Deformed-helix Ferroelectric Liquid Crystal Cells Research Articles

27‐3: Deformed Helix Ferroelectric Liquid Crystal Based Threshold‐Less Passive Vibration Sensing System

In this article, we proposed a passive vibration sensing system using a threshold‐less deformed helix ferroelectric liquid crystal (DHFLC) cell. Combining the DHFLC cell, piezoelectric film, and optