new route toward the design and realization of tunable optical metasurfaces with improved functionalities that have potential applications in numerous technologically important fields, ranging from telecommunications to consumer technology, security, and defense.

2. RESULTS

A. Design

An illustration of our proposed hybrid dielectric/PCM meta-surface is shown in Fig. 1(a). In this case, it consists of an array of Si/Ge2Sb2Te5 nanodisks arranged in a square lattice on top of a SiO2 substrate. The choice of Ge2Sb2Te5 (GST for short) as the PCM in our design is made for two main reasons; first, it has very

Fig. 1. (a) Schematics of the proposed hybrid silicon/PCM metasurface, consisting of arrays of silicon/GST nanodisks on a SiO2 substrate in this example. (b) Refractive index (left) and the absorption coefficient (right) of amorphous GST (a-GST), crystalline GST (c-GST), and (amorphous) silicon. The spectral region of interest is highlighted in yellow: in this region, n and k of a-GST and silicon are closely matched. (c) Generic scheme of the device working principle: the hybrid silicon/GST cylinders effectively behave as silicon-only when the GST is amorphous, and the resonant modes supported by the array (thus its optical response) can be modified on demand by switching the GST layer between its amorphous and crystalline states.

attractive and well-known switching properties (such as the fast and repeatable switching, high optical contrast between states, etc., as described in the first paragraph), and second, its complex refractive index in the amorphous state, as shown in Fig. 1(b), matches very well the refractive index of (amorphous) silicon in the A = 1300 nm to A = 1600 nm window [31,42]. After crystallization of the GST, however, an increase of the refractive index n and absorption coefficient k takes place, with an overall increment of An ~ 1.6 and Ak ~ 1.1 in the spectral region of interest here. Therefore, our hybrid Si/GST nanodisks effectively behave as silicon-only cylinders when the GST is amorphous, but the resonant modes supported by the array (thus its optical response) can be modified on demand by switching the GST layer between its amorphous and crystalline states [Fig. 1(c)]. Importantly, due to the very small volume of PCM used in our approach, whether a mode is affected by switching of the PCM layer is critically dependent on, and can be controlled by, its particular placement within the overall resonator structure. This opens up the opportunity of selective and independent control of the metasurface modes by strategically positioning the PCM layer at the anti-nodes of the electric field.

To highlight the independent, reconfigurable control of meta-surface resonances that is possible with the hybrid high-index dielectric/PCM concept, we now design, fabricate, and experimentally characterize devices suitable for simultaneous and tunable filtering/switching in the O and C telecommunications bands (1320 nm and 1550 nm, respectively). To create a metasurface with resonances in both these bands, we capitalize on the morphological dependence of Mie resonances of dielectric disks [16-19], which

enables their mutual tuning through the change of the disk aspect ratio. Design and analysis of the devices was carried out employing finite element methods (FEM) using the commercial software package Comsol Multiphysics (see Supplement 1 Section 1 for a detailed description ofFEM models).

The unit cell design of our devices is summarized in Fig. 2, where the period Л is 850 nm, the cylinder thickness tcyl is 195 nm, and the cylinder radius r is 666 nm. As shown in the reflectance spectrum R of Fig. 2(a), employing cylinders made exclusively of lossless Si (i.e., without the GST inclusion) results in two spectrally separated resonances falling at the O and C telecommunication bands, associated to the electric (ED) and magnetic (MD) dipole modes of a single disk [15,16], thus creating a dual-band filtering behavior. Replacing Si by GST [thus yielding all-GST nanodisks, see Fig. 2(b)] results in severe damping of the resonances in both amorphous (a-GST) and crystalline (c-GST) states, due to high PCM volume leading to large optical losses across the spectrum [as shown previously in Fig. 1(b)]. Indeed, such relatively high optical losses for the all-PCM approach lead to relatively low device efficiencies [39,40], similar in fact to those obtained with plasmonic phase-change metasurfaces [30-33]. Thus, an alternative approach to the use of direct structuring of PCMs to provide all-dielectric metasurfaces is needed, since the latter (1) does not provide any improvement, from an optical performance point of view, with respect to plasmonic PCM-based metasurfaces [39,40] and (2) requires large PCM volumes for which reversible switching becomes very challenging, if at all possible [26]. Remarkably, as we show in Fig. 2(c), by introducing an ultra-thin amorphous GST layer (tgst = 15 nm) in the middle of a silicon resonator body,

Fig. 2. (a) Schematics and dimensions of the unit cell of an array of cylinders fully made of silicon (top), and corresponding reflectance spectrum (bottom), showing a dual-band-filtering behavior where MD and ED are located at the O and C telecommunication bands. (b) Schematics and dimensions of the unit cell of arrays with direct structuring of GST (top), and corresponding reflectance spectrum for amorphous and crystalline states (bottom), where the optical response is degraded due to the presence of relatively high dielectric losses. (c) Schematics and dimensions of the unit cell considering a hybrid silicon/GST cylinder design (top), and corresponding reflectance spectrum (bottom) for amorphous and crystalline phases. The optical performance of silicon-only cylinders is maintained (green line), and selective control of the ED can be achieved via crystallization of the GST later (red line). (d) and (e) Electromagnetic field distribution of the electric (d) and magnetic (e) resonances for amorphous and crystalline phases of the GST.

