Локальное лазерное управление физико-химическими свойствами композиционных тонких плёнок с плазмонными наночастицами тема диссертации и автореферата по ВАК РФ 00.00.00, кандидат наук Андреева Ярослава Михайловна

• Андреева Ярослава Михайловна
• кандидат наук
• 2023

Оглавление диссертации кандидат наук Андреева Ярослава Михайловна

Оглавление

Реферат

Synopsis

Введение

ГЛАВА 1. Обзор литературы

1.1. Оптические нанокомпозиционные материалы

1.2. Тонкоплёночные нанокомпозиционные материалы

1.3. Взаимодействие электромагнитного излучения с нанокомпозиционными средами

1.4. Лазерные методы создания и управления свойствами оптических нанокомпозиционных материалов

1.5. Выводы по Главе

ГЛАВА 2. Изучение процессов роста наночастиц серебра в тонких золь-гель плёнках при непрерывном лазерном воздействии

2.1. Материалы и оборудование

2.1.1. Подготовка экспериментальных образцов тонких пористых плёнок

2.1.2. Внедрение ионов серебра в тонкие плёнки

2.1.3. Схема лазерной обработки нанокомпозиционных плёнок непрерывным излучением

2.2. Лазерная модификация физико-химических свойств нанокомпозиционного материала на основе SiO2 плёнок с наночастицами серебра

2.2.1. Исследование процессов роста наночастиц серебра в нанокомпозиционных плёнках SiO2 под действием непрерывного лазерного излучения

2.2.2. Исследование спектральных характеристик нанокомпозиционного материала на основе SiO2 c серебром после непрерывного лазерного воздействия

2.3. Лазерная модификация физико-химических свойств нанокомпозиционного TiO2:Ag в пределах лазерного трека

2.4. Лазерная модификация физико-химических свойств нанокомпозиционного материала TiO2:Ag при построчном сканировании

2.5. Выводы к Главе

ГЛАВА 3. Изменения свойств нанокомпозиционных тонких плёнок с серебром при лазерном воздействии импульсами наносекундной длительности

3.1. Материалы и методы

3.2. Изменение оптических свойств плёнок ТЮ2 при помощи лазерной модификации размеров наночастиц серебра в их составе

3.3. Формирование периодических структур в тонких плёнках TiO2:Ag при лазерном воздействии импульсами наносекундной длительности

3.3.1. Режимы лазерного формирования периодических структур в плёнках ТЮ2Л

3.3.2. Характеризация периодических структур в плёнках ТЮ2:Л§, сформированных лазерным излучением наносекундной длительности импульса

3.4. Оптические свойства периодических структур на основе плёнок TiO2 с наночастицами серебра

3.4.1. Спектральные свойства периодических структур

3.4.2. Фотолюминесценция периодических структур на основе плёнок ТЮ2Л

3.5. Выводы по Главе

ГЛАВА 4. Модификация свойств плёнок SiO2 с серебром при обработке в поле двулучевой интерференционной картины

4.1. Особенности лазерной обработки с использованием двулучевой интерференционной схемы

4.2. Формирование периодических структур в плёнках SiO2:Ag

4.3. Оптические свойства периодического рельефа в нанокомпозиционной плёнке SiO2:Ag

4.4. Выводы по Главе

ГЛАВА 5 Изучение процессов роста наночастиц золота в композиционных плёнках

5.1. Материалы и методы

5.1.1. Подготовка нанокомпозиционных плёнок TiO2 с частицами золота

5.1.2. Фемтосекундная лазерная обработка плёнок

5.2. Процессы роста наночастиц золота в плёнках TiO2 при фемтосекундном лазерном воздействии

5.3. Оптические свойства нанокомпозиционного материала TiO2:Au

5.4. Механизмы образования нанокомпозиционного материала TiO2:Au

5.4.1. Механизмы роста наночастиц золота в плёнке TiO2 под воздействием фемтосекундного излучения

5.4.2. Механизмы образования полостей в плёнке TiO2 с золотыми наночастицами под воздействием фемтосекундного излучения

5.5. Выводы по Главе

ЗАКЛЮЧЕНИЕ

СПИСОК СОКРАЩЕНИЙ И УСЛОВНЫХ ОБОЗНАЧЕНИЙ

БЛАГОДАРНОСТИ

СПИСОК ИСПОЛЬЗОВАННЫХ ИСТОЧНИКОВ

Приложение А

Приложение Б

Приложение В

Приложение Г

Реферат

Общая характеристика диссертации

Введение диссертации (часть автореферата) на тему «Локальное лазерное управление физико-химическими свойствами композиционных тонких плёнок с плазмонными наночастицами»

Актуальность темы.

Наночастицы и нанокластеры металлов уже долгое время вызывают интерес исследователей, так как из-за своих размеров обладают уникальными магнитными, оптическими, каталитическими, а также функциональными свойствами, такими как антибактериальность, усиление сигналов рамановской спектроскопии и проч. Такие структуры широко применяются в медицине, биосенсорах, устройствах фотоники, солнечной энергетике и наноэлектронике [1-7]. Однако, наночастицы металлов могут быть неустойчивы и со временем агломерируют в кластеры больших размеров или окисляются [8]. Для стабилизации частиц их можно вводить в инертные матрицы, в качестве которых могут использоваться полимеры, пористые стёкла или оксиды ^Ю2, ТЮ2, А1203). Материалы, объединяющие в себе два или более компонента с различными физико-химическими и функциональными свойствами получили название композиционных. При этом в случае, если фракция одного из материалов имеет малые размеры (например, металлические наночастицы и наноструктуры), то такие материалы принято называть нанокомпозиционными.

Нанокомпозиционные материалы, являющиеся предметом исследования в данной диссертационной работе, состоят из материала матрицы в виде тонких плёнок диоксида кремния и диоксида титана, полученных золь-гель методом, и допантов в виде наночастиц благородных металлов, обладающих плазмонными свойствами, а именно частиц серебра и золота. Выбор такой комбинации материалов не случаен. Плёнки БЮ2 и ТЮ2 с толщиной порядка 200 нм практически прозрачны в видимом диапазоне длин волн, термически и химически устойчивы, поэтому подходят для многих оптических применений. Наночастицы серебра и золота в свою очередь обладают эффектом локализованного плазмонного резонанса в видимом диапазоне, поэтому в зависимости от их размеров и формы спектральный отклик материала, содержащего такие частицы, будет меняться.

Таким образом спектр применения наночастиц серебра включает в себя как различные оптические элементы и сенсоры, так и нанофотонные устройства. Наночастицы золота также обладают этим эффектом, кроме того, они практически не окисляются и применяются в фотокатализе и сенсорике.

Разработка современных устройств и гаджетов диктует свои требования к новым материалам, в том числе расширение спектральных диапазонов работы, удешевление производства и увеличение разрешения. Помимо этого, для многих применений в связи с миниатюризацией устройств требуется прецизионное изменение оптических свойств материала, которое может быть достигнуто путём использования наночастиц с определёнными размерами и их распределением. Для управления свойствами нанокомпозиционных материалов в данной работе предлагается применять лазерные источники, которые позволяют локально и точно менять размеры, форму и распределение наночастиц на поверхности и в объеме материала, и тем самым контролируемо управлять его оптическими свойствами. Однако для прецизионной модификации свойств материала необходимо понимать взаимосвязи между режимами лазерного воздействия и структурой полученного материала. Для этого требуется изучить механизмы роста наночастиц и модификации физико-химических свойств материала матрицы под действием лазерного излучения.

Таким образом была сформулирована цель диссертационной работы -исследование лазерно-индуцированных процессов роста плазмонных наночастиц в нанокомпозиционных тонких пленках диоксида титана и диоксида кремния и механизмов изменения их физико-химических свойств.

Задачи работы:

• изучить особенности модификации нанокомпозиционных плёнок при непрерывном и импульсном лазерном воздействии;

• определить зависимости физико-химических свойств нанокомпозиционных материалов от режимов лазерного воздействия;

• изучить основные физические процессы, происходящие при взаимодействии лазерного излучения с нанокомпозиционными материалами разного состава;

• изучить взаимосвязи оптических свойств сформированных нанокомпозиционных материалов с размерами и распределением наночастиц в их составе.

Научная новизна работы:

1. Впервые экспериментально обнаружен и объяснен рост наночастиц серебра как в плёнке диоксида титана, так и в стеклянной подложке под действием непрерывного лазерного излучения с длиной волны 405 нм, связанный с диффузией атомов серебра из плёнки в подложку и их дальнейшей агломерацией, при этом в стеклянной подложке средний диаметр наночастиц значительно превышает диаметр наночастиц в плёнке.

2. Показана возможность и объяснены особенности плазмон-поляритонного механизма формирования периодических структур с различными периодом и ориентацией в нанокомпозиционных плёнках на основе диоксида титана с серебром при воздействии импульсного излучения наносекундной длительности.

3. Описаны механизмы формирования периодического рельефа на поверхности нанокомпозиционных тонких плёнок на основе диоксида кремния с серебром при воздействии на него двулучевого интерференционного поля пикосекундных импульсов и впервые показана возможность локального управления оптическими свойствами полученного материала за счёт варьирования периода и направления полученных структур.

4. Впервые показана возможность локального управления морфологией и оптическими свойствами нанокомпозиционных тонких плёнок на основе диоксида титана с наночастицами золота в составе, реализованная с помощью излучения фемтосекундной длительности импульса за счёт фотовосстановления ионов золота и их дальнейшей коалесценции. При этом процессы роста сопровождаются

образованием полостей вокруг частиц большего размера, образованных вследствие термомеханических напряжений при быстром нагреве.

Теоретическая значимость работы состоит в описании механизмов взаимодействия лазерного излучения с нанокомпозиционными тонкими плёнками на основе диоксида кремния и диоксида титана с плазмонными наночастицами, сформулированных путём анализа и объяснения экспериментальных закономерностей роста таких наночастиц и изменения морфологии и структуры материала под действием лазерного излучения.

Практическая значимость работы состоит в разработке физико-технической базы для создания элементов оптики, плазмоники, фотоники, сенсорики на основе нанокомпозиционных тонких плёнок различного состава за счёт локальной и управляемой лазерной модификации их оптических и физико -химических свойств.

Положения, выносимые на защиту:

Положение 1: Лазерное облучение тонких плёнок ТЮ2 с наночастицами серебра, нанесённых на стеклянные подложки, в непрерывном режиме с нижеследующими параметрами (длиной волны X = 405 нм, плотностью мощности q в диапазоне 42,1 кВт/см2 - 68,8 кВт/см2 и эффективным временем воздействия > 0,125 с) вызывает рост размеров наночастиц в плёнке и ускорение диффузии атомов серебра в подложку (по мере достижения ею температуры размягчения), которая приводит к появлению наночастиц в ней, причём средний диаметр наночастиц в подложке превышает средний диаметр наночастиц, сформированных в плёнке в 2 раза и более.

Положение 2: При воздействии серии лазерных импульсов наносекундной длительности с длиной волны X = 355 нм, следующих с частотой порядка сотен кГц на пористых плёнках ТЮ2 с наночастицами серебра образуются поверхностные периодические структуры двух типов, имеющие электромагнитную природу. При этом по мере увеличения плотности энергии и количества импульсов в зоне воздействия структуры 1 типа с периодом порядка X и вектором решётки д,

параллельным вектору напряженности электрического поля Е (д II Е), сменяются промежуточными структурами, являющимися комбинацией структур 1 и 2 типа, и переходят к структурам 2 типа с периодом порядка Х/2 и вектором решётки 6 ± Ё. Регистрация структур 1 типа связана с частичной денсификацией плёнки ТЮ 2, а регистрация структур 2 типа с циклом плавления-затвердевания материала плёнки.

