Resonant Light-Matter Coupling to Chiral Liquid Crystal Phases
Year: 2024
Type: dissertation
Abstract: Chirality is an essential property in sciences, e.g. in the fields of biology, chemistry, and physics. It provides insights in relation to symmetry and fundamental properties relevant for quantum optics, plasmonics and molecules. Due to its non-local properties, chirality is directly related to topology and the occurrence of phase factors in physics. Light carrying an orbital angular momentum (OAM) is also referred to as structured light, twisted light, vortex light or simply OAM light and offers ideal conditions for research on these topics. This is owed to its helical wavefront and tunable magnitude of orbital angular momentum. In the following thesis, we will follow the question under which conditions a strong light-matter coupling onto the chiral phase of matter can be established. We have used liquid crystals (LCs) as a model system for several reasons. In particular, we are interested whether these material systems are capable of receiving or collecting OAM from light. Chiral nematic LCs have a repetitive molecular arrangement after each pitch length, p. The optical properties are similar to photonic crystals, permitting a localization of circularly polarized light. Chiral nematic LCs have long-range attractive dipole-dipole interactions that compete with short-range repulsive interactions. The chiral nematic phase of LCs has a high polarizability, anisotropic optical properties and show birefringence. These unique physical and optical properties make this material system a perfect candidate to probe fluctuation processes by Raman scattering. To gain insight into the dispersion relation of possible quasi-particles involved in the light-matter interactions, we conduct scattering vector dependent Raman experiments in backscattering and transmission geometry. Here, we find evidence for roton-like quasi-particle (QP) contributions that we attribute to the transfer of OAM to the matter. Roton QPs are usually related to long-range dipole-dipole interactions competing with anisotropic short-range repulsion. They have been observed in dipolar Bose-Einstein condensates (BECs), suprafluid helium, and 3D meta-materials. Our observations are substantiated by transmitted laser power measurements at different magnitudes of OAM, where a noticeable decrease of the laser power is observed with right-handed circularly polarized light (RH CPL) carrying an OAM magnitude of l = 2. At the same excitation, roton-like QP behavior is observed in our Raman scattering data. We assign this laser power drop and roton-like dispersion to the resonant conditions between the helical wavefront of OAM with l = 2 and the helically arranged directors of the chiral nematic LC. Within this thesis, the properties of chiral nematic phases of LCs have been tailored by modifying the composition and by adding plasmonic and metallic nanoparticles. In this way, the pitch length has been exactly adapted to the excitation wavelength. Besides the evidence regarding OAM transfer to chiral nematic phase of LCs, our research also enables the distinction between contributions from spin angular momentum (SAM) and OAM within an inelastic light scattering process.
Author: Müllner, Silvia
Language: English
Subfield: Atomic and Molecular Physics, and Optics
Field: Physics and Astronomy
Domain: Physical Sciences
Open Access status: green
OpenAlex ID: W6945169131
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