(a) a-GST, TM (b) c-GST. TM (C) a-GST, TE (d) e-GST, TE R

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Fig. 3. (a) and (b) Angular reflectance under TM polarization for (a) amorphous and (b) crystalline states, showing splitting of the MD with the angle of incidence, and cancellation of the ED in the crystalline phase. (c) and (d) Angular reflectance under TE polarization for (c) amorphous and (d) crystalline states, showing a dispersionless behavior of the MD, and cancellation of the ED in the crystalline phase.

we can recover the dual-band resonant behavior achievable with silicon-only cylinders (apart from a small residual absorption; see Supplement 1 Section 2A). Moreover, crystallization of the GST layer reveals a quite different scenario [Fig. 2(c), red line]; here, the ED resonance is strongly attenuated, while the MD resonance remains mostly unaffected. The nature of such behavior can be readily explained by looking at the hybrid nanodisk near-field distributions at the resonant frequency of ED and MD modes shown in Figs. 2(d) and 2(c). Figure 2(d) (left) shows the characteristic electric field profile of an electric resonance when the GST is amorphous, with a strong enhancement of the electric field modulus in the center of the disk (see color bar) surrounded by magnetic current loops (red cones). After crystallization, the GST layer, being intentionally placed in the electric field antinodes of the mode, results in attenuation ofthis particular resonance, essentially due to an abrupt increase of the GST refractive index and absorption coefficient [Fig. 2(d), right]. Correspondingly, Fig. 2(e) (left) shows the typical magnetic field profile of a magnetic dipole mode, with a strong enhancement of the magnetic field modulus in the disk center (see color bar), surrounded by electric field loops (white cones). Here, the electric fields interaction with the GST layer is much weaker, leading to almost no attenuation of this mode upon switching to its crystalline phase [as shown in Fig. 2(e) (right) and in more detail in Supplement 1 Section 2.A].

The above-described results clearly show that our hybrid Si/GST metasurface can operate as a reconfigurable dual-band to mono-band spectral filter, via selective cancelation of one of the resonant modes (ED), without altering the other (MD). With the GST layer in the amorphous state, both O and C band wavelengths (1320 and 1550 nm) are reflected, whereas switching the GST to its crystalline state results in a single-band filter in which, effectively, only the O band is reflected (with the C band being transmitted instead). The absolute modulation depth in reflection (Mdr = Ram — Rcr) is 72% at X = 1550 nm (relative modulation contrast |Ram — Rcr|/Rcr = 100%). In transmission, the modulation depth at the same wavelength is Mdt = Tm — Tcr = 65% (see Supplement 1 Section 2B).

Finally, we also investigated the robustness of our device performance against changing the angle of incidence. For this purpose, we calculated reflection for a range of angles of incidence 9 going from —13° to 13° in steps of AO = 0.5° for both transverse electric (TE) and transverse magnetic (TM) polarization states. Figures 3(a) and 3(b) show the angular dependence of the reflectance spectra under TM excitation for a-GST and c-GST, respectively. It can be seen from Fig. 3(a) that the mode associated with the ED resonance of the disk is dispersionless (i.e., remains stationary when varying 9), whereas the MD-associated mode

for non-zero angles of incidence splits into two separate Bloch modes with a high dispersion and opposite sign of group velocity due to strong interaction between the disks (see the eigenmode analysis in Supplement 1 Section 2C and [43,44]). Crystallization of the GST layer results, as expected, in the cancellation of the ED mode for every angle of incidence, while the splitting of the MD mode is conserved. For TE-polarized excitation [see Figs. 3(c) and 3(d) for a-GST and c-GST cases, respectively], both ED- and MD-associated modes remain unaffected by the oblique incidence, while maintaining the characteristic cancellation of the ED mode after GST crystallization. These results suggest that our meta-surfaces can therefore have additional features (such as tunable multiband filtering) upon exciting the device at different angles under TM polarization. On the other hand, angular robustness could be achieved by simply changing the incident polarization state to TE.

B. Hybrid Silicon/GST Fabrication and Characterization

The hybrid Si/PCM device designs shown in Fig. 2(c) were fabricated in areas of 100 ^m x 100 ^m using the magnetron thin-film sputter deposition, e-beam lithography, and etching techniques, as described in the methods section (a schematic flowchart of the whole process can be found in Supplement 1 Section 3). A scanning electron microscope (SEM) image of a typical as-fabricated device is depicted in Fig. 4(a), showing measured nanodisk diameters of 668 nm extremely close (in fact within the measurement error) of the target design diameter [as in Fig. 2(c)] of 666 nm. The devices were optically characterized using back focal plane spectroscopy (described in the methods section and Supplement 1 Section 4) to obtain the experimental reflectance spectra. Results, for normal incidence, are shown in Fig. 4(b) and show excellent agreement with the simulated results previously shown in Fig. 2(c). For a-GST [green curves in Fig. 4(b)], a reflectance peak of 79% corresponding to the electric dipole resonance is clearly observed at X = 1540 nm (cf. 80% at X = 1550 nm from numerical simulations). As expected, a second reflectance peak corresponding to the magnetic dipole is located experimentally at a shorter wavelength of X = 1380 nm, with a strength of R = 83% (cf X = 1320 and R = 93% obtained numerically). The reflectance spectrum taken after (hot plate) crystallization (red curve) confirms the predicted absence of the ED peak at X = 1540 nm, with the MD nearly unaffected by the phase transition. An absolute experimental contrast (modulation depth) between phases of 70% was obtained at X = 1540 nm, very close to that predicted by our numerical models (i.e., 72% atX = 1550 nm).