Положение 3: Формирование периодического рельефа, ориентированного вдоль направления полос двулучевой интерференционной картины, в нанокомпозиционных тонких плёнках SiO2 с наночастицами серебра полем пикосекундного лазерного излучения с длиной волны X =355 нм при частоте следования импульсов порядка десятков Гц происходит при плотности энергии, достаточной для размягчения плёнки SiO2 за счёт теплопередачи от нагретых наночастиц в матрицу. При повышении плотности энергии до значений, достаточных для перехода к конвективным процессам теплообмена (за счет снижения вязкости стекла) и для начала испарения, происходит формирование периодического рельефа с вектором решетки, перпендикулярным направлению вектора напряженности электрического поля лазерного излучения.

Положение 4: При фемтосекундном лазерном воздействии на плёнку TiO2 с наночастицами золота с плотностями энергии порядка десятков мДж/см2 на длине волны 515 нм в килогерцовом диапазоне частот следования импульсов при их количестве в зоне воздействия N > 100000 происходит увеличение диаметра наночастиц до 60 нм, сопровождающееся образованием полостей в плёнке вокруг частиц за счёт высоких скоростей деформации матрицы диоксида титана при ее тепловом расширении.

Все научные положения были обоснованы и доказаны экспериментально, что подробно рассмотрено в настоящей диссертационной работе.

Достоверность научных достижений

Достоверность полученных в диссертации результатов подтверждается повторяемостью экспериментальных результатов, использованием современных высокоточных методов исследования вещества, а также подробным анализом всех

результатов с учётом общепринятых методов обработки. Кроме того, проведённые расчёты и оценочные исследования согласуются с экспериментальными данными и не противоречат результатам, полученным другими авторами в схожих исследованиях.

Внедрение

Данная диссертационная работа может быть отнесена к фундаментальным и поисковым исследованиям. Полученные результаты в части разработки методов локального управления оптическими свойствами нанокомпозиционных тонких плёнок, а также создания прототипов оптических элементов на их основе использовались в ходе выполнения научно-исследовательских работ по теме «Разработка методов создания оптических растровых структур на основе тонких нанокомпозитных золь-гель плёнок» в рамках гранта Российского фонда фундаментальных исследований № 19-32-90247\19 (договор от 22.08.2019). Результаты в части характеризации и описания механизмов роста плазмонных структур в составе плёнок под действием лазерного излучения были использованы в научно-исследовательской работе по теме «Фото-термо-химический синтез наночастиц и наноструктур с прогнозируемыми плазмонными свойствами», в рамках гранта Российского научного фонда № 19-79-10208 (соглашение от 19.08.2019). Результаты работ по методам записи периодических решёток на поверхности плёнок на основе ТЮ2 с серебряными частицами применялись в ходе выполнения работ по теме ««Разработка методов управляемого формирования термохимических лазерно-индуцированных поверхностных периодических структур для современных устройств фотоники» в рамках гранта Российского научного фонда № 21-79-10241.

В результате работы были предложены лазерные технологии изменения оптических свойств нанокомпозиционных материалов, а также концепции оптических элементов на основе нанокомпозиционных тонких плёнок с плазмонными частицами. Данные концепции были детально проработаны и сделаны выводы о границах применимости предложенных технологий. Также были разработаны и протестированы экспериментальные образцы оптических элементов

на основе нанокомпозиционных тонких плёнок. Уровень внедрения описанных выше результатов работ можно условно отнести к стадии разработки технического предложения в соответствии с ГОСТ 2.103-2013.

Личный вклад автора

Выбор направления научных исследований, постановка цели и задач диссертационной работы, а также разработка плана исследований осуществлялись совместно с научным руководителем. В части проведения практической работы соискатель лично принимал участие во всех описанных экспериментальных исследованиях, в том числе проводил сборку и настройку использованных оптических схем, лично проводил изготовление экспериментальных образцов, а именно пропитку и сушку образцов пористых плёнок. Также аспирант лично проводил измерение спектральных характеристик плёнок после обработки, присутствовал при проведении электронной и атомно-силовой микроскопии, лично производил обработку полученных экспериментальных данных. Анализ, интерпретация и описание полученных зависимостей проводилось лично аспирантом при участии научного руководителя, научных консультантов, а также совместно с соавторами публикаций по тематике диссертации. Высокий личный вклад автора работы подтверждается большим количеством представленных устных докладов на российских и международных конференциях, а также первым авторством большинства публикаций, перечисленных ниже.

Апробация работы

Результаты, полученные в ходе диссертационной работы, были апробированы на международных и российских конференциях в виде устных и стендовых докладов:

1. International Conference on Ultrafast Optical Science (UltrafastLIght-2018), сентябрь 2018, Москва, Россия. Доклад «Registration of periodical structures in Ag-doped sol-gel films by interference of picosecond laser pulses».

2. International Conference on Ultrafast Optical Science (UltrafastLIght-2020), октябрь 2020, Москва, Россия. Доклад «TiO2:Au nanocomposite formation under femtosecond laser irradiation».

3. VII international conference Modern nanotechnologies and nanophotonics for science and industry 2018, ноябрь 2018, Суздаль, Россия. Доклад: «Experimental and numerical study of laser-induced metal nanoparticle formation in thin porous films».

4. VII Всероссийский конгресс молодых ученых, апрель 2018, Санкт-Петербург, Россия. Доклад: «Формирование изолирующих барьеров в силикатных пористых плёнках при лазерной обработке».

5. VIII Всероссийский конгресс молодых ученых, апрель 2019, Санкт-Петербург, Россия. Доклад: «Изучение диффузионного роста наночастиц Ag / Au в поле лазерного излучения для управления оптическими свойствами композитов».

6. IX Всероссийский конгресс молодых ученых, апрель 2020, Санкт-Петербург, Россия. Доклад: «Лазерное формирование и свойства нанокомпозитного материала на основе диоксида титана с наночастицами золота и серебра».

7. XLVII научная и учебно-методическая конференция Университета ИТМО, февраль 2019, Санкт-Петербург, Россия. Доклад: «О возможности сквозной лазерной денсификации пористых золь-гель пленок».

8. XLIX научная и учебно-методическая конференция Университета ИТМО, февраль 2020, Санкт-Петербург, Россия. Доклад: «Лазерно-индуцированный рост Au наночастиц в тонких золь-гель плёнках».

9. 50 научная и учебно-методическая конференция Университета ИТМО, февраль 2021, Санкт-Петербург, Россия. Доклад: «Исследование процессов лазерного формирования нанокомпозитных тонких пленок с наночастицами серебра».

10. International Symposium "Fundamentals of Laser Assisted Micro- and Nanotechnologies" (FLAMN-19), июнь - июль 2019, Санкт-Петербург, Россия. Доклад: «Laser-induced growth and degradation of gold nanoparticles in titanium dioxide porous film».

11. IV International Conference on Metamaterials and Nanophotonics METANANO 2019, июль 2019, Санкт-Петербург, Россия. Доклад: «Laser writing of nanocomposite metasurfaces».

12. The 14th International Congress on Artificial Materials for Novel Wave Phenomena (Metamaterials'2020), октябрь 2020, Нью-Йорк, США. Доклад: «Fabrication and Optical Properties of Plasmonic Nanocomposite Structures».

13. XXXVI International Conference on Equations of State for Matter (ELBRUS 2021)", февраль-март 2021, Чегет, Россия. Доклад: «Laser control of TiO2-based nanocomposites optical properties».

14. XXXVII Fortov International Conference on Equations of State for Matter (ELBRUS 2022) (ELBRUS 2022)", февраль-март 2022, Чегет, Россия. Доклад: «Laser formation of periodical structures on composite thin films».

15. XXXVIII Fortov International Conference on Equations of State for Matter (ELBRUS 2023) (ELBRUS 2023)", февраль-март 2023, Чегет, Россия. Доклад: «Nonlinear optical response of TiO2:Ag-based nanocomposite gratings».

Структура и объем диссертации

Диссертация состоит из Введения, 5 глав, разделённых на подразделы в соответствии с результатами экспериментальных или теоретических исследований, Заключения, Приложений 1 - 4.

Во введении сформулированы основные термины, связанные с тематикой диссертационной работы, рассмотрены вопросы актуальности исследования нанокомпозиционных материалов в целом, сформулированы преимущества лазерных методов модификации свойств таких материалов. В конце введения сформулированы цели и задачи для настоящей диссертационной работы. Кроме того, обосновано соответствие тематики исследования и полученных научных результатов паспорту специальности «Лазерная физика».

В Главе 1 проведен обзор актуальной научной и научно-технической литературы по видам и методам создания нанокомпозиционных материалов, описаны существующие способы модификации свойств нанокомпозиционных материалов. Также в главе описаны свойства золь-гель плёнок диоксида титана и диоксида кремния, являющихся объектом исследования в диссертационной работе.

В Главе 2 рассмотрены методы локального лазерного управления свойствами нанокомпозиционных тонких плёнок на основе диоксида кремния и диоксида титана с наночастицами серебра. В частности:

В разделе 2.1 были описаны методы изготовления нанокомпозиционных тонких плёнок на основе диоксида кремния и диоксида титана. И те, и другие плёнки были получены методом золь-гель синтеза и нанесены на стеклянную подложку методом окунания с постоянной скоростью. После нанесения плёнки подвергались отжигу в муфельной печи при температуре более 340 °С. Таким образом формировались мезопористые плёнки толщиной порядка 170 - 200 нм, которые и были использованы в качестве экспериментальных образцов в данной работе.

Внедрение частиц в пористые плёнки производилось путём пропитки пористой матрицы в растворах различных металлов. Для получения материала с наночастицами серебра плёнки в течение 30 минут пропитывали в водном растворе диамминсеребра, после чего промывали в чистой воде и сушили в начале в струе азота, а затем на воздухе в течение 24 часов. После пропитки плёнки приобретали светло-жёлтый оттенок, свидетельствующий о наличии малых кластеров и нанокристаллов серебра в плёнке. Непосредственно перед лазерной обработкой плёнки подвергали воздействию излучения УФ лампы (X = 240 нм) для того, чтобы сформировать малые наночастицы серебра ^ < 10 нм), тем самым обеспечить начальное резонансное поглощение лазерного излучения материалом.

Лазерная обработка плёнок проводилась при помощи непрерывного полупроводникового лазера с длиной волны 405 нм, имеющего двухмодовое распределение интенсивности по пятну. В ходе экспериментов варьировались скорость сканирования и плотность мощности лазерного излучения.

В разделе 2.2 исследованы процессы роста наночастиц серебра в тонких плёнках SiO2 в зависимости от режимов лазерного воздействия, а также оптические свойства полученных материалов. На рисунке 1 а представлены СЭМ снимки таких плёнок после лазерного воздействия с плотностью мощности q = 109 - 135 кВт/см2 и скорости сканирования Vск = 200 мкм/с, и гистограммы распределения

наночастиц по размерам для этих режимов. Было выявлено, что в рассмотренном диапазоне с увеличением плотности мощности средние размеры наночастиц уменьшаются.

