Optical second-harmonic generation in polar nematic liquid crystalline phasesLovšin, Matija (Avtor) Drevenšek Olenik, Irena (Mentor) Sebastián Ugarteche, Nerea (Komentor) ferroelectric nematic liquid crystalssecond harmonic generationferroelectric nematic fluidsnonlinear opticssecond-order susceptibilityMaker fringes methodpolarization patterning2D SHG active arraysFerroelectric 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.20262026-07-17 08:15:07Doktorsko delo/naloga184918VisID: 160683COBISS_ID: 285876995sl