Photonics and topology of liquid crystal dispersions

Mur, Maruša (author), Muševič, Igor (mentor)

In the thesis we examine three separate problems in liquid crystal dispersions. We study photonic properties of liquid crystal droplets and topological properties in a liquid crystal film. We design two different photonic micro-devices out of liquid crystal droplets, acting as microresonators, and investigate formation of topological defects in a liquid crystal film of a passively driven nematic. In the first topic we study stimulated emission depletion (STED) as a mechanism for all-optical control of light. We investigate STED effect in bulk liquid crystals and check how the depletion depends on liquid crystal phase and STED beam parameters. Later we use STED principle to create an all-optical switch, based on a dyed liquid crystal droplet. We produce tapered polymeric waveguides to couple light in and out of the droplet and show that STED can switch off whispering gallery mode resonances inside this microresonator, working on a sub-nanosecond timescale. In the second topic we study droplets, made of a ferromagnetic nematic liquid crystal, and their response to external magnetic field. When exposed to the field, the director configuration of a droplet acting as a microresonator gets distorted, which affects its optical properties. We show that whispering gallery mode lasing, supported by the microresonator, can be tuned by the applied field. The resonance wavelengths shift linearly with the magnetic field. The direction of the shift depends on whether the magnetic field is applied in the circulation plane of the whispering gallery modes or not. In the third topic we investigate a phenomenon of continuous topological defect creation and annihilation in a system of smectic liquid crystal, coming in contact with aqueous solution of 2-hydroxy-2-methylpropiophenone photoinitiator. We first research experimental geometry and characterize the system. Photoinitiator diffusion causes flows and liquid crystal melting, resulting in a passively driven nematic system. Later we analyze the movement of defects and the underlying flow. Finally we study defect formation and compare it to the theoretical proposal.

• Language: English
• Keywords: optical switch, stimulated emission depletion (STED), all-optical light control, whispering gallery mode laser, ferromagnetic nematic droplet, magnetic field tuning, topological defect formation, passively driven nematic
• Work type: Doctoral dissertation
• Typology: 2.08 - Doctoral Dissertation
• Organization: FMF - Faculty of Mathematics and Physics
• Year: 2019
• PID: 20.500.12556/RUL-106861
• COBISS.SI-ID: 3298916
• Publication date in RUL: 22.03.2019

Secondary language

• Language: Slovenian
• Title: Fotonika in topologija tekočekristalnih disperzij

Abstract

V disertaciji raziskujemo tri ločene probleme s področja tekočekristalnih disperzij. Preučujemo fotonske lastnosti tekočekristalnih kapljic in topološke lastnosti tankega tekočekristalnega filma. Predstavimo dve različni fotonski mikro-napravici, osnovani na tekočekristalni kapljici, ki deluje kot mikroresonator. Poleg tega raziščemo tvorjenje topoloških defektov v filmu pasivno gnanega nematika. V prvem delu preučujemo možnost uporabe principa deplecije s stimulirano emisijo (STED, ang. stimulated emission depletion) za nadzor svetlobe s svetlobo. Raziščemo, kako je deplecija s stimulirano emisijo v tekočekristalnih vzorcih odvisna od tekočekristalne faze in od parametrov STED žarka. Kasneje princip STED-a uporabimo za razvoj povsem optičnega preklopnika, ki ga sestavlja kapljica obarvanega tekočega kristala, sklopljena s polimernimi optičnimi valovodi, po katerih v kapljico in iz nje vodimo svetlobo. Kapljica deluje kot mikroresonator, v katerem se ojači svetloba z resonančnimi frekvencami. Pokažemo, da s STED-om lahko ugašamo "whispering gallery" resonance v mikroresonatorju in da preklapljanje poteka na sub-nanosekundni skali. V drugem delu disertacije preučujemo kapljice feromagnetnega nematika in njihov odziv na zunanje magnetno polje. Le-to spremeni konfiguracijo direktorja v kapljici, kar vpliva na njene optične lastnosti. Kapljice delujejo kot mikroresonatorji, v katerih lahko pride do "whispering gallery" laserske emisije. Prek spremembe efektivnega lomnega količnika magnetno polje povzroči spektralni premik laserskih črt. Premik je linearno odvisen od magnetnega polja, smer premika pa je odvisna od tega, če polje kaže v ravnini kroženja svetlobe ali ne. V tretjem delu preiskujemo pojav kontinuiranega nastajanja in anihilacije topoloških defektov v sistemu smektičnega tekočega kristala, ki pride v kontakt z vodno raztopino fotoiniciatorja (2-hidroksi-2-metilpropiofenon). Fotoiniciator, ki prehaja v tekoči kristal, ga raztaplja in v njem povzroča tokove, tako da dobimo pasivno gnan nematik. V nematskem filmu, ki nastane na vodni površini, analiziramo gibanje defektov in z uporabo mikro-delcev še gibanje tokov. Nazadnje preučimo še nastanek defektov.

• Keywords: optični preklopnik, deplecija s stimulirano emisijo (STED), povsem optični nadzor svetlobe, "whispering gallery" laser, kapljice feromagnetnega nematika, uglaševanje z magnetnim poljem, nastanek topoloških defektov, pasivno gnan nematik