Рисунок 1 - Нанокомпозиционные тонкие плёнки на основе SiO2 с серебром после

лазерной обработки: СЭМ снимки поверхности и гистограммы распределения наночастиц при Vск = 200 мкм/с, q = 109,0 - 135,0 кВт/см2- а; спектры отражения плёнок в зависимости от плотности мощности излучения при Vск = 200 мкм/с - б; спектры отражения в зависимости от скорости сканирования лазерного излучения

при q = 140,1 кВт/см2 - в

При анализе спектральных свойств плёнок, обработанных с разной плотностью мощности излучения и скоростью сканирования (рисунок 1 б, в) было выявлено, что в зависимости от режима наблюдается смещение пика плазмонного резонанса наночастиц серебра в пределах 440 - 510 нм. Такое смещение может быть вызвано как увеличением размеров частиц за счёт коалесценции (на начальных этапах) и оствальдовской переконденсации при повышении температуры, так и с дефрагментацией наночастиц при высоких плотностях мощности. Кроме того, на процессы роста частиц также оказывает влияние положительная обратная связь по коэффициенту поглощения материала, изменяющегося в зависимости от интенсивности лазерного воздействия [9]. Однако однозначных зависимостей положения резонансного пика от режима

лазерного воздействия обнаружить не удалось. Вероятно, данный факт объясняется тем, что диоксид кремния имеет широкую запрещённую зону, таким образом количество свободных электронов для фотовосстановления ионов серебра может быть недостаточно. Свободные носители заряда в этом случае генерируются преимущественно благодаря дефектам структуры плёнки, которые имеют случайное распределение, в связи с чем процессы роста частиц серебра при прямом непрерывном воздействии сложно прогнозировать. Поэтому в работе был предложен более точный метод изменения оптических свойств плёнок на основе диоксида кремния, описанный подробно в главе 4.

В разделе 2.3 представлены данные по лазерно-индуцированному росту наночастиц серебра в плёнках ТЮ2 в пределах одного лазерного трека. Данные исследования позволили определить основные спектральные зависимости от скорости сканирования и мощности излучения, исключив влияние перекрытия между соседними лазерными треками. Было выявлено, что при плотности мощности излучения, превышающей 42,1 кВт/см2 в пределах лазерного трека формируются две характерные области с разными спектральными характеристиками (рисунок 2): (1) центральная область, имеющая визуально светло-жёлтый цвет, спектр которой имеет характерный максимум на длине волны Хтах= 380 нм и его положение практически не зависит от скорости сканирования лазерного воздействия в рассмотренных диапазонах (рисунок 2 а); (и) краевые области, спектральный максимум которых смещается в длинноволновую область с увеличением плотности мощности д = 42,1 - 101,5 кВт/см2 (рисунок 2 б) или уменьшением скорости сканирования излучения в диапазоне Уск = 100 - 800 мкм/с (рисунок 2 г). В первом случае лазерное воздействие не приводит к изменению среднего размера наночастиц, поэтому положение максимума остаётся неизменным, тогда как смещение максимума спектра в красную область говорит о росте наночастиц серебра при повышении температуры.

Рисунок 2 - Спектры отражения областей лазерных треков в нанокомпозиционном материале на основе ТЮ2 записанных при различных параметрах лазерного воздействия: q = 101,5 кВт/см2, Уск =100 - 800 мкм/с - а, б;

Уск =100 мкм/с, q = 42,1 - 101,5 кВт/см2 - в, г

При низких плотностях мощности (42,1 кВт/см2 и менее) центральная область трека также имеет визуально наблюдаемый цвет, при этом в данной области с увеличением мощности цвет приближается к светло-жёлтому, а спектральный максимум отражения смещается в фиолетовую область (рисунок 2 в). Смещение спектра в коротковолновую область, говорит об уменьшении среднего размера наночастиц в составе нанокомпозиционного материала. Такой противоречивый на первый взгляд эффект связан с повышением температуры наночастиц в центре трека при больших плотностях мощности до температур, достаточных для их разрушения (дефрагментации) на меньшие кластеры [10, 11].

Рисунок 3 - Нанокомпозиционные плёнки ТЮ2:А§ после лазерной обработки в пределах лазерного трека: внешний вид лазерных треков, записанных при скорости сканирования 100 мкм/с и плотности мощности в диапазоне 42,1 -75,3 кВт/см2 - а; спектры комбинационного рассеяния в центре и на краях этих треков (буквы А, В и Я соответствуют пикам фаз анатаза, брукита и рутила соответственно) - б. Фигурами на рисунках показаны области, в которых

измерялись спектры

Для того, чтобы определить, происходила ли модификация матрицы диоксида титана при непрерывном лазерном воздействии, были измерены спектры комбинационного рассеяния (КРС) материала в центре (области, обозначенные звёздочками) и на краях лазерного трека (области, обозначенные кругами) (рисунок 3). До лазерной обработки плёнка ТЮ2 была аморфной. После лазерной обработки в центральной области при плотности мощности 42,1 кВт/см2, а также в краевых областях, где наблюдался цвет, были детектированы характерные пики анатаза (обозначены буквой А на спектрах) в областях 146 - 641 см-1, интенсивность которых увеличивалась с повышением плотности мощности, а также пики, характерные для ТЮ2 в фазе брукита (буква В на спектрах) (рисунок 3 в). В центре трека с повышением плотности мощности излучения наблюдалось появление характерного пика рутила (буква Я на спектрах) в районе 143 см-1 и исчезновение пиков, связанных с брукитом (рисунок 3 б). Таким образом, можно сделать выводы о том, что лазерное воздействие приводило к

кристаллизации плёнки TiO2, при этом температуры, достигаемые в центре и на краях трека, были различны. В центре трека материал нагревается до более высоких температур, что приводит к исчезновению пиков, связанных с брукитом, а значит диоксид титана в этом случае полностью переходит в высокотемпературные фазы анатаза и рутила. Обычно при нагревании материала в печи этот процесс начинается при температурах выше 600 °С и полностью завершается при отжиге при температуре 700 °С. На краях трека интенсивности ниже, и температура недостаточна для полного перехода диоксида титана из фазы брукита в фазы анатаза и рутила, следовательно температура на краях в рассмотренных режимах лазерного воздействия не достигала 700 °С [12, 13].

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13. Chen T., Chen G., Xing S., Wu T., Chen H. Scalable routes to janus Au-SiO2 and ternary Ag-Au-SiO2 nanoparticles // Chemistry of Materials. 2010. V. 22. N 13. P. 3826-3828. doi: 10.1021/cm101155v

14. Сгибнев Е., Никоноров Н.В., Игнатьев А.И., Стародубов Д.С. Люминесцентные свойства кластеров серебра, сформированных методом ионного обмена в фото-термо-рефрактивном стекле // Научно-технический вестник информационных технологий, механики и оптики. 2016. Т. 16. № 6. С. 1031-1037. doi: 10.17586/2226-1494-2016-16-6-1031-1037

15. Kavetskyy T., Kravtsiv M.M., Telbiz G.M., Nuzhdin V.I., Valeev V.F., Stepanov A.L. Surface plasmon resonance band of ion-synthesized Ag nanoparticles in high dose Ag:PMMA nanocomposite films // NATO Science for Peace and Security Series B: Physics and Biophysics. 2018. P. 43-47. doi: 10.1007/978-94-024-1298-7_5

16. Makarov G.N. Laser applications in nanotechnology: nanofabrication using laser ablation and laser nanolithography // Physics-Uspekhi. 2013. V. 56. N 7. P. 643-682. doi: 10.3367/UFNe.0183.201307a.0673

17. Nadar L., Sayah R., Vocanson F., Crespo-Monteiro N., Boukenter A., Sao Joao S., Destouches N. Influence of reduction processes on the colour and photochromism of amorphous mesoporous TiO2 thin films loaded with a silver salt // Photochemical and Photobiological Sciences. 2011. V. 10. N 11. P. 1810-1816. doi: 10.1039/c1pp05172e

18. Garcia M.A. Surface plasmons in metallic nanoparticles: fundamentals and applications // Journal of Physics D: Applied Physics. 2011. V. 44. N 28. P. 283001. doi: 10.1088/0022-3727/44/28/283001

19. Liu Z., Destouches N., Vitrant G., Lefkir Y., Epicier T., Vocanson F., Bakhti S., Fang Y., Bandyopadhyay B., Ahmed M. Understanding the growth mechanisms ofAg nanoparticles controlled by plasmon-induced charge transfers in Ag-TiO2 films // Journal of Physical Chemistry C. 2015. V. 119. N 17. P. 9496-9505. doi: 10.1021/acs.jpcc.5b01350

References

1. Pathak T.K., Kumar V., Purohit L.P. Sputtered Al-N codoped p-type transparent ZnO thin films suitable for optoelectronic devices. Optik,

2016, vol. 127, no. 2, pp. 603-607. doi: 10.1016/j.ijleo.2015.10.013

2. Znaidi L., Touam T., Vrel D., Souded N., Yahia S.B., Brinza O., Fischer A., Boudrioua A. AZO thin films by sol-gel process for integrated optics. Coatings, 2013, vol. 3, no. 3, pp. 126-139. doi: 10.3390/coatings3030126

3. Arya S., Saha S., Ramirez-Vick J.E., Gupta V., Bhansali S., Singh S.P. Recent advances in ZnO nanostructures and thin films for biosensor applications: Review. Analytica Chimica Acta, 2012, vol. 737, pp. 1-21. doi: 10.1016/j.aca.2012.05.048

4. Li M., Chokshi N., DeLeon R.L., Tompa G., Anderson W.A. Radio frequency sputtered zinc oxide thin films with application to metal-semiconductor-metal photodetectors. Thin Solid Films, 2007, vol. 515, no. 18, pp. 7357-7363. doi: 10.1016/j.tsf.2007.03.026

5. Sidorov A.I., Tung N.D., Van Vu N., Nikonorov N.V. Synthesis and characterization of oriented silver nanospheroids in glass. Plasmonics, 2019, vol. 14, no. 4, pp. 979-983. doi: 10.1007/s11468-018-0883-3

6. Wong Z.J., Wang Y., O'Brien K., Rho J., Yin X., Zhang S., Fang N., Yen T.-J., Zhang X. Optical and acoustic metamaterials: superlens, negative refractive index and invisibility cloak. Journal of Optics,

2017, vol. 19, no. 8, pp. 84007. doi: 10.1088/2040-8986/aa7a1f

7. Zhang G., Li G., Zhang Y., Wang X., Cheng G. Method of encapsulating silver nanodots using porous glass and its application in Q-switched all solid-state laser. Optics Express, 2019, vol. 27, no. 4, pp. 5337-5345. doi: 10.1364/OE.27.005337

8. Stalmashonak A., Abdolvand A., Seifert G. Metal-glass nanocomposite for optical storage of information. Applied Physics Letters, 2011, vol. 99, no. 20, pp. 201904. doi: 10.1063/1.3660740

9. Stalmashonak A., Seifert G., Abdolvand A. Ultra-Short Pulsed Laser Engineered Metal-Glass Nanocomposites. Springer, 2013, 70 p., SpringerBriefs in Physics. doi: 10.1007/978-3-319-00437-2