Fig. 4. Measured optical response of the fabricated meta-devices. (a) SEM image of (part of) a typical as-fabricated hybrid silicon/GST all-dielectric metasurface device, here showing six unit cells. (b) Experimentally obtained reflectance spectra for the as-fabricated device with the GST layer in both amorphous and crystalline states. (c)-(f) Experimentally measured angular reflectance spectra: (c) and (d) under TM excitation when the GST is (c) amorphous and (d) crystalline; (e) and (f) under TE excitation when GST is (e) amorphous and (f) crystalline. The experimental angular reflectance spectra show good agreement with simulation [as in Figs. 3(c)-3(f)] and confirm robustness of device performance against the angle of incidence for TE illumination, but dispersion of the MD-associated mode with the angle for TM illumination.

The optical performance of our hybrid metasurfaces at oblique incidence was also experimentally investigated, under both TE and TM incidence, and compared to our numerical simulations. Thus, Figs. 4(c)-4(f) show the experimentally measured reflectance spectra as a function of the angle of incidence under both TM and TE polarization and for amorphous and crystalline states of the GST layer. As predicted by simulations shown in Figs. 3(a) and 3(b), under TM illumination, our as-fabricated metasurfaces exhibit splitting with the angle of incidence of the MD and no dispersion of the ED mode [Fig. 4(a)], along with the cancellation of the ED mode upon GST crystallization [Fig. 4(b)]. Under TE illumination, again, an excellent agreement between the simulations [Figs. 3(c) and 3(d)] and experiment [Figs. 4(c) and 4(d)] was achieved, showing robustness at oblique incidence (i.e., ED

and MD reflection peaks remain in the same spectral position at every angle), and the ED resonance at 1550 nm is completely suppressed for the c-GST phase over the entire range of incident angles measured (here to ±12.5°).

C. Optically Induced, Multilevel, Reversible Switching of Hybrid Silicon/PCM Metasurfaces

Our novel hybrid all-dielectric/PCM metasurface approach also enables rapid, reversible, multilevel, and non-volatile switching of the resonances, which we demonstrate here by employing femtosecond laser scans [45-47]. We use pulses with a central wavelength of 1050 nm, as described in the methods section. At this wavelength, the losses of both amorphous and crystalline

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Fig. 5. (a) Experimental demonstration of multilevel tuning of the hybrid silicon/GST metasurface. Starting from the as-deposited amorphous state (bottom panel), the metasurface is first switched to the crystalline state (Scan 1) by excitation with a train of laser pulses with a fluence of 1.28 mJ/cm2. Multilevel switching from the fully crystalline state back to amorphous GST (Scans 2 to 7) was carried out using a single-pulse regime with fluences linearly increasing from 6.4 up to 19.2 mJ / cm2. (b) FEM simulation of the multilevel switching process (see Supplement 1 Section 7).

GST are reasonably high, while the absorption in silicon is negligible, which ensures very effective heating of GST inclusion. Importantly, we successfully demonstrate a reversible and multilevel tuning of the GST phase between fully crystalline and fully amorphous states, which in turn leads to the possibility of fine control over the spectral position and amplitude ofthe metasurface resonance.

Experimental results are given in Fig. 5(a) where microreflectiv-ity measurements at various stages of the multilevel switching cycle (here seven states, but 10 states also demonstrated in Supplement 1 Section 5) are shown. The switching to the crystalline state from the initially as-deposited amorphous state [Scan 1 in Fig. 5(a)] was performed using multipulse excitation (repetition rate 80 MHz) with a fluence of 1.28mJ/cm2. Multilevel switching from crystalline GST back to amorphous GST [Scans 2 to 7 in Fig. 5(a)] was carried out using a single-pulse regime with fluences linearly increasing from 6.4 up to 19.2 mJ/cm2. Figure 5(a) clearly indicates a gradual restoration and blue shift of the ED resonance as the GST layer is gradually re-amorphized. In fact, we were able to reconstruct the multilevel switching process via COMSOL simulations [shown for comparison in Fig. 5(b)], which suggest that the multilevel tuning is achieved by redistribution of the hot

spots at the laser pump wavelength, resulting in different regions being re-amorphized after each scan (details of these simulations can be found in Supplement 1 Section 6).

3. DEVICE SCALABILITY: TOWARD HIGH-EFFICIENCY VIVID NON-VOLATILE COLOR GENERATORS

As a final example of the broad general scope of our hybrid all-dielectric/PCM metasurface concept, we show its potential for tunable color generation, which could have applications for non-volatile digital paper, transmissive or reflective displays, electronic signage, and reconfigurable spectral filtering. Indeed, phase-change materials in the form of resonant cavities and/or combined with plasmonic metasurfaces have been already successfully employed to produce tunable color generation [41, 48-50]. However, most of the reported approaches are limited to operating in reflection only [41,49], and performance is limited by plasmonic and dielectric losses coming from metallic parts and phase-change materials, respectively. As we will show in this section, employing

Fig. 6. (a) Refractive index and absorption coefficient of TiO2 (rutile), amorphous and crystalline Sb2S3 (a — Sb2S3 and c — Sb2S3), taken from [41,51]. (b) Schematics of the hybrid TiO2/Sb2S3 nanocylinder, showing geometrical dimensions and the PCM distribution in the optimized structure. (c) Reflectance (left) and transmittance (right) spectra for amorphous and crystalline Sb2S3 states. Insets show the resultant color, based on chromaticity calculations employing a standard D65 illuminant. (d) Electric field distribution for amorphous (top) and crystalline (bottom) Sb2S3, excited at a wavelength of X = 520 nm, confirming attenuation of the ED mode after crystallization. (e) Chromaticity objective coordinates (black dots) and obtained coordinates (blue dots) in reflection (left), and subsequent resulting coordinates in transmission (right).

hybrid phase-change dielectric metasurfaces could lead to high-efficiency, reconfigurable structural color generation, operating simultaneously in reflection and transmission.

