Design of complex material structures for light control

Ropač, Peter (author), Ravnik, Miha (mentor)

This thesis explores the design and optimization of complex material structures for controlling light, with a particular focus on soft-matter systems such as liquid crystals. The research utilizes parametric and inverse design approaches to create various optical devices. The work is divided into four main research directions. The first two focus on the parametric and inverse design of solid-state and liquid crystal photonic components. The third concentrates on liquid crystal diffraction gratings, while the final research direction focuses on computer-generated holograms. As part of the first research direction, the parametric design of slow-light dual-periodic photonic crystals is presented, enabling arbitrarily high group indices using a single design parameter. The research topic continues with the inverse design of two-dimensional photonic structures with asymmetric transmission properties, achieved via topology optimization for both silicon and organic platforms. The results are three devices with relatively high asymmetric transmission in a range of wavelengths. The second research focus is the design of novel three-dimensional liquid crystal optical waveguides and their incoupling/outcoupling terminations, which are designed through substrate anchoring patterns that define the director configuration, using a new versatile method based on signed distance functions. Additionally, reconfigurable liquid crystal devices and networks, designed using both parametric methods and specially developed material-constrained topology and signed distance-based shape optimization, are presented, focusing on different low-loss reconfigurable beam-splitting, deflecting, light-focusing, and routing devices. The penultimate research topic introduces highly dispersive liquid crystal diffraction gratings based on geometric phase optics. These are designed via specially developed parametric anchoring patterns that allow for fine-tuning of the diffraction spectra and continuous tuning via external electric fields between a highly dispersive and transmissive state. The topic evolves into the material-constrained topology optimization of soft-matter diffraction gratings, which are designed to diffract light at specific angles. The design approach uses a new method to impose liquid crystal elastic energy related material constraints, which would otherwise be very computationally expensive. The final research topic of the thesis focuses on material-constrained computer-generated holograms, which are designed using a custom topology optimization-based algorithm. Furthermore, multi-target and multi-image holograms for data encoding, encryption, and color holograms, which are optimized with the new method and adhere to the material constraints of liquid crystals, are presented. The thesis bridges theoretical modeling and physical implementation, advancing the fields of inverse design, soft-matter, and solid-state photonics.

• Language: English
• Keywords: photonics, inverse design, machine learning, liquid crystals, waveguides, holograms, diffraction gratings, photonic crystals
• Work type: Doctoral dissertation
• Typology: 2.08 - Doctoral Dissertation
• Organization: FMF - Faculty of Mathematics and Physics
• Year: 2025
• PID: 20.500.12556/RUL-174323
• COBISS.SI-ID: 251641091
• Publication date in RUL: 01.10.2025

Secondary language

• Language: Slovenian
• Title: Modeliranje kompleksnih struktur za nadzorovanje poti svetlobe

Abstract

Doktorska naloga raziskuje načrtovanje in optimizacijo kompleksnih materialnih struktur za nadzor svetlobe s posebnim poudarkom na sistemih mehke snovi, kot so tekoči kristali. Uporabljena sta tako parametrični kot inverzni pristop načrtovanja za ustvarjanje različnih optičnih naprav. Delo je razdeljeno na štiri glavne raziskovalne sklope. Prva dva se osredotočata na parametrično in inverzno načrtovanje fotonskih komponent v trdni snovi in tekočih kristalov. Tretji sklop se posveča uklonskim mrežicam iz tekočih kristalov, medtem ko se zadnja smer osredotoča na računalniško generirane holograme. Prva smer se začne s parametričnim načrtovanjem dvo-periodičnih fotonskih kristalov za počasno svetlobo, ki omogočajo poljubno visoke grupne lomne količnike. Nadaljuje se z inverznim načrtovanjem dvodimenzionalnih fotonskih struktur z asimetrično prepustnostjo, načrtovanih s topološko optimizacijo za silicijeve in organske platforme. Rezultat so tri naprave z relativno visoko asimetrično prepustnostjo pri različnih valovnih dolžinah. Druga smer obravnava nove tridimenzionalne optične valovode iz tekočih kristalov in njihove zaključke, ki so načrtovani preko vzorcev sidranja na substratih. Ti vzorci so generirani z novo posplošeno metodo na osnovi funkcij predznačene razdalje. Poglavje predstavlja tudi rekonfigurabilne naprave in vezja iz tekočih kristalov, načrtovane z uporabo tako parametričnih metod kot tudi topološke optimizacije in optimizacije oblike z materialnimi omejitvami na osnovi funkcij predznačene razdalje. Predstavljene so različne rekonfigurabilne naprave za delitev, odklanjanje, fokusiranje in usmerjanje žarkov z nizkimi izgubami. Predzadnji raziskovalni sklop uvaja uklonske mrežice iz tekočih kristalov, ki temeljijo na geometrijski fazi. Te so zasnovane s posebej razvitimi parametričnimi vzorci sidranja, ki omogočajo fino nastavitev uklonskega spektra in z uporabo zunanjih električnih polj zvezen prehod med visoko disperzijskim in prepustnim stanjem. Tema se razvije v topološko optimizacijo uklonskih mrežic iz mehke snovi z materialnimi omejitvami, ki so načrtovane, da uklonijo svetlobo pod določenimi koti. Pristop k načrtovanju uporablja novo metodo za uvedbo materialnih omejitev, povezanih z elastično energijo tekočih kristalov, kar bi bilo sicer zelo računsko zahtevno. Zadnja raziskovalna smer se osredotoča na računalniško generirane holograme, ki so generirani z uporabo novega algoritma na osnovi topološke optimizacije in so ustrezno prilagojeni lastnostim materiala. Poleg tega so predstavljeni tudi večciljni in večconski hologrami za zapisovanje podatkov, šifriranje in barvne holograme, ki so prilagojeni materialnim lastnostim tekočih kristalov. Doktorska naloga se osredotoča na približevanje teoretičnega modeliranja in fizične implementacije ter tako prispeva k področjem inverznega načrtovanja in fotonike v mehki in trdni snovi.

• Keywords: fotonika, inverzno načrtovanje, strojno učenje, tekoči kristali, valovodi, hologrami, uklonske mrežice, fotonski kristali