10. Royon A., Bourhis K., Bellec M., Papon G., Bousquet B., Deshayes Y., Cardinal T., Canioni L. Silver clusters embedded in glass as a perennial high capacity optical recording medium. Advanced Materials, 2010, vol. 22, no. 46, pp. 5282-5286. doi: 10.1002/adma.201002413

11. Langlet M., Sow I., Briche S., Messaoud M., Chaix-Pluchery O., Dherbey-Roussel F., Chaudouet P., Stambouli V. Elaboration of an Ag°/TiO2 platform for DNA detection by surface enhanced Raman spectroscopy. Surface Science, 2011, vol. 605, no. 23-24, pp. 20672072. doi: 10.1016/j.susc.2011.08.007

12. Kawata S., Ichimura T., Taguchi A., Kumamoto Y. Nano-Raman scattering microscopy: resolution and enhancement. Chemical Reviews,

2017, vol. 117, no. 7, pp. 4983-5001. doi: 10.1021/acs.chemrev.6b00560

13. Chen T., Chen G., Xing S., Wu T., Chen H. Scalable routes to janus Au-SiO2 and ternary Ag-Au-SiO2 nanoparticles. Chemistry of Materials,

2010, vol. 22, no. 13, pp. 3826-3828. doi: 10.1021/cm101155v

14. Sgibnev Y.M., Nikonorov N.V., Ignatiev A.I., Starodubov D.S. Luminescent properties of silver clusters formed by ion exchange method in photo-thermo-refractive glass. Scientific and Technical Journal of Information Technologies, Mechanics and Optics, 2016, vol. 16, no. 6, pp. 1031-1037. (in Russian). doi: 10.17586/2226-1494-2016-16-6-1031-1037

15. Kavetskyy T., Kravtsiv M.M., Telbiz G.M., Nuzhdin V.I., Valeev V.F., Stepanov A.L. Surface plasmon resonance band of ion-synthesized Ag nanoparticles in high dose Ag:PMMA nanocomposite films. NATO Science for Peace and Security Series B: Physics and Biophysics,

2018, pp. 43-47. doi: 10.1007/978-94-024-1298-7_5

16. Makarov G.N. Laser applications in nanotechnology: nanofabrication using laser ablation and laser nanolithography. Physics-Uspekhi, 2013, vol. 56, no. 7, pp. 643-682. doi: 10.3367/UFNe.0183.201307a.0673

17. Nadar L., Sayah R., Vocanson F., Crespo-Monteiro N., Boukenter A., Sao Joao S., Destouches N. Influence of reduction processes on the colour and photochromism of amorphous mesoporous TiO2 thin films loaded with a silver salt. Photochemical and Photobiological Sciences,

2011, vol. 10, no. 11, pp. 1810-1816. doi: 10.1039/c1pp05172e

18. Garcia M.A. Surface plasmons in metallic nanoparticles: fundamentals and applications. Journal of Physics D: Applied Physics, 2011, vol. 44, no. 28, pp. 283001. doi: 10.1088/0022-3727/44/28/283001

19. Liu Z., Destouches N., Vitrant G., Lefkir Y., Epicier T., Vocanson F., Bakhti S., Fang Y., Bandyopadhyay B., Ahmed M. Understanding the growth mechanisms of Ag nanoparticles controlled by plasmon-induced charge transfers in Ag-TiO2 films. Journal of Physical

20. Kreibig U., Vollmer M. Optical Properties of Metal Clusters. Springer-Verlag, 1995. 535 p. doi: 10.1007/978-3-662-09109-8

21. Sancho-Parramon J., Bosch S., Abdolvand A., Podlipensky A., Seifert G., Graene H. Effective medium models for metal-dielectric composites: An analysis based on the spectral density theory // Proceedings ofSPIE. 2005. V. 5963. P. 596320. doi: 10.1117/12.625125

22. Cavaliere E., Benetti G., Van Bael M., Winckelmans N., Bals S., Gavioli L. Exploring the optical and morphological properties of Ag and Ag/TiO2 nanocomposites grown by supersonic cluster beam deposition // Nanomaterials. 2017. V. 7. N 12. P. 442. doi: 10.3390/nano7120442

Chemistry C, 2015, vol. 119, no. 17, pp. 9496-9505. doi: 10.1021/acs.jpcc.5b01350

20. Kreibig U., Vollmer M. Optical Properties of Metal Clusters. Springer-Verlag, 1995, 535 p. doi: 10.1007/978-3-662-09109-8

21. Sancho-Parramon J., Bosch S., AbdolvandA., Podlipensky A., Seifert G., Graene H. Effective medium models for metal-dielectric composites: An analysis based on the spectral density theory. Proceedings of SPIE, 2005, vol. 5963, pp. 596320. doi: 10.1117/12.625125

22. Cavaliere E., Benetti G., Van Bael M., Winckelmans N., Bals S., Gavioli L. Exploring the optical and morphological properties of Ag and Ag/TiO2 nanocomposites grown by supersonic cluster beam deposition. Nanomaterials, 2017, vol. 7, no. 12, pp. 442. doi: 10.3390/nano7120442

Авторы

Варламов Павел Викторович — инженер-исследователь, Университет ИТМО, Санкт-Петербург, 197101, Российская Федерация, Scopus ID: 57193017775, ORCID ID: 0000-0002-1266-8855, [e-mail]

Михайлова Юлия Витальевна — лаборант, Университет ИТМО, Санкт-Петербург, 197101, Российская Федерация, ORCID ID: 0000-0002-0147-5361, [e-mail]

Андреева Ярослава Михайловна — инженер-исследователь, Университет ИТМО, Санкт-Петербург, 197101, Российская Федерация, Scopus ID: 56971143200, ORCID ID: 0000-0002-0582-3159, [e-mail]

Сергеев Максим Михайлович — кандидат технических наук, научный сотрудник, Университет ИТМО, Санкт-Петербург, 197101, Российская Федерация, Scopus ID: 55624732300, ORCID ID: 0000-0003-2854-9954, [e-mail]

Authors

Pavel V. Varlamov — Research Engineer, ITMO University, Saint Petersburg, 197101, Russian Federation, Scopus ID: 57193017775, ORCID ID: 0000-0002-1266-8855, [e-mail]

Julia V. Mikhailovna — Laboratory Assistant, ITMO University, Saint Petersburg, 197101, Russian Federation, ORCID ID: 0000-0002-0147-5361, [e-mail]

Yaroslava M. Andreeva — Research Engineer, ITMO University, Saint Petersburg, 197101, Russian Federation, Scopus ID: 56971143200, ORCID ID: 0000-0002-0582-3159, [e-mail]

Maxim M. Sergeev — PhD, Scientific Researcher, ITMO University, Saint Petersburg, 197101, Russian Federation, Scopus ID: 55624732300, ORCID ID: 0000-0003-2854-9954, [e-mail]

14th International Congress on Artificial Materials for Novel Wave Phenomena - Metamaterials 2020

New York, USA, Sept. 28th - Oct. 3rd, 2020

Effective Medium Approach for Investigation of Optical Properties of Nanocomposites

with Plasmonic Nanoparticles

P. Varlamov Y. Andreeva1, M. Sergeev1, F. Vocanson 2, T. Itina 1,2

1 ITMO University, Laser photonics and optoelectronics department, 49 Kronverksky pr., 197101, Saint

Petersburg, Russia

2 University of Lyon, Laboratoire Hubert Curien, UMR CNRS 5516/UJM, Saint-Etienne 42000, France

[e-mail]

Abstract - We present an approach based on effective medium theory in the Bruggeman-Bergman approximation for modeling of optical properties of nanocomposites based on TiO2 and SiO2 films with Ag nanoparticles. The importance of volume fraction of metal phase for spectral characteristics of such nanocomposite is demonstrated and confirmed. The increase of this parameter leads to the redshift of the spectral peak and an increase of the reflection amplitude. The numerical calculations demonstrate a good agreement with the experimental data for laser-obtained TiO2- and SiO2-based nanocomposite films with Ag nanoparticles.

I. INTRODUCTION

Nanocomposite materials based on semiconductor matrices, e.g. TiO2, ZnO, AZO with plasmonic nanoparticles become more and more attractive for various applications such as photogalvanic devices, solar cells, photocatalysis, and SERS. For the numerical modeling of such systems, different approaches are employed, including generalized Mie theory and direct solutions of Maxwell equations (solved either by FDTD or FEM) [1]. Despite the obvious advantages of each method, they all have their own restrictions and limitations.

Optical properties of nanocomposite materials differ due to the plasmonic properties of noble metal nanoparticles, which depend on the shape, size, and dispersion of nanoparticles, as well as on the optical properties of surrounding media [2, 3]. The effective medium theory in various approximations [4, 5] allows us to consider the nanoparticles properties and the matrix surrounding them. In most approximations, the volume fraction of the metal phase is supposed to be utilized, however, experimental determination of this parameter is rather difficult, and the calculation methods are not fully described in the literature.

In this paper, we consider an effective medium theory in the Bruggeman-Bergman approximation for calculations of the spectral characteristics of composite materials based on solid-state thin film with spherical silver nanoparticles. The volume fraction of the metal phase for determining the spectral characteristics is calculated taking into account the size dispersion of nanoparticles. We considered all the metallic inclusions to be conventionally divided into three ensembles: nanoclusters (atomic radius - 1 nm), seeds (1 - 3 nm), and nanoparticles (3 - 50 nm). The effect of nanoparticles dispersion and the radius on volume fraction of the metal phase and spectral characteristics is demonstrated. The simulation results are compared with experimental data of nanoparticles size distribution in TiO2 and SiO2 films containing silver nanoparticles and irradiated by UV laser.

II. Results and Discussion

A. Modelling details

The spectral characteristics are determined by the dielectric function, which in the case of a composite material with spherical nanoparticles is determined using the model of effective media, and in the Bruggeman-Bergman approximation is expressed as follows:

978-1-7281-6104-4/20/$31.00 ©2020 IEEE X-102

14th International Congress on Artificial Materials for Novel Wave Phenomena - Metamaterials 2020

New York, USA, Sept. 28th - Oct. 3rd, 2020

s w = ^ ())

1 J

iu,ynp )

Smat (^¡{Smat (W) - S M] ~ U

du

(1)

where g(u,vnp) is a spectral density function, UR/L(vnp) is right and left function of volume fraction, emat(X) is the permittivity of the surrounding medium, emat(X) is the permittivity of the nanoparticles determined by the following equation:

ffl

1

1

a + ir. a + ir

(2)

where eb(X) is the permittivity of the bulk metal, hmpi = 9.2 eV is the plasma frequency, rb = 3.464-1013 s-1 is the electron scattering rate. r coefficient is obtained using the following formula:

r = rh +-

4 Vf_ 3 R

(3)

where is a Fermi velocity, R is an average radius of spherical nanoparticles.

