Optical second-harmonic generation in polar nematic liquid crystalline phases

Lovšin, Matija (author), Drevenšek Olenik, Irena (mentor), Sebastián Ugarteche, Nerea (comentor)

Ferroelectric nematic liquid crystals (FNLCs) are oriented fluids with macroscopic electric polarization. Their polarity breaks inversion symmetry and enables second-order nonlinear optical (NLO) effects such as second harmonic generation (SHG). As the FNLCs were discovered less than a decade ago, we are currently in the early stages of their research. Despite their potential for photonics, their NLO properties are poorly understood, and the goal of this thesis was to show the impact of molecular structure on the second-order susceptibility values in selected archetypical FNLC molecules, thereby demonstrating the importance of molecular design for efficient NLO materials, and to assess the potential of using surface anchoring constraints to create thin-film patterned NLO structures. Molecular characteristics of the FNLCs, such as their donor and acceptor groups, define their second-order susceptibility $\chi^{(2)}$. We evaluated the values of $\chi^{(2)}$ for several compounds. For each material, we measured by symmetry allowed components of $\chi^{(2)}$ via the Maker fringes method, together with their temperature dependence and the relevant values of the refractive indices. In addition to the ferroelectric nematic ($\mathrm{N_F}$) phase, we also analyzed the recently discovered ferroelectric smectic A ($\mathrm{SmA_F}$) phase and the polar chiral smectic C ($\mathrm{SmC_P^H}$) phase. The measured values were compared with the calculated ones based on their molecular properties. Taking advantage of the flexoelectric properties of FNLCs, we performed SHG studies to show that by creating splay-shaped surface alignment, the electric polarization can be guided by the splay deformation. In this way, we studied different possible polarization geometries. We also report studies of some fundamental properties of the $\mathrm{N_F}$ phase: the tendency to reduce splay in the bulk of the cell, the tendency to form $\pi$-twisted structure to reduce electrostatic energy, and the formation of electrically charged topological defects. Building on that knowledge, we aimed to create spatially modulated SHG active structures in which the chosen SHG active areas are controlled by the incident beam polarization. We explored structures with different patterns, cell thicknesses and materials. Using a combination of polarizing optical microscopy (POM), SHG microscopy and computational simulations, we showed that the combination of periodic splay-shaped surface alignment and the spontaneous $\pi$-twist of the FNLC material along the cell thickness, resulting from the balance between the flexoelectric and electrostatic energy, presents an effective modular system with a large contrast in SHG intensity between adjacent regions.

• Language: English
• Keywords: ferroelectric nematic liquid crystals, second harmonic generation, ferroelectric nematic fluids, nonlinear optics, second-order susceptibility, Maker fringes method, polarization patterning, 2D SHG active arrays
• Work type: Doctoral dissertation
• Typology: 2.08 - Doctoral Dissertation
• Organization: FMF - Faculty of Mathematics and Physics
• Year: 2026
• PID: 20.500.12556/RUL-184918
• COBISS.SI-ID: 285876995
• Publication date in RUL: 17.07.2026

Secondary language

• Language: Slovenian
• Title: Optično frekvenčno podvajanje v polarnih nematskih tekočekristalnih fazah

Abstract

Feroelektrični nematski tekoči kristali (FNTK) so urejene tekočine z makroskopsko električno polarizacijo. Zaradi polarnosti niso inverzno simetrični, kar omogoča nelinearne optične (NLO) pojave drugega reda, kot je optično frekvenčno podvojevanje (ang. second harmonic generation - SHG). Ker so bili FNTK odkriti pred manj kot desetletjem, smo trenutno v zgodnji fazi raziskav na tem področju. Kljub njihovemu potencialu v fotoniki, so njihove NLO lastnosti trenutno še slabo raziskane. Cilj te doktorske naloge je bil raziskati vpliv molekulske strukture na parametre povezane s procesom SHG (nelinearno susceptibilnost drugega reda $\chi^{(2)}$) in proučiti možnosti strukturiranja površinske ureditve z namenom ustvarjanja NLO vzorcev na tanki plasti. Molekulske lastnosti FNTK, kot so donorska in akceptorska funkcionalna grupa, določajo njihovo susceptibilnost drugega reda $\chi^{(2)}$. Določili smo vrednosti $\chi^{(2)}$ različnim FNTK materialom. Pri vsakem materialu smo s pomočjo metode Makerjevih oscilacij izmerili več komponent $\chi^{(2)}$, njihovo temperaturno odvisnost in ustrezne vrednosti lomnih količnikov. Poleg feroelektrične nematske ($\mathrm{N_F}$) faze smo proučevali tudi pred kratkim odkriti feroelektrično smektično A ($\mathrm{SmA_F}$) fazo ter polarno kiralno smektično C ($\mathrm{SmC_P^H}$) fazo. Meritve smo primerjali z izračuni, ki so temeljili na njihovih molekularnih lastnostih. Na podlagi njihovih fleksoelektričnih lastnosti smo pokazali, da lahko s pahljačastimi oblikami ureditve na površini celice vodimo smer električne polarizacije. Na ta način smo proučevali več različnih geometrij polarizacije. Pri raziskavi smo opisali tudi nekaj temeljnih lastnosti feroelektrične nematske faze: težnjo k zmanjšanju pahljačaste deformacije v notranjosti celice, težnjo k tvorbi $\pi$-zasukov zaradi zmanjšanja elektrostatične energije in nastanek električno nabitih topoloških defektov. Na osnovi teh raziskav smo želeli ustvariti prostorsko modulirane SHG aktivne strukture, pri katerih SHG aktivna območja kontroliramo s polarizacijo vpadnega žarka. Proučevali smo strukture z različnimi vzorci, debelinami celic in materiali. S kombinacijo polarizacijske optične mikroskopije (POM), SHG mikroskopije ter simulacij smo pokazali, da kombinacija periodične pahljačasto oblikovane ureditve na površini celice in spontanega $\pi$-zasuka vzdolž debeline celice, ki je posledica ravnovesja med fleksoelektrično in elektrostatično energijo, predstavlja učinkovit modularni sistem z velikim kontrastom v SHG intenziteti med različnimi območji.

• Keywords: feroelektrični nematski tekoči kristali, optično frekvenčno podvajanje, feroelektrične nematske tekočine, nelinearna optika, susceptibilnost drugega reda, metoda Makerjevih oscilacij, vodenje polarizacije, 2D SHG aktivne strukture