The basic structure here again consists of a lossless dielectric nanocylinder with embedded PCM layers. Here, for the PCM layer, we use the material Sb2S3 (SS23 for short), which has low losses (in the amorphous phase) in the visible spectrum (as compared to say GST in particular) [41]. For the dielectric of the nanocylinders, we use T1O2 here (rutile [51]), which has a near matching of refractive index to amorphous SS23 [see Fig. 6(a)]. The overall structure is shown in Fig. 6(b) (note that we use MgF2 here as a substrate to avoid the generation of diffraction orders across the entire visible spectrum, due to its low refractive index, nMgF2 = 1.38). As in the case of the Si/GST nanocylinder design described above, when the SS23 is in its amorphous (i.e., low optical loss) state, this hybrid metasurface can be tuned to generate high-efficiency reflectivity resonances. As shown in Fig. 6(c) (left), a sharp reflectance peak of a 60% efficiency is excited at A = 520 nm when the SS23 PCM is amorphous, which results in a vivid primary additive color in reflection (here green). After crystallization, however, the resonance is shifted and damped due to an abrupt increase of the refractive index and absorption coefficient, resulting in a pale, close to white, color appearance. Again, as in the case for the Si/GST devices described previously, such behavior was achieved by positioning the SS23 material coincident with the electric field maxima under resonance, as shown in Fig. 6(d). It is worth highlighting, however, that here we did not directly choose the position and thicknesses of the tunable layers, but instead, we allowed an optimization tool (running in MATLAB) to find the optimum position for the generation of a strong green color in reflection. It is reassuring to note that the optimization routine placed the PCM layers at precisely the same positions that we would have chosen considering the location offield maxima.

Figure 6(e) shows the chromaticity color coordinates obtained in the reflection and transmission after the optimization of the cylinder geometrical parameters (a detailed description of the optimization process and color space calculations can be found in Supplement 1 Section 7), confirming pure green (reflection) and magenta (transmission) colors when the SS23 layer is amorphous, and a close to white state after crystallization. Additional colors could be generated via appropriate optimization of the cylinder geometry and the position of the PCM inclusions using the same optimization routine described in Supplement 1 Section 7. Furthermore, alternative PCMs exhibiting low losses in the visible spectrum, such as gallium lanthanum sulphide [52], are available, as well as other high-index dielectrics, such as hydrogenated silicon, thus increasing even more the design degrees offreedom when it comes to color generation.

4. METHODS

A. Device Fabrication

Arrays of nanodisks were fabricated on 1 cm x 1 cm SiO2 substrates, previously cleaned with acetone and rinsed in isopropyl alcohol. First, a silicon/GST/silicon tri-layer stack was deposited using a magnetron sputtering system (Nordiko). RF sputtering in an argon atmosphere (50 sccm) with a plasma power of200 W was used to deposit the top and bottom silicon layers. DC sputtering was employed for the GST deposition, under the same atmosphere and a plasma power of 20 W. The chamber pressure and base

vacuum for both processes were 8.5 x 10-2 and 1.0 x 10-5 Pa, respectively.

Next, the samples were covered with an adhesion layer (Ti-Prime) using a spinner at 4000 rpm for 20 s, with subsequent post-baking at 90° C for 5 min. A negative resist (ma-N 2403) was then spin-coated at 2500 rpm for 60 s and post-baked at 90°C for 10 min. Finally, a thin layer of conductive resist (Elecktra) was spin-coated to ease the charge dissipation during e-beam lithography (2500 rpm for 50 s, post-baking at 90°C for 2 min).

The required array pattern was then transferred to the resist via e-beam lithography (NanoBeam nB4), and subsequent development in MF-319 solution for 45 s was carried out to remove the unexposed areas. After lithography, the sample was post-baked at 90°C for 5 min to increase the hardness of the remaining exposed areas.

Finally, the samples were etched in a SF6 and O2 plasma mixture using an inductively coupled plasma-reactive ion etching (ICP-RIE) system. ICP (300 W) was used to create high-density plasma, which was then accelerated toward the sample by the RIE (200 W) component to achieve directional etching. A low pressure of 2 Pa was used to avoid frequent collisions inside the plasma cloud.

B. Optical Characterization

Optical properties of the metasurfaces were characterized using a back focal plane spectroscopy setup (a detailed description of this setup can be found in Supplement 1 Section 4). Sample excitation was provided by a linearly polarized beam from a supercontinuum laser source (Fianium SC400-6), which was carefully filtered and attenuated to ensure that GST inclusions do not change phase during the experiment. The beam was then focused on the sample using a 10x objective (Mitutoyo M Plan Apo NIR, NA = 0.26). The end facet of a multimode fiber (50 |im core, NA = 0.22) was manually scanned along the two main axes of the reflected light spot in the back focal plane of the optical setup with a step of 100 |m. Light collected by the fiber was then sent to a spectrometer (Horiba LabRAM HR800) equipped with water-cooled CCD (Andor iDus InGaAs) with a sensitivity range up to 1650 nm. This provided the metasurface reflectance spectra for both TE and TM polarizations, with an angle of incidence resolution of approximately 1 deg. The spectra were normalized to the reflectance of a protected silver mirror (Thorlabs, >97.5% reflectance in the spectral range ofinterest).

C. Femtosecond Optical Switching

Optical-switching experiments were carried out using a femtosecond laser source (Yb3+ Avesta TEMA-150, central wavelength 1050 nm, pulse duration 150 fs). To facilitate fast switching of large areas of the metasurface, the laser was focused on the sample with a 5 cm achromatic doublet lens that provided a smooth laser intensity profile in the focal plane (with a laser spot FWHM of approximately 15 | m). The sample was then scanned with respect to the laser beam to achieve switching of even larger areas of the metasurface. For these studies, we used the metasurface with a slightly lower diameter of the disks (311 nm) so that the position of the ED peak for both crystalline and amorphous states fitted into the spectral range of detection of our InGaAs CCD (up to 1650 nm).