The key parameter of determining the optical properties of a nanocomposite is a volume fraction of the metal phase vnp. To determine it, the initial distribution of all metallic inclusions was separated into three ensembles: nanoclusters (rat - 1 nm), nucleuses (1 - 3 nm), and nanoparticles (3 - 50 nm). We considered only nanoparticles to influence the spectral characteristics of the nanocomposite. Thus, the volume fraction is determined from the following equation:

4n

J C (r)dr,

(4)

where rat is an atomic radius of metal, Ca is the atoms size distribution:

(r) = KcCA

i V

r

Cp (r),

(5)

where Cp is the initial nanoparticles size distribution, and Kc is the normalizing constant that is calculated as:

jr = J ffl cp (r)dr + J tJ Cp (r)dr + J W Cp (r)dr .

c V at J V at J V at J

(6)

V —

np

3

r

np

B. Optical properties of the nanocomposite

As it follows from expressions (4) - (6), the initial size distribution of nanoparticles Cp affects the volume fraction of the metal phase. The distribution of nanoparticles, in turn, is determined by the nanoparticles' dispersion and the nanoparticles' radii. The increase of these parameters leads to the growth of the volume fraction of the metal phase (Figure 1). Such a change leads to a change in spectral characteristics, in particular, to an increase of the reflection maximum amplitude and its shift to the IR region of the spectrum.

The calculated spectral characteristics depending on the initial size distribution of nanoparticles were compared with experimental data. For this, TiO2- and SiO2-based nanocomposite films with silver nanoparticles were synthesized by laser treatment with continuous wave irradiation in the visible range (X = 405 nm). The parameters of the nanoparticles size and distributions were controlled by the scanning speed and laser power. According to SEM measurements and experimentally obtained reflection spectra, with the increase of the

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14th International Congress on Artificial Materials for Novel Wave Phenomena - Metamaterials 2020

New York, USA, Sept. 28th - Oct. 3rd, 2020

nanoparticles size, the reflection maximum is redshifted. On the other hand, the broadening of nanoparticles distribution lead to a slight shift of spectral maxima. These experimental observations are agree with our theoretical conclusions.

Fig. 1. Theoretically modelled influence of nanoparticles distribution on the volume fraction and reflection of nanocomposite: (a) effect of nanoparticles dispersion 0NP; (b) effect of nanoparticles mean radius ArNp. Sectors on concentrations graphs: the orange is for nanoclusters, the pink is for seeds and the white is for nanoparticles.

III. Conclusion

As a result of the study, we have revealed the influence of the distribution parameters of silver nanoparticles doped into a solid-state matrix on optical characteristics of such nanocomposite. Both the nanoparticle dispersion broadening with a constant nanoparticle radius, and the increase of nanoparticles mean radius at a constant dispersion scattering are shown to lead to the increase of the volume fraction of the metal phase. In turn, the change of the volume fraction affects the spectral characteristics. Namely, the amplitude of reflection spectra maximum has been increased with its simultaneous redshift with the increase of the volume fraction. The calculated dependences are consistent with the experimental data obtained for TiO2 and SiO2 films containing silver nanoparticles with different size distributions. This model can be used to describe the optical properties of different nanocomposite materials based on semiconductor and dielectric matrices with noble metal nanoparticles with a relatively low volume fraction of the metal phase (no more than 20%).

Acknowledgement

The reported study was supported by the Russian Science Foundation (Project No. 19-79-10208).

References

[1] F. M. Kahnert, "Numerical methods in electromagnetic scattering theory," Journal of Quantitative Spectroscopy and Radiative Transfer, vol. 79, pp. 775-824, 2003.

[2] A. Trügler, Optical properties of metallic nanoparticles, Springer, Springer International Publishing Switzerland, 2011.

[3] K.-S. Lee and M.A. El-Sayed, "Gold and silver nanoparticles in sensing and imaging: sensitivity of plasmon response to size, shape, and metal composition," The Journal of Physical Chemistry B, vol. 110, pp. 19220-19225, 2006.

[4] S. Kasap and P. Capper, Springer handbook of electronic and photonic materials, 2nd ed., Springer, Springer, Springer International Publishing AG, 2017.

[5] J. Sancho-Parramon, S. Bosch, A. Abdolvand, A. Podlipensky, G. Seifert, H. Graener, "Effective medium models for metal-dielectric composites: an analysis based on the spectral density theory," in Advances in Optical Thin Films II, vol. 5963, p. 596320, International Society for Optics and Photonics, 2005.

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nanomaterials

Article

Laser Fabrication of Highly Ordered Nanocomposite Subwavelength Gratings

Yaroslava Andreeva , Alexander Suvorov1, Evgeniy Grigoryev 2 , Dmitry Khmelenin 3 , Mikhail Zhukov 4 , Vladimir Makin 5 and Dmitry Sinev1

Institute of Laser Technologies, ITMO University, 197101 Saint Petersburg, Russia Interdisciplinary Resource Center for Nanotechnology of Research Park of SPbSU, Saint-Petersburg State University, 199034 Saint Petersburg, Russia

Federal Scientific Research Center "Crystallography and Photonics" RAS, 119333 Moscow, Russia Laboratory of Scanning Probe Microscopy and Spectroscopy, Institute for Analytical Instrumentation RAS, 198095 Saint Petersburg, Russia

Institute for Nuclear Energy (Branch), Peter the Great St.Petersburg Polytechnic University, Sosnovy Bor City, 188541 Leningrad Oblast, Russia

Correspondence: [e-mail] (Y.A.); [e-mail] (D.S.)

©

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Citation: Andreeva, Y.; Suvorov, A.; Grigoryev, E.; Khmelenin, D.; Zhukov, M.; Makin, V.; Sinev, D. Laser Fabrication of Highly Ordered Nanocomposite Subwavelength Gratings. Nanomaterials 2022,12, 2811. https://doi.org/10.3390/ nano12162811

Academic Editors: Marcela Socol and Nicoleta Preda

Received: 27 July 2022 Accepted: 11 August 2022 Published: 16 August 2022

Publisher's Note: MDPI stays neutral with regard to jurisdictional claims in published maps and institutional affiliations.

Copyright: © 2022 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/).

Abstract: Optical nanogratings are widely used for different optical, photovoltaic, and sensing devices. However, fabrication methods of highly ordered gratings with the period around optical wavelength range are usually rather expensive and time consuming. In this article, we present high speed single-step approach for fabrication of highly ordered nanocomposite gratings with a period of less than 355 nm. For the purpose, we used commercially available nanosecond-pulsed fiber laser system operating at the wavelength of 355 nm. One-dimensional and two-dimensional nanostructures can be formed by direct laser treatment with different scan speed and intensity. These structures exhibit not only dispersing, but also anisotropic properties. The obtained results open perspectives for easier mass production of polarization splitters and filters, planar optics, and also for security labeling.

Keywords: nanocomposite films; nanogratings; TiO2; silver nanoparticles; surface plasmonpolaritons; direct laser writing

1. Introduction

Periodical structures and nanogratings are advantageous for optics and photonic

devices [1,2], sensors [3-5], SERS substrates [6,7], security labeling [8], changing of wetting properties [9], antibacterial efficiency [10], and other perspective applications. However,

the formation of highly regular gratings with small period is a tricky goal for modern industry due to its physical and optical limitations. Several approaches were proposed, including lithography [11], nanoinscribing [12], surface wrinkling [13], and also laser-based holographic [14,15] and interference [10,16-19] patterning methods. Although promising, these methods sometimes struggle with providing the necessary flexibility, for instance, in recording the free-shape patterns on curved substrates, the task that can be conducted easily by using the direct laser writing [10,20].

Laser-induced formation of periodical structures on metals probably is one of the most versatile and well studied field of direct laser writing techniques. Various issues of laser-induced periodic surface structures (LIPSS) formation relating to the influence of pulse duration [21], laser processing parameters [21-24], material [21,22,25], and polarization [23,26,27] were widely discussed. In the paper of [28], it was shown that accumulation effects could drastically affect on the period and regularity of LIPSS. The pulse number reduces LIPPS period due to the change of optical constants and roughness of pretreated area. Another work, [25], emphasizes the key role of the material choice: the more free path length

5

St

Nanomaterials 2022,12,2811. https://doi.org/10.3390/nano12162811

https://www.mdpi.com/journal/nanomaterials

of surface plasmon polariton for the material at the given wavelength, the less regular gratings form. It was also demonstrated that the orientation of polarization vector along scanning direction [29], elliptical polarization [26,27], as well as that a relatively large size of laser spot [25] and ablation [26] can reduce regularity of LIPSS.

The properties of the resulting grating will obviously depend not only on size and shape of structural elements, but also on the material properties. Recently, the perspectives of hybrid materials such as metal-dielectric nanostructures [30-32], organic-inorganic materials [33,34], and different nanocomposite materials [8,35,36] have been demonstrated. In some cases, the properties of the periodic lattices (the period of the grating and its height) can be combined with the material properties, thereby expanding the possibilities of their application. It was recently demonstrated that a periodic self-organization of plasmonic nanoparticles within a composite medium can exhibit unique optical effects due to the coupling of lattice modes with plasmon resonances of nanoparticles [8,37]. Similar effects were demonstrated in other studies [32,38-40] on periodic arrays of plasmonic and dielectric nanostructures. The optical response of such materials will differ greatly depending on the lattice parameters as well as on the material and dimensions of nanostructures due to the coupling of lattice resonances with electric and magnetic resonances of nanoscale structures. That allows us to use such hybrid structures for coloration, polarization elements, molecular sensing, and nonlinear optics. Thus, there is a great potential hidden behind different hybrid materials.

In this article, we demonstrate that the fast fabrication of highly regular subwavelength gratings consists of titanium dioxide film with small silver nanoparticles. It was shown that nanosecond-pulsed UV exposure within a certain parametric range leads to the formation of one-dimensional and even two-dimensional periodical structures with periods of less than 355 nm. The resulting nanocomposite periodic gratings exhibit anisotropic behavior when viewed in crossed-polarized light: not only the intensity, but also the position of the spectral maxima depends on the rotation angle of the grating. Control throughout the formation of such gratings combined with over processing parameters allows the creation of colorful images with hidden symbols and also paves the way to fast fabrication of various optical elements.

2. Materials and Methods

Thin porous film of titanium dioxide impregnated with ions, molecular clusters, and silver nanoparticles (up to 10 nm in diameter) were used as the samples for research. Previously we also investigated silica-based nanocomposit film in terms of nanogratings formation [41]. However, here, we concentrate on titanium dioxide based films since they, being semiconductor, simplify photoreduction processes and find applications in such promising areas as photovoltaics and sensing. The porous TiO2 film of about 170-180 nm thick was deposited on a glass substrate by dip coating into the sol-gel solution. Porous films were than soaked in aqueous ammoniacal silver nitrate solution (0.5 M) for 30 min. After that, the films were rinsed by pure water and dried for 24 h in dark. Right before laser treatment, the film was irradiated by UV lamp (A = 240 nm) to start reduction of silver ions, which ensured the initial absorption of laser irradiation. Similar preparation process of nanocomposite films was described in detail in [42,43].

The gratings were fabricated by commercially available nanosecond-pulsed UV laser system (Laser Center Inc., Saint Petersburg, Russian Federation) The third harmonic of an ytterbium fiber laser (A = 355 nm) with a pulse duration of 1.5 ns and a pulse repetition rate from 150 to 300 kHz was utilized. The output radiation had a linear polarization and the Gaussian intensity distribution. The beam was focused onto the sample surface with an F-theta lens in a spot with a diameter of d0 = 30 ц-m. Scanning over the surface was carried out using a system of two-axis galvanometric scannators with a variable speed. The study of the surface morphology for the samples before and after treatment was carried out using scanning electron microscopy with the Auriga Laser System-crossbeam SEM-FIB

workstation (Carl Zeiss). The surface relief was studied by atomic force microscopy in the semi-contact mode using an NTEGRA Prima (NT-MDT) microscope.

