Raman scattering of plane-wave and twisted light off chiral molecular liquids

TL;DR

This study investigates how different phases of liquid crystals and the use of twisted light influence quasi-elastic Raman scattering spectra, revealing phase-dependent spectral features and potential angular momentum transfer.

Contribution

It provides new experimental insights into the effects of crystal phase and twisted light on Raman spectra, especially in chiral nematic phases, and explores angular momentum transfer mechanisms.

Findings

Chiral phase shows reduced linewidth Lorentzian spectra.

Iridescence regime correlates with specific scattering features.

Twisted light reveals angular momentum transfer effects.

Abstract

We present an experimental study of the quasi-elastic Raman scattering (QES) of plane-wave and twisted light by liquid crystals. Depending on their temperature, these crystals can exhibit isotropic, nematic and chiral nematic phases. The question is addressed of how the phase of a crystal and the state of incident light can affect the quasi-elastic energy spectra of the scattered radiation, whose shape is usually described by the combination of Lorentzian and Gaussian components. Special attention is paid to the \textit{chiral phase}, for which the Raman QES spectrum is dominated by a Lorentzian with reduced linewidth, pointing to diminished disorder and configurational entropy. Moreover, this phase is also known for a regime of iridescence (selective backscattering) which arises when the wavelength of incident light becomes comparable with the chiral pitch length. Detailed measurements, performed in this \textit{resonant} regime and by employing twisted light, carrying various projections of the orbital angular momentum (OAM), have indicated a low-energy scattering surplus depending on OAM. We argue that this observation might indicate a transfer of angular momentum between light and the liquid crystal.