5. CONCLUSION

We have introduced and experimentally validated a new concept in all-dielectric optical metasurfaces based on a hybrid combination of high-index dielectric building blocks with embedded subwavelength inclusions ofchalcogenide phase-change materials. By using this hybrid approach, we are able not only to provide on-demand dynamic control of light amplitude, but also to deliver a very high efficiency of operation over a very wide spectral range by a judicious material choice. We demonstrated the flexibility and universality of our approach by the design and development of hybrid metasurfaces for applications as switchable spectral filters in the near-infrared and dynamic color generation in the visible spectrum.

For the spectral filtering application, we proposed and experimentally demonstrated Si/Ge2Sb2Te5 hybrid metasurfaces consisting of arrays of low-aspect-ratio nanocylinders in which the magnetic and electric dipole resonances are specifically designed to be located at the telecommunication O and C bands (X = 1320 nm and 1550 nm, respectively). This meta-device has dual-band filtering/modulation capabilities when the Ge2Sb2Te5 layer is amorphous, but switches to a single-band configuration after crystallization, due to individual cancellation of the ED mode only (made achievable via strategic location of the PCM inclusions). Fabricated devices performed very much in line with theoretical (numerical) simulations, with an experimental contrast in reflection of 70% at 1540 nm obtained, using a GST layer of only 15 nm in thickness (~X0/100). This device has, to the best of our knowledge, the highest contrast/volume relationship reported for any PCM-based optical metasurface. Perhaps more importantly, the ability to use ultra-thin PCM layers in our hybrid metasurface approach is a critical factor in terms of ensuring a successful re-amorphization process, which requires cooling rates of the order of tens of degrees per nanosecond, rates that are unachievable when using large PCM volumes (due to the relatively low thermal conductivity of phase-change materials) [23,24,31]. Indeed, the use ofultra-thin PCM layers enabled us to write, using a femtosecond laser, reversible multilevel states in the phase-change layer, thus delivering the capability for ultra-fine dynamic control over the spectral position and amplitude of the metasurface resonance.

Finally, we have also demonstrated hybrid dielectric/PCM metasurfaces for operation in the visible spectrum, here for tunable non-volatile color generation in reflection and/or transmission. For this purpose, we used subwavelength inclusions ofthe low-loss (when in the amorphous phase) PCM Sb2S3 [41] in combination with the high-index dielectric rutile (TiO2) to generate vivid and high-efficiency structural colors (by engineering the spectral position of the reflectivity resonances). Switching the Sb2S3 to its lossy crystalline state results in resonance damping/shifting, broadening the reflectance and transmittance spectra, thus generating a colorless (white) state. Our hybrid metasurfaces therefore also show potential for the realization of non-volatile digital paper, transmissive or reflective displays, and spectral filtering in the visible regime.

In summary, we have introduced a novel concept in dynamically reconfigurable optical metasurfaces, comprising a hybrid combination of high-index dielectrics and chalcogenide phase-change materials. Our approach can provide a practicable platform for the realization of a new generation of active optical metasurfaces

with new and improved functionalities, with potential applications in numerous technologically important fields, ranging from telecommunications to consumer technology, and security and defense.

Funding. Engineering and Physical Sciences Research Council (EP/L015331/1, EP/M015130/1, EP/M015173/1); Russian Science Foundation (19-72-10086); Russian Foundation for Basic Research (18-32-00527).

Acknowledgment. C.D.W. acknowledges funding of the EPSRC Chalcogenide Advanced Manufacturing Partnership (ChAMP) and Wearable and Flexible Technologies (WAFT) projects. C.R.d.G. acknowledges funding via the EPSRC CDT in metamaterials. Laser-driven switching experiments were performed with the support ofRussian Science Foundation. I.S., P.T., and A.M.A. acknowledge the support from the RFBR for angle-resolved reflectance measurements. A.M.A acknowledges support from the EPSRC Impact Acceleration Account. The authors are grateful to Andrey Bogdanov for useful discussions.

Disclosures. The authors declare no conflicts of interest. See Supplement 1 for supporting content.

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Research Article

Rewritable and Tunable Laser-Induced Optical Gratings in Phase-Change Material Films

Pavel I. Trofimov, Irina G. Bessonova, Petr I. Lazarenko, Demid A. Kirilenko, Nikolay A. Bert, Sergey A. Kozyukhin, and Ivan S. Sinev*

Cite This: ACS Appl. Mater. Interfaces 2021, 13, 32031-32036

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ABSTRACT: Laser-induced periodic surface structures (LIPSS) can be fabricated in virtually all types of solid materials and show great promise for efficient and scalable production of surface patterns with applications in various fields from photonics to engineering. While the majority of LIPSS manifest as modifications of the surface relief, in special cases, laser impact can also lead to periodic modulation of the material phase state. Here, we report on the fabrication of high-quality periodic structures in the films of phase-change material Ge2Sb2Te5 (GST). Due to considerable contrast of the refractive index of GST in its crystalline and amorphous states, the fabricated structures provide strong spatial modulation of the optical properties, which facilitates their applications. By changing the excitation laser wavelength, we observe the scaling of the grating period as well as transition between formation of different types of LIPSS. We optimize the laser exposure routine to achieve large-scale high-quality phase-change gratings with controllable period and demonstrate their reversible tunability through intermediate amorphization steps. Our results reveal the prospects

of fast and rewritable fabrication of high-quality periodic structures for photonics and can serve as a guideline for further development of phase-change material-based optical elements.