The optical properties of the obtained structures were studied in the regime of crossed polarizers in reflection and transmission modes. For this purpose, a ZEISS Axio Imager A1.m (Carl Zeiss) optical microscope equipped with a rotary object stage was used. To study the spectral properties of the obtained structures, an AvaSpec-ULS4096CL-EVO (Avantes) fiber spectrometer was used. An optical fiber with a diameter of 300 ^m was connected to one of the observation channels of the microscope. Thus, the system made it possible to measure the spectra from a region on a sample with a diameter of about 200 pm. To investigate the structure of the nanocomposite gratings, a high-angle annular dark field scanning transmission electron microscopy (HAADF-STEM) and energy-dispersive X-ray analysis were carried out using an electron microscope (Tecnai Osiris, FEI). Cross-sections of the film were prepared using scanning electron microscope (Scios DualBeam, FEI).

3. Results

3.1. Laser Fabrication of Nanocomposite Gratings

The formation of periodic gratings in a nanocomposite film was carried out by singlestep direct laser writing. Focused laser beam scanned the surface line by line with different scan speed and overlap between the neighboring scan lines. The pulse duration in the experiment did not change and was equal to t = 1.5 ns. To determine the optimal laser processing parameters, under which the formation of periodic nanogratings occur, arrays of squares were recorded with different laser power density and scanning speeds in the range of F = 21-75 mJ/cm2 and Vsc = 50-500 mm/s correspondingly, for two values of the pulse repetition rate f = 150 and 300 kHz and overlap n = 100 and 200 lin/mm. To compare experimental results, the parameter number of pulses in the area of influence was used, which was determined as N = f ■ n ■ d0/Vsc. The resulting arrays were examined by scanning electron microscopy. The development of structures was studied in detail when increasing the laser fluence in the range of F = 21-56 mJ/cm2 for constant N = 540 pulses per spot (Figure 1a), and for constant F = 27 mJ/cm2 varying N from 54 to 270 pulses (f = 300 kHz, Vsc = 100-500 mm/s) (Figure 1b).

-I-1-1-1-1-1-1—

54 60 68 77 135 180 270

Number of pulses, N

Figure 1. Evolution of periodical nanostructurs formation on TiO2:Ag film: (a) With the increase of power density for N = 540; (b) With the increase of N for F = 27 mJ/cm2. Double arrow is a direction of EE. Gratings with g| | EE and G ± EE are identified for F = 21-25 mJ/cm2 and N = 54-68.

Far from any random combination of laser processing parameters, a periodic relief was formed on the film surface. For instance, for N = 90-540 and laser fluence F = 21-31 mJ/cm2 (Figure 1a, Table A1) LIPSS were formed with the average period of 327 ± 10 nm and a grating vector g (which is a normal to the grating direction) parallel to the electric field vector EE. For higher repetition rate f = 300 kHz when F = 27-67 mJ/cm2 and N = 54-90, the formation

of a periodical structures was also observed on the top of the film (Figure 1b, Table A1). It worth knowing that within the investigated range of laser processing parameters, there were no spallation, destruction or ablation of the film observed. The regularity of LIPSS can be expressed via parameter DLOA (the dispersion in the LIPSS orientation angle), which was analyzed by the fast Fourier transform (FFT) of the images. For the obtained structures the average DLOA 59 was 13 ± 4°. In some cases, it was possible to reduce DLOA 59 up to 7°. The processing of SEM images using FTT also showed the presence of a periodic grating G perpendicular to the main relief g with the grating vector orientation G 1 E. The period of these high-frequency LIPSS was found to be 186 ± 8 nm. Here and further small letters g and d are related to normally oriented LIPSS whereas capital letters G and D aree used for abnormally orientated LIPSS. The statistics over the periods of emerging structures are presented in detail in Table A1 in Appendix A.

Studying the surface of the obtained periodic gratings was also carried out by atomic force microscopy (AFM) in the semi-contact mode. Three-dimensional relief maps of the surface of gratings fabricated at N = 540, F = 21 mJ/cm2 and 23 mJ/cm2 are shown in Figure 2a,b respectively.

Figure 2. Surface topology of the periodic gratings obtained in various laser processing modes. Three-dimensional relief maps obtained using AFM and the corresponding surface relief profiles: (a) N = 540, F = 21 mJ/cm2; (b) N = 540, F = 23 mJ/cm2.

The surface profiles of periodic gratings showed that the height of structures with a period of 327 ± 11 nm was about 35 ± 12 nm. At the same time, we did not observe any noticeable changes in the average level of the surface relief. Taking into account the film initial thickness, one can conclude that the formation of periodic structures was not associated with hydrodynamic effects and film melting. It should be noted that periodically distributed inhomogeneities were observed on the top of the ridges, the height of which was about 14 ± 5 nm (profiles 2 and 4 in Figure 2).

Then, transmission electron microscopy was applied to investigate the structure of the nanocomposite film in detail. The thin lamella with the dimentions of about 10 x 2 x 0.1 |im was cut perpendicularly to the grating vector g by the focused ion beam. HAADF STEM image and EDX map for titanium (red) and silver (green) are demonstrated in Figure 3. In the vicinity of the grating height minimum, the densified area was

obsearved. The film thickness in this area was about 140 ± 10 nm compared to initial value of 175 ± 7nm.

Figure 3. SEM, HAADF STEM and EDX map images of the structure (i) (N = 540, F = 21 mJ/cm2). Dash-dotted outline shows the densification area.

In contrast to previously reported results [8,44], the formation of periodic nanogratings were not accompanied by the growth of silver nanoparticles and their self-organization.

3.2. Optical Properties and Applications

The recorded structures were studied by optical microscopy in a scheme with crossed polarizers. The reflection and transmission spectra of structure (i) (N = 540, F = 21 mJ/cm2) and structure (ii) (N = 540, F = 23 mJ/cm2) measured in this mode are shown in Figure 4a,b correspondingly.

Figure 4. Optical properties of periodic nanocomposite gratings: microgimages of squares and reflection and transmission spectra for different rotation angles of periodic gratings in the crossed-polarizers: (a) structure (i) N = 540, F = 21 mJ/cm2; (b) structure (ii) N = 540, F = 23 mJ/cm2.

Both the intensity of the transmitted and reflected light and the position of the spectral maxima depended on the angle of rotation of the structure. The intensity of gratings was maximal when rotated at the angles of 45° and 135°. At the codirectional position of the periodic structure vector with one of the axes of the polarizers, the intensity of the spectra was minimal (Figure 4a,b). The reflection and transmission spectra of structure (i) exhibited pronounced peaks, the position of which depended on the rotation angle. With the rotation from 0 to 90 degrees, a reflection maximum was observed at a wavelength of 628 nm, while

the spectra also contained a peak at a wavelength of about 515 nm. With further rotation through the angles from 105 to 180 degrees, the peak in the region of 515 nm became the most intense. Similar behavior were also observed in the transmission spectra of structure (i). The visually observed color of the structures during their rotation also changed from blue at 0-90° to green at 105-180°.

The spectra of structure (ii) were significantly different. For rotation angles other than 0°, 90°, and 180°, the reflection spectrum had a maximum at wavelengths of 500-550 nm. For 0°, 90°, and 180° the reflection spectrum was uniform and much less intense. The picture is completely different for the transmission spectra of structure (ii). Regardless of the angle of rotation, the structures have a relatively low transmittance; however, the effect of shifting the spectral minimum to the short-wavelength region with a change in the angle of the structure rotation by 0-90 and 105-180 degrees was also traced.

The described behavior in the reflection and transmission of the gratings indicate that the recorded reliefs have both polarization and dispersion properties. In addition, the degree of manifestation of these properties is seriously affected by the orientation of the structures relative to the polarizer axis.

Examples of possible applications of the described nanocomposite gratings are shown in Figure 5. The combination of different laser exposure modes made it possible to record various color images with hidden security symbols. For instance, one of the sectors on the color wheel was made as the periodic structure (i). When observed in transmitted and reflected light, each sector has its own color. However, if the structure is illuminated from a small angle, this sector exhibits dispersion properties. Such a color hologram can be used as a hidden security label, while observing the surface in crossed polarizers introduces an additional degree of protection against counterfeiting.

Figure 5. Examples of nanocomposite gratings applications: a color spectrum, where each sector was obtained with different laser processing parameters (transmission and reflection photos for different angles of illumination); ITMO University Logo—photos in reflection and scattering modes.

4. Discussion

The formation of structures with a period of the order of the wavelength, perpendicular to the polarization of radiation, is associated with the excitation of a surface plasmon polaritons (SPP) at the interface between the nanocomposite film and air. It is important to note that previously SPP excitation on TiO2:Ag films were reported only for femtosecond laser processing [45,46]. For nanosecond- and longer irradiation, interference of waveguide modes with incident laser beam was the main proposed mechanism of periodic structures formation [8,44].

The conditions for the appearance of such a wave at the boundary between the active media and air are well known and met when Re £1 < -1, where £1 is the dielectric

permittivity of the medium. Such conditions can be reached than the free carrier density in the media increases due to laser induced excitation, therefore TiO2 film acts as metallic.

The free carrier density N* required for the SPP excitation of pure TiO2 can be assumed according the Drude model [47] using following equation (see Appendix B for details):

N* (Re{£ film ) + + 72 )e 0^*ptme

Ne > g2 (1)

where Re(efnm = 8.8384 [48]) is the real part of dielectric permittivity for TiO2 film, &> is the laser frequency, 7 ~ 1014c-1 is the damping constant [49], £0 is the dielectric permittivity, Kpt ~ 0.01 is the optical effective mass of free carriers [49], me is the free electron mass, and e is the electron charge.

Thus, the estimated critical free carrier density for SPP excitation in the TiO2 film—air interface is found to be n* > 6.8 x 1020 cm-1. For pure TiO2 film laser fluence of about 0.3-0.5 J/cm2 is needed [49]. However, in the case of processing a composite film, laser radiation is absorbed not only on titamium dioxide but also on silver nanocrystals and nanoparticles. Moreover, the irradiaton wavelength 355 nm (3.49 eV) is close to the bandgap for TiO2 film (3.43-3.49 eV [50]), so the absorption efficiency is relatively high. Therefore, electron density increases due to the photoexcitation of TiO2 matrix and photoemission of hot electrons from silver nanoparticles into the environment. The latest process is even more effective due to the photoreduction of Ag ions [43] and enhanced absorption of silver for the given wavelength. The contribution of photoexcitation and photoemission processes can be magnified by accumulation effects for a large number of pulses per spot (more than 500) acting at high repetition rate [28]. In addition, due to the impregnation conditions of the film, the concentration of nanoclusters and silver particles on the surface is higher than in the bulk of the film, which also leads to rapid metallization of the interface. These factors create conditions for the excitation of SPP.

The SPP, in turn, interferes with the incident radiation and forms a standing wave and a grating g| |E, the period of which is defined as d = A/q, where A is the wavelength, q is the real part of the complex refractive index of the air-TiO2:Ag interface for SPP. Thus, for structures with a period of d = 327 nm, q is 1.086.

Periodic modulation of the field near the boundary affects the conditions for heating, thus modifying the film, which in turn influences the absorption of subsequent pulses and changes surface morphology. The process of periodic grating g formation occurs through the positive feedback loop of surface absorption enhancement via growing relief height. Therefore, accumulation effects for large number of pulses lead to the decrease of LIPSS formation threshold and affect their regularity.

