Dynamical optical response of nematic liquid crystal cells through electrically driven Fréedericksz transition: influence of the nematic layer thickness

Abstract

A dynamical optical characterization of planar nematic liquid-crystal cells electrically driven through the Fréedericksz transition is presented. Our method involves applying voltage steps with different starting voltage close to the Fréedericksz threshold. Measurements are performed on cells with various thickness, from a few microns up to 180µm, and highlight the transient molecular disorder occurring close to the Fréedericksz transition. We show that the transient disorder affects the molecular arrangement mainly in the reorientational plane of the splay motion induced by the planar cell geometry. Moreover, a disorder quantification in terms of optical transmission losses and temporal dynamics enables us to picture the Fréedericksz transition. This characterization provides the identification of the electrical driving conditions for which the effect of the reorientational disorder is minimized. When comparing cells with various thicknesses, it results that thick cells are characterized by a much smoother transition with respect to the conventional step-like Fréedericksz transition of the thin cells, hence, thick cells can be dynamically driven over a large range of voltages, even below the Fréedericksz threshold. The results are discussed in view of novel electro-optical applications of thick layers of nematics. As an example, the experimental conditions for realizing a rapid birefringence scan and the achievement of a large and tunable group delay for femtosecond pulses are presented.

Nanoscale hyperspectral imaging of tilted cholesteric liquid crystal structures

“…We have thus demonstrated the ability to exploit the birefringence of thick nematic cells (200µm), and to control them dynamically. In order to avoid diffusion and optical losses due to the Freédericksz transition in thick nematic layers, 68 we exploit the transient relaxation of the molecules. By applying a bias voltage onto the cell, LC molecules are brought out of equilibrium, then, the voltage is switched off, and LC molecules are dynamically driven back to their rest position.…”

Broadband Spectral Domain Interferometry for Optical Characterization of Nematic Liquid Crystals

“…The mechanism relies on a tunable change of the LC optical index, either due to electrically-induced molecular orientation, or thermal-control of the nematic order [1,2]. Increasing the mixture birefringence together with the liquid crystal layer thickness (e.g., above 50 µm) enables enhancing the induced phase-shift and thus develop innovative applications [3,4]. Recently, electrically-addressed large cell gap nematics have attracted interests for terahertz phase shifters [5,6], high-resolution hyperspectral imaging [7][8][9][10], phase and group delay control of ultrashort pulses trains [11], tunable angular shearing in wedge-shaped cells [12] and terahertz vortex beam generators [13], just to cite a few.…”

“…SDI combined with Fourier analysis is a powerful metrology tool, already developed for optical coherent tomography [26], bulk group index measurement [27], phase response of metasurfaces [28] or ultrashort pulse characterization [29,30]. In previous publications, we have shown that common-path SDI performed with femtosecond pulses is suitable to achieve real-time tracking of the electro-optical changes in birefringence of nematics [3,11]. Here, we demonstrate that such a method, combined with broad spectral bandwidth (above 200 nm) and a low-coherence Michelson interferometer, takes advantage of the high chromatic dispersion of thick nematics.…”

Electro-Optical Switching of Dual-Frequency Nematic Liquid Crystals: Regimes of Thin and Thick Cells

“…Indeed, in the majority of the reported cases, this range of the liquid crystal cell gap was on the order of tens of micrometers or less [5]. Recently, advanced [6] and emerging non-display applications of liquid crystals have resulted in a growing interest to develop and study thicker cells [7]. Thick liquid crystal cells are characterized by a higher phase shift according to Equation (1).…”