KEYWORDS: laser-induced periodic surface structures, phase-change materials, GST, optical gratings, femtosecond laser pulses

■ INTRODUCTION

Nanofabrication technology based on laser processing is a rapidly developing field that can replace costly, low-throughput, and time-consuming lithography techniques that are routinely used for patterning. With the use of ultrashort-pulsed lasers, structuring of virtually all types of materials down to the micro- and nanometer scale is readily available.1 Careful choice of the laser exposure parameters allows the engineering of artificial structures inspired by natural surfaces that exhibit unique wetting, mechanical, and optical properties;2 direct laser writing through multiphoton polymerization enable the fabrication of complex three-dimensional nanostructures that are beyond the capabilities of the most advanced planar technologies.3,4 In terms of the scalability and cost-effectiveness, laser-induced surface patterning is among the most promising methodologies. Recent years witnessed intensive research of the laser-induced periodic surface structures (LIPSS or ripples) that are formed in the focal spot of a linearly polarized laser beam on a multitude of different substrates (metals,5 semiconductors6 and dielectrics7). The exact mechanism of LIPSS formation is still under debate and strongly depends on the material and the laser exposure regime.8 10 The most widely accepted theory that describes the process was developed in 1980s by Sipe et al. It characterized the inhomogeneous absorption of the radiation

induced by the surface roughness via the so-called efficacy factor n. Since then, the theory was developed12 and found many experimental evidences.13,14 Generally, LIPSS are classified by the correlation between their period A and the laser wavelength X [high (HSFL) and low (LSFL) spatial frequency LIPSS] and orientation of the pattern with respect to the laser polarization (parallel or perpendicular). Strongly absorbing substrates, such as metals, tend to exhibit LSFLs with A ~ X oriented perpendicularly to the beam polarization (LSFL-type I).15 Formation of such types of LIPSS is usually associated with the spatially nonuniform electromagnetic energy absorption profile defined by the interference of the incident wave with surface plasmon polaritons (SPPs) excited through the surface roughness. For transparent dielectrics, LIPSS are predominantly generated with smaller periods A ~ X/n parallel to the beam polarization (LSFL/HSFL type II) and originate from the so-called radiative remnant absorption channels.16 Semiconductors can support various LIPSS types

Received: May 7, 2021 Accepted: June 16, 2021 Published: June 30, 2021

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© 2021 American Chemical Society

32031

Figure 1. Generation of gratings parallel to the laser polarization. (a) Full spectral dependence of the period of the generated phase gratings. (b-e) AFM images of phase gratings imprinted in 50 nm GST film with linearly polarized laser pulses of different wavelengths. The polarization direction is horizontal (shown with arrow below the images).

as absorption of intense laser pulses and subsequent generation of critical electron density can turn the material into a metallic (SPP-active) state. Finally, HSFL with deep subwavelength periods are usually manifested in irradiation regimes close to the material damage threshold.18,19 An alternative theory based on the matter reorganization process was developed to describe their formation.20

Mainly, LIPSS formed even in the subablation regimes provide strong modification of the surface relief that is amplified during the multipulse irradiation. However, in special cases for materials capable of sustaining different phase states under normal conditions, it is possible to achieve phase-state modulation instead. This concept has already been demonstrated for silicon.21,22 A phase-change grating formation regime is achieved below the ablation threshold and potentially allows for reversible laser processing as it does not involve removal of the material. However, the relatively small (up to 8%) optical contrast between amorphous and crystalline silicon in the visible (>500 nm) and near-infrared regions and its high melting temperature limit the applicability of such phase gratings for photonics.

A stronger optical contrast between the phase states is available for phase-change materials (PCMs). Importantly, the phase-state switching of the chalcogenide family of PCMs, in particular, GeSbTe-based alloys, is nonvolatile (as opposed to volatile PCMs, such as VO2) and is routinely induced by short laser pulses,23 25 which makes these materials readily compatible with the LIPSS concept. Indeed, femtosecond laser-induced formation of the phase-state modulation in GeSbTe films on tungsten was demonstrated in recent works.26,27 Earlier works also reported formation of laser-induced structures in GST during the processing with nanosecond laser pulses28 and also in other PCMs such as GeTe.29 However, up to now, to the best of our knowledge, spectral dependence of the parameters of phase-change LIPSS as well as large-scale fabrication of such structures have not been explored.

Here, we demonstrate the high-speed fabrication of phase-change gratings Ge2Sb2Te5 (GST) via femtosecond laser writing. We show how both the period and the direction of the gratings can be controlled by the choice of the laser wavelength and study the morphology of the gratings formed for a broad range of excitation wavelengths (700-2200 nm). We also confirm the rewritability of the gratings enabled by the

reversibility of the GST phase change, which shows great promise of laser-induced phase change gratings for tunable optical devices.