Formation of a periodically modulated relief with a period of about 186 ± 8 nm with G 1 E occurred on the ridges and in the grooves of the major periodical relief. Such structures are associated with the modes called wedge and channel plasmon-polariton modes [51]. These modes are localized near the interface between two media, propagate along extended groves or ridges in two opposite directions, and then mutually interfere. The period of formed structures is defined as D = A/2£, where A is the wavelength, £ is the real part of the complex refractive index of the interface air- TiO2:Ag for channel (wedge) plasmon-polariton modes. For structures such as £ = 0.954, which is almost equal to n for SPP.

It have been previously demonstrated in [25] that the less SPP decay length of the material LSPP, the more ordered and regular structures appear under laser irradiation. This value can be expressed as:

1

LSPP = 2M(f) (2) where ft is the SPP wave number given by [52]:

o ^ £ air£ m

p = 7 V (3)

L V £air + £m

where c is the speed of light, eair is the dielectric permittivity of air, e fnm is the dielectric permittivity of a medium.

For instance, for A = 355 nm SPP free path in silver (e'= -2.0435 and e"= 0.28156 [53]) LSPPAg ~ 0.32 pm. Taking into account the critical value of free carrier density N* for TiO2 excitation, the dielectric constants of the film are e'*= —1 and e*" = 0.41951. Substituting these threshold values, we obtain L*Spp = 0.034 pm, which is in order of magnitude smaller than for Ag film.

The experimentally obtained grating relief depth was estimated to be 20% of the film thickness, i.e., ^35 ± 12 nm. In the regions of minima of the interference pattern, the surface level did not change significantly. Moreover, transmission electron microscopy revealed the increase of Ti and Ag concentration in the regions of interference maxima after laser exposure. Thus, the mechanism of the remnant relief recording is local densification of the porous film due to local laser heating.

Although it is difficult to determine the optimal depth of the relief due to the lack of information about the optical constants of the composite medium, for most cases, complete suppression of the reflection of laser radiation occurs when h ~ A/8 [54,55]. For 355 nm wavelength the estimated optimal depth is ^44 nm, therefore the depth of the formed composite gratings is close to optimal.

5. Conclusions

In this article, we demonstrated the fabrication of highly ordered periodical nanocom-posite gratings by direct UV laser writing. Formation of periodical gratings with the period of about d = A/n = 327 ± 10 nm and DLOA S9 of 7-8° occurred when laser fluence is relatively low, i.e., less than F = 31 mJ/cm2 and scan speed is about 50-250 mm/s (equal to 540-108 pulses per spot). Gratings formation is related to excitation of surface plasmon-polaritons on the boundary between the nanocomposite film and air under UV ns laser irradiation. The regularity of the gratings is supposed to be insured by the low free path of SPP in the composite material for the given wavelength. At a higher fluence range of up to 67 mJ/cm2 and a lower number of laser pulses per spot, two-dimensional periodical structures are formed. The grooves and ridges of the main relief were found to be modulated by the structures with a smaller period ~D = A/2£ = 186 ± 8 nm. This grating is produced by the mutual interference of wedge and channel plasmon-polariton modes propagating in the opposite directions. However, this kind of 2D periodical gratings is less uniform and elongated. It is worth noting that the considered experimental phenomenon of laser-induced formation of polarization-dependent ordered periodic structures on TiO2:Ag film is a rare example of the universal polariton model realization under nanosecond laser exposure of dielectrics.

The obtained gratings exhibit optical anisotropy when viewed using crossed-polarizers. Not only the intensity of the transmitted and reflected light, but also spectral peaks depend on the orientation of the gratings relative to the polarizers axes. Such properties can be used for the production of planar optical elements, polarization elements and also for hidden security labels.

The proposed laser fabrication method provides faster, easier and more flexible patterning, compared to the conventional interference-based methods [17-19], as it allows the single-step self-organization of highly regular throughout nanocomposite gratings with a performance of about 0.4-0.9 cm2/min. The results show that by using commercially available single-beam scanning system instead of multiple-beam optical scheme, we can reduce the equipment's complexity, ease the spatial tolerance requirements and produce large-area gratings simply by overlapping the irradiated areas.

Author Contributions: Conceptualization, Y.A. and D.S.; methodology, Y.A.; validation, D.S. and V.M.; investigation, Y.A., D.K., E.G. and M.Z.; writing—original draft preparation, Y.A.; writing— review and editing, Y.A., A.S. and V.M.; visualization, Y.A. and A.S.; supervision, D.S. and V.M.; project administration, D.S. All authors have read and agreed to the published version of the manuscript.

Funding: This research was funded by RSF grant number 21-79-10241.

Institutional Review Board Statement: Not applicable. Informed Consent Statement: Not applicable. Data Availability Statement: Not applicable.

Acknowledgments: In part of TEM investigations this work was supported by the Ministry of Science and Higher Education of Russian Federation within the State assignment Federal Scientific Research Centre «Crystallography and Photonics» of Russian Academy of Sciences and was performed using the equipment of the Shared Research Centre of the FSRC «Crystallography and Photonics» of RAS. The authors are grateful to the Interdisciplinary Resource Center for Nanotechnology of St. Petersburg State University for the study of the surface by scanning electron microscopy. Authors acknowledge Francis Vocanson from Laboratoire Hubert Curien, St.Etienne, France for preparation of titanium dioxide thin porous films. Authors also acknowledge the Laser Center Ltd. for the UV laser system. The authors express their gratitude to the ITMO University Core Facility Center "Nanotechnologies".

Conflicts of Interest: The authors declare no conflict of interest. Abbreviations

The following abbreviations are used in this manuscript:

AFM Atomic force microscopy

SEM Scanning electron microscopy

HAADF STEM High-angle annular dark-field scanning transmission electron microscopy

EDX Energy-dispersive X-ray spectroscopy

LIPSS Laser-indused periodic surface structures

SPP Surface plasmon-polariton

Appendix A. Statistical Analysis of the Period and DLOA of the Nanocomposite Gratings

Table A1. Statistical data of the period end DLOA of the obtained gratings depending on number of pulses N and Laser fluence F.

N, pulses F, mJ/cm2 d,nm (g||E) DLOA 56, ° D, nm (G ± E)

54 27 335 19 179

54 30 331 12 184

54 36 335 18 171

54 27 333 16 187

60 30 330 10 194

60 36 342 12 198

60 45 342 8 181

60 52 330 14

60 67 328 15 192

77 27 337 13

77 28 327 8 190

90 27 326 6

90 37 302 14

108 23 326 11

108 28 329 15

108 31 321 16

135 25 330 14

135 28 303 16

180 23 332 17

180 25 340 15

180 28 324 8

270 23 324 17

270 28 315 16

540 21 324 8 192

540 23 316 15

540 25 316 7

Appendix B. Optical Properties Estimations for the Irradiated Material

Changes of optical properties of TiO2 film are assumed to be caused by free carrier response for laser excitation. The dielectric constant of such excited media can be expressed as [47]:

£* = £ film + Ae Drude (A1)

Dielectric constant shift caused by free carriers generation AeDrude can be described then by the Drude model [47]:

/ wv \2 1

Ae Drude = , , ■ 1 (A2)

1 + ^

where w is the excitation frequency, Wp = y £o^B^ is the plasma frequency, e0 is the

dielectric permittivity, ^*pt ~ 0.01 is the optical effective mass of free carriers, me is the free electron mass, e is the electron charge, Ne is the free carrier density, td = 1/ Y is the Drude dumping time.

Taking into account the plasma frequency, expression Equation (A2) can be rewritten

as:

. =__e2 Ne . e2Nej

Ae Drude = e 0 m*Bptme (w2 + y2 ) +1 £0 m*Bptme w(w2 + y2) (A3)

Knowing that real part of dielectric constant of the material should be less than —1 for

SPP excitation, one can write the following inequality:

e2N

Re(e*) = Re(efilm)----^-^ < —1 (A4)

v 7 v fllm' £0m**ptme(w2 + y2)

Thus, the threshold or critical value of free carrier density required for excitation of

SPP can be expressed as it is shown in Equation (1).

Using the estimated critical value of N*B' , the real and imaginary parts of the excited material dielectric constant are: Re e* = —1 and Im (eB) = Im(efilm) + Im(AeDrude) = 0.27851 [48] + 0.141 (from Equation (A3) = 0.41951. These values are used to estimate the SPP free path in the excited TiO2 film according to Equation (2).

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Optics and Lasers in Engineering 124 (2020) 105840

Contents lists available at ScienceDirect

Optics and Lasers in Engineering

journal homepage: www.elsevier.com/locate/optlaseng

Picosecond laser writing of Ag - Si02 nanocomposite nanogratings for optical filtering

Yaroslava Andreevaa'*, Vladislav Kovala, Maksim Sergeeva, Vadim P. Veikoa, Nathalie Destouchesb, Francis Vocansonb, Hongfeng Mab, Anton Loshachenkoc, Tatiana E. Itinaa,b

a ITMO University, Saint Petersburg 197101, Russia

b Laboratoire Hubert Curien, UMR CNRS 5516/UJM-Saint-Etienne/Univ. Lyon, Saint-Etienne 42000, France c St Petersburg University, Saint Petersburg 199034, Russia

A R T I C L E I N F O

A B S T R A C T

Keywords:

Laser surface structuring Laser fabrication Nanoparticles Nanocomposites Silica glass

In this article, we propose a novel approach for dielectric nanograting fabrication by direct laser patterning. Possibilities of a well-controlled ultra-short laser recording of Ag-SiO2 nanocomposite gratings and their optical properties are examined. The mechanisms of laser processing involve silver nanoparticle growth in nanoporous silica glass films and laser interference-based formation of a periodic grating-like nanorelief. It is shown that laser energy should stay below the surface "grooves" formation threshold for laser-inscription of the interference-based grating. Otherwise, another periodic structure oriented parallel to the incident laser polarization appears to erase the interference pattern. The parameter windows required for the controlled fabrication of the obtained structures are determined. The required threshold decay with the number of applied laser pulses is explained by a similarity in the roles of the nanoparticle absorption and surface defect accumulation typically leading to such dependencies. The optical properties of the obtained gratings are shown to depend on the angle between the incidence plane and the grating direction. When these directions coincide, a signal enhancement with a period-dependent blue-shift is revealed in the diffuse scattering spectra. When these directions are perpendicular, the signal is less enhanced, and a red shift is observed. The observed results are promising in short laser fabrication of different optical components, such as reflective optical filters.

1. Introduction

Various surface patterns as well as micro- and nanostructures are traditionally of great practical interest in optics and photonics, where they are used for light manipulation, encoding and decoding optical information, for optical switching, interferometry, SERS, sensors, etc. [1-4]. Additionally, the unique optical properties of the periodic surface structures, such as gratings and metasurfaces, further extend the range of their applications to security, chemistry, biology, and medicine [5].

Numerous studies are focused on the surface nanostructures design and considerable progress in this field was achieved [6,7]. However, a well-controlled fabrication of such structures still represents a great challenge. For this, various methods were proposed ranging from bottom-up, such as colloidal and cluster chemistry [8], to top-down, including nanoparticle-assisted photo-lithography and multi-photon polymerization. However, nanoparticle manipulation at nanoscale and fabrication of well-organized or periodic nanoparticle-containing struc-

tures are still challenging. In fact, many bottom-up procedures are particularly hard to control because of self-assembly that is often involved.