■ EXPERIMENTAL SECTION

Phase-change LIPSS were fabricated on thin (50 and 100 nm) amorphous GST films on a sapphire substrate. The substrate material was justified by the relatively high thermal conductivity of sapphire, which facilitates the reamorphization of GST while being transparent in the spectral region of interest. The emission of an optical parametric amplifier (ORPHEUS, Light Conversion) pumped by an Yb femtosecond laser source (PHAROS, Light Conversion) with a pulse duration of «290 fs and a repetition rate of 1 MHz was used for the fabrication of gratings. The laser beam was focused on the film using a 10X long working distance objective (Mitutoyo 10X M Plan Apo NIR). To erase the imprinted LIPSS patterns, the emission from a continuous wave (CW) laser was used (Coherent OBIS LX/LS Laser, central wavelength 532 nm). The laser beam was focused on the film using an achromatic doublet lens (Thorlabs AC254-050-A). Line patterns and two-dimensional gratings with a size of 50 X 50 ^m2 were fabricated by scanning the sample with a piezo stage (PI). As the laser power threshold and focusing was optimized for each wavelength, we consider the laser spot size as diffraction-limited (e.g., 2.6 ^m for 1100 nm and 4.7 ^m for 2000 nm wavelength). The scan speeds were varied within 15-20 ^m/s for 700-1250 nm wavelengths and increased to 200-400 ^m/s for 1400-2000 nm wavelength region. Higher scan speeds that were required to get high-quality structures for longer wavelengths are likely stipulated by worse heat dissipation characteristic for larger laser focal spots. Laser-induced modifications of the surface relief were measured with an atomic force microscope (AIST-NT).

The internal LIPSS structure was studied using a transmission electron microscope (JEOL JEM-2100F, accelerating voltage 200 kV, point resolution 0.19 nm). Spatial distribution of the elemental composition in the films was characterized in TEM with energy-dispersive X-ray spectroscopy (EDX) using a Bruker XFlash 6TI30 spectrometer. Specimens for TEM were prepared by the focused ion beam technique.

■ RESULTS AND DISCUSSION

Narrow band-gap semiconductors, such as GST, demonstrate strong dispersion of the optical properties in the visible and near-infrared ranges. Therefore, unlike metals or wide bandgap dielectric materials, GST is expected to offer strong spectral dependence of the surface morphology of the laser-treated surface. Recent works26,27 demonstrated that GST can support various types of LIPSS depending on the laser

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Figure 2. Phase gratings imprinted in 50 nm GST film at shorter wavelengths. (a) Spectral dependence of the period of phase gratings generated perpendicularly to laser polarization. Blue and red points represent data for scans performed along and perpendicular to the laser polarization direction, respectively. (b) AFM image of phase grating with dual periodicity imprinted in 50 nm GST film with laser pulses at a wavelength of 780 nm. (c) AFM image of the phase grating imprinted at 700 nm. The direction of laser polarization is horizontal (shown with arrow below the images).

Polarization

Figure 3. Bright-field TEM images of the sections of phase change gratings imprinted into 50 nm GST film at (a) 1850 nm laser wavelength and (b,c) 760 nm laser wavelength. Section directions are shown with pink lines in the AFM scans on the right. The inset shows the zoomed-in area of the boundary between amorphous and crystalline phases for different types of LIPSS

wavelength that is used. However, the possibility of fine control over the surface morphology and the transition between the different LIPSS formation regimes have not been explored in detail yet.

We study the spectral dependence of the phase-change LIPSS formed on a 50 nm GST film on sapphire by femtosecond laser pulses focused on the film with a 10X objective. A piezoscanner was used to pattern prolonged areas of the film instead of single spots, which resulted in the formation of large-scale phase gratings with a size of 50 X 50 ^m2. For basic characterization of the imprinted patterns, we used AFM. Despite the absence of ablation, crystallization of GST is accompanied by approximately 6.5% decrease of the material volume.30 Therefore, crystallized areas of the gratings should be manifested as recesses on the film surface, which allows direct characterization of the grating parameters with AFM.

We investigate the periodic surface patterns formed for a broad range of laser wavelengths (from 700 to 2000 nm) and reveal the transition between distinct regimes of LIPSS

formation. Across the whole wavelength range, the optimal average laser power that enabled the formation of high-quality gratings was around 2 mW. Taking into account the laser beam scanning speed, we estimated the average number of pulses per point at (25-33) X 104. AFM data show that for longer laser wavelengths (900-2000 nm), the ripples are imprinted parallel to the laser pulse polarization, which is consistent with HSFL type II regime of LIPSS formation.10 Examples of the measured AFM maps are shown in Figure 1b-e and reveal high-quality periodic patterns with an average recess depth of 1.5 nm (slightly less than what is expected for the reported values for the volume changes for GST upon crystallization, which may indicate additional material reorganization processes). The full spectral dependence of the periods of the structures extracted from the measured AFM data via fast Fourier transform is summarized in Figure 1a. It reveals almost linear increase of the LIPSS period with the wavelength. However, the observed spectral dependence does not fully correlate with the HSFL type II (or HSFL-||) model A ~ X/n, even with account for GST refractive index dispersion (Figure

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Figure 4. Optical images of the consecutive stages of phase-change grating rewriting process in two modifications. The initial gratings [stage 1; panels (a,d)] in both cases are written on a 50 nm GST film deposited on sapphire substrate and exposed to a two-stage reamorphization procedure. The writing process was executed using 2000 nm laser wavelength with horizontal polarization (HP). In the first modification (a-c), the grating is rewritten with the same polarization but different laser wavelength (1850 nm). In the second modification (d-f), the new grating is formed at the same laser wavelength (2000 nm), but using vertical polarization (VP). (b,e) demonstrate the same area of the sample with LIPSS gratings partially erased through two-stage reamorphization. The erased area is enclosed with a red dashed line. (c,f) Show the same area after rewriting with stage 2 LIPSS (the processed area is marked with blue dashed line). *"Two-stage reamorphization" routine involved crystallization of the patterned area with CW laser at 532 nm with VP and subsequent amorphization of the same area using single femtosecond laser pulses (1 Hz, 290 fs, 1500 nm, HP).

1a, black curve, see Supporting Information Figure S1 for the refractive index data). This may be indicative of additional refractive index changes due to ultrafast charge carrier generation by intense femtosecond laser pulses and subsequent spectral shift of the plasma frequency. The observed deviations can also be influenced by the thickness of the film and the resulting efficiency of heat dissipation. In our case, it is confirmed by data for thicker (100 nm) GST film (see Supporting Information Figure S2a-e), which manifests smaller periods that can be associated with low thermal conductivity of GST. Notably, some of the fabricated structures show "wave-like" features in the topography (Figure 1d). These features, however, are manifested randomly and are most likely associated with local defects of the film.

For shorter excitation wavelengths, on the contrary, the ripples are imprinted orthogonally to the polarization direction, which suggests a LSFL type I (or LSFL-^) formation mechanism (Figure 2). Interestingly, in a narrow spectral region (700-800 nm for 50 nm GST film on sapphire) both metallic and dielectric types of LIPSS coexist, which leads to the formation of complex surface morphologies (Figure 2b). The observed periods closely follow the A ~ X relation, as expected for this type of LSFL. We also found that in this wavelength range, the period and morphology of the structures slightly depend on the direction of scanning, and a more stable pattern emerges when the scan is performed along the LSFL-^ ripples (orthogonal to laser polarization). Figure 2 summarizes the data for both scan directions.

Modulation of the GST phase between the ridges and recesses of the generated ripple pattern is readily visible in the optical images of the large period structures that demonstrate clear optical contrast of the ripples (see Supporting

Information Figure S3). To gain further insights into the laser-induced GST phase distribution, we study the internal structure of the fabricated gratings using high-resolution TEM. TEM images of thin lamellas prepared from the gratings fabricated in a 50 nm GST film on sapphire for two characteristic laser wavelengths—1850 and 760 nm—are shown in Figure 3a. They demonstrate the modulation of the phase between the lines of the grating and reveal important peculiarities of different types of generated phase-change LIPSS. In HSFL-|| generated parallel to the laser polarization (Figure 3a), the crystallized region is uniform throughout the entire GST film depth. On the contrary, for the LSFL-^ observed at 760 nm (Figure 3b) the crystallized region is localized toward the GST-air interface, which is consistent with the LIPSS model based on the interference of surface waves, and, in particular, SPPs. HSFL-|| observed at the same wavelength (Figure 3c) demonstrates a more uniform crystallization profile. Note that due to the large scale of the images, the surface relief modulation is barely visible in the TEM data. At the same time, electron diffraction images and EDX maps (Supporting Information Figure S4) confirm both the stability of the element composition between the areas of different phase and the high purity of both amorphous and crystalline phases. EDX images also demonstrate no noticeable surface oxidation of the film, which suggests that this process is either suppressed or extremely slow in our case.

Notably, for structures made on thicker GST films (100 nm), laser treatment regimes that yielded high-quality gratings did not provide full-depth crystallization of the films, which was confirmed by TEM (Supporting Information Figure S2f,g). Different contrast between amorphous/crystalline GST areas and sapphire substrate in the TEM images is

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associated with different cut angles of the lamellas with respect to the sapphire crystalline lattice, which in turn resulted in different optimal angles for TEM imaging.

To finalize our concept of tunable and rewritable optical gratings, we study the reversibility of the LIPSS formation process. We perform the rewriting of the gratings via a multistage process illustrated in Figure 4. We start by cycling the phase state of a 50 nm GST film on sapphire via crystallization with a CW 532 nm laser and subsequent reamorphization with single 290 fs laser pulses at 1500 nm. Then, we scan the resulting reamorphized film with femtosecond laser pulses at 2000 nm, thus forming a well-defined phase grating with a period of »650 nm (LIPSS stage 1, Figure 4a,d). To prepare the film for the second stage of patterning, we cycle its phase state once again through full crystallization with CW 532 nm laser and subsequent reamorphization with single femtosecond laser pulses at 1500 nm (Figure 4b,e). Finally, we demonstrate the rewriting of a grating on top of the cycled area (LIPSS stage 2) either with a different period using femtosecond laser pulses at 1700 nm (Figure 4c) or with a different direction using femtosecond laser pulses at 2000 nm with orthogonal polarization (Figure 4f). The measured optical images of the processed film demonstrate the pronounced spatial modulation of reflectivity that confirms the formation of stage 2 phase gratings. The grating periods (500 and 623 nm for Figure 4c,f, respectively) determined from the optical images are in good agreement with the data for gratings imprinted on the as-deposited films (see Figure 1c,e).

We found that the initial reamorphization step is crucial for successful rewriting of the gratings. Stage 1 LIPSS imprinted on the as-deposited amorphous GST film produce a very well-pronounced surface profile, which is sustained and even amplified during the following reamorphization cycle (see Supporting Information Figure S5). This strongly interferes with the formation of the gratings during the imprint of stage 2 LIPSS. The initial phase cycling process, on the other hand, induces small corrugation on the film surface (see Supporting Information Figure S6), which, on the one hand, suppresses the consequent amplification of the detrimental LIPSS-associated surface relief and, on the other hand, does not interfere with the formation of periodic phase modulation within the film, as the optical images in Figure 4 clearly demonstrate.

■ ASSOCIATED CONTENT [s* Supporting Information

The Supporting Information is available free of charge at https://pubs.acs.org/doi/10.1021/acsami.1c08468.

Optical images of phase-change gratings, AFM and TEM results for the thicker GST film, and AFM images of the reamo rphized film (PDF)

■ AUTHOR INFORMATION Corresponding Author

Ivan S. Sinev — School of Physics and Engineering, ITMO University, 197101 St. Petersburg, Russia; © orcid.org/ 0000-0002-4246-7747; Email: [e-mail]

Authors