To solve this issue, a well-optimized laser treatment is particularly promising, namely because of the control possibilities that lasers can provide. In fact, lasers are known to be a powerful surface structuring tool [9] that can also be used for growing and tailoring the properties of nano-objects [10,11], and, particularly, for nanoparticle formation in solid matrices [12].

Several mechanisms can cause the formation of laser-induced periodic surface structures (LIPSS) on different materials. Among them, an interference of the incident light with surface plasmon polariton waves can lead to the formation of low spatial frequency LIPPS (LSFL, or ripples) on metals and other plasmonic materials. The period of these structures is defined by laser wavelength and free electron density in the target [13,14]. In addition, high-spatial frequency structures (HSFL) with a period much smaller than laser wavelength were often observed at the

* Corresponding author.

E-mail addresses: [e-mail] (Y. Andreeva), [e-mail] (T.E. Itina). https://doi.Org/10.1016/j.optlaseng.2019.105840

Received 27 May 2019; Received in revised form 22 August 2019; Accepted 22 August 2019 0143-8166/© 2019 Elsevier Ltd. All rights reserved.

periphery of a Gaussian laser spot [15-17]. These HSFL usually appear at lower laser fluences and at large pulse numbers. Commonly, they are perpendicular to laser polarization and were attributed to the interference effect between several scattering modes appearing due to surface bumps and holes. On the other hand, much larger periodic structures, also known as grooves, are often formed in the middle of the Gaussian laser beam upon material melting. Such structures are mostly parallel oriented to laser polarization and are attributed to thermo-capillary waves, instabilities, or other thermodynamic phenomena [14,18]. While these structures are less sensitive to the external factors such as vibrations, the period and orientation of these structures is often hard to control since it depends on many factors, such as refractive index and physical properties of the material, laser fluence, pulse number, surface irregularities and etc [16].

On the other hand, interference schemes based on diffractive optical elements are widely used in direct laser structuring because of their compactness, reliability and simple implementation [19-21]. With short and ultrashort laser pulses, these setups are particularly promising due to the high spatial symmetry of the diffracted beams since the difference in the optical paths should be much less than the pulse duration and close to zero [19]. To achieve a smooth control over the interference pattern, it is also possible to replace one of the lenses with a prism. However, such schemes provide the fixed configuration of the beams.

Laser processing of transparent materials by beam interference further extended the possibilities of both surface and volume nanostruc-turing. The main advantage of this approach is in better control over the periodicity. Additional possibilities related to nanoparticle presence [22,23] and laser beam focusing in bulk were also studied [24]. The photo-controlled organization of metallic NPs in TiO2 and SiO2 films was also demonstrated [12]. Additionally, self-organized nanostructures were shown to be formed by scanning CW laser irradiation [25] and even femtosecond laser pulses [26].

Laser beam interference had already been shown to be promising for the controlled synthesis of silver nanoparticles (Ag NPs) in mesoporous TiO2 films [27]. As a result, periodic structures consisting of highly ordered arrays of NPs were formed with period, comparable to laser wavelength. The advantage of such 2D ordered nanoparticle arrays is in their sensitivity to the incident light, which depends on both the angle of incidence and the polarization direction [28,29].

However, well-controlled synthesis of metallic NPs in nanostructures as well as laser modifications SiO2 surface relief remain unstudied. In fact, the concentration of free electrons is very small in mesoporous SiO2 matrices. These electrons are essential for the efficient ion reduction that allows nanocluster formation. As electron donors, polyatomic organic compounds or metallic salts can be used. For instance, a solution of silver nitrate in ethylene glycol was dissolved in such matrices [30,31]. Upon laser irradiation, ethylene glycol is decomposed, so that free electrons are released and can be involved in the reduction of silver ions. It should be emphasized that the synthesis of Ag NPs in SiO2 films becomes extremely difficult in the absence of such an electron donor.

This work aims to demonstrate the possibilities of a controlled synthesis of periodic structures in porous SiO2 films prepared by the solgel method and impregnated with silver nitrate. For this, we use a laser beam interference set-up with two picosecond lasers operating in a scanning regime [32]. In the experiments performed, laser irradiation not only leads to silver nanoparticle formation and manipulation, but also to the periodic surface structuring of nanocoposite films. The resulted periodic surface structures have several potential applications, namely as sensors, polarization splitters, couplers, etc. Additionally, such structures can be useful for molecular detection, photocataly-sis [12,33] as well as for diffracting deflector used in solar cells [34]. The revealed structure formation mechanisms are examined and explained. Finally, interesting optical properties of the obtained structures are discussed.

Fig. 1. Experimental setup used for controlled direct laser interference patterning of thin nanocomposite film.

2. Materials and methods

2.1. Samples preparation

Mesoporous silica films were prepared following an evaporation induced self assembly (EISA) route as described in [27,35]. Briefly, the sol-gel formulation was prepared by mixing tetraetoxysilane (TEOS) as silica precursor, with hydrochloric acid solution as catalyst, with tri-bloc copolymer F127 as structuring agent dissolved in ethanol. The films were deposited by dip-coating them on cleaned glass substrates before calcination at 673.15 K to remove the copolymer templates and form interconnected mesopores. Next, the films were soaked in an ionic silver nitrate solution 0.5 M (water/EtOH 1:1) for 1h before rinsing and drying at room temperature. The resulting film thickness is around 180 nm. The average pore size in the films is ~ 7 ± 3 nm; the pore volume is ~ 10%.

2.2. Laser treatment

The experimental setup used for laser micro processing by a two-beam interference field is shown in Fig. 1.

The processing schematics consists of a laser source, a block of mirrors, a phase diffraction grating with a period p = 30 |im, a lens with a focal length f1 , a spatial filter, an aspherical lens with a focal length f2 , and a three-coordinate positioning system. Regarding the laser source, we used the third harmonic of picosecond Nd:YAG laser X = 355 nm at the repetition rate of f = 10 Hz and a pulse duration equal to x = 30 ps. The average energy was varied from 3 to 60 J and the number of pulses for one laser spot N was taken in the range from 1 to 1000. The optical transmission of the optical scheme was about 17%

To determine the modification threshold, laser processing was conducted in a stationary mode (without movements of the sample along X and Y axis). Taking into account the pulse repetition rate v = 10 Hz and the spot diameter d = 98 |im, the scanning speed corresponding to this processing mode was determined as follows: V = dv/N.

To create a stable, double-beam interference pattern in the focal plane, a laser beam was split by a phase grating, and two identical beams of the first interference order were subtracted by a spatial filter. Next, the beams were focused by the second lens creating an interference pattern at the intersection [21,36]. In the case of a confocal scheme, the angle between two beams is changed to manage the period of the pattern, which is determined as [37]:

Л = X¡ 2 sine,

(1)

where A is the period of the interference pattern, X is the wavelength, 8 is the angle between two beams. The angle 8 is expressed through the distance between the diffraction maxima of ± 1 order and focal length f2 as follows:

в = arctg(L±l /2/2) .

(2)

order and f2 is a focal length of the second lens. Hence, the easiest way

where L ± 1 is the distance between the diffraction maximums of ± 1

Fig. 2. Nanograting formation. SEM image of the boundary zone with the increase of peak laser energy (a). SEM images of nanogratings with Aj = 540 nm obtained for different laser fluences: interference pattern recorded at F = 0.15 J/cm2 (b); surface grooves obtained at F = 0.35 J/cm2 (c); film destruction occurred at F = 0.68 J/cm2 (d). White double arrow corresponds to the polarization direction, green arrow shows scanning direction. Here, the depth of nanostructures varies with laser fluence, but does not exceed initial film depth.

to change the angle between the interfering beams is to manipulate it with the focal distances of the lenses. In our setup, the first lens was changed to obtain different periods of the interference field. To create interference patterns with the period of about 400 and 500 nm, we used lenses with focal distances of 8 and 9,7 mm, respectively. In both cases, the first micro lens with 300 mm focal distance was used. The expected (calculated) periods of the nanostructures were 415 and 516 nm, but due to non-ideal optical scheme and optical losses on the components the actual nanopatterns with the periods of 410 and 540 nm occurred. In contrast to the interference lithography methods based on the long-term exposure [38], the direct laser interference patterning is much less sensitive to vibrations coming from the optical stages and does not strictly require special fringe-locking systems. However, to increase the accuracy we used vibration-isolated optical table and high-accuracy linear translation stages with direct-drive servomotors.

2.3. Characterization

To investigate the resulting structures on the nanoscale, a CrossBeam workstation Zeiss AURIGA as a scanning electron microscope (SEM) was used. The microscope has relatively high special resolution (images were carried out using SE2 detector at acceleration voltage 10-15 kV, beam current 600 pA) and good image contrast. Optical properties were characterized by using a microscope-spectrophotometer MSFU LOMO. The transmittance and reflectance spectra in the visible wave range as well as diffusion reflectance spectra were measured in circular areas of about 50 (im in diameter. Carl Zeiss Axio Imager A1M optical microscope with the magnification up to 160 X was used for visual analysis of the obtained structures in both reflected and transmitted light modes. The microscope is equipped with additional 1/4 plate allowing observations in the polarization contrast mode.

3. Results and discussion

3.1. Nanograting fabrication

The mesoporous SiO2 thin films are activated by the presence of small Ag clusters and subjected to the scanning laser interference field. The direction of the interference pattern is parallel to the scanning direction. We use two identical laser beams and the absorption is enhanced by the formation and growth of silver nanoparticles (NPs). Consequently, different nanogratings were recorded with a constant scanning speed of VSr = 1.6 (m/s and various laser fluences. Fig. 2 demonstrates SEM microimages of the nanogratings.

The obtained nanostructures turned out to be oriented differently as a function of laser fluence. For lower laser fluences, structures were aligned along the scanning direction, as expected for the interference pattern (Fig. 2b). With the increase in laser fluence, however, instead of the interference pattern based structure, another one appeared and was oriented along laser polarization (Fig. 2c). Further increase in laser

fluence above a certain threshold obviously leaded to film damage (Fig. 2d). The two indicated types of periodic structures can be merged because of the Gaussian laser beam profile. As it is shown in Fig. 2a at laser fluence between 0.15 J/cm2 and 0.35 J/cm2 it is possible to obtain the non-regular pattern, where the parallel oriented structures prevail in the center and the double-beam interference pattern occurs at the border.

An interference pattern is formed at smaller laser energy. With the increase in energy, the periodic structure appears to be parallel to laser polarization, contrary to our laser interference pattern. For instance, this effect can be observed in the central part of the laser track if peak laser fluence is too large. Such periodic self-organization is often observed in laser processing of various surfaces, including metals, semiconductors, dielectrics, ceramics, polymers, and nano-composites [13-16,39-42]. Possible formation scenarios range from wave interference [39] to various instabilities and thermo-capillary waves [43]. The prevailing formation mechanisms depend on the laser-irradiated material, as well as on the fabricated structure orientation and periodicity. On the one hand, periodic sub-wavelength structures, such as "ripples", are typically formed at lower laser flu-ence and mostly oriented perpendicular to laser polarization. These small structures are attributed to the plasmonic effects [42,44]. On the other hand, larger "grooves" are formed at higher fluences and typically oriented parallel to laser polarization. These grooves are formed due to thermo-capillary waves, instabilities, or other thermodynamic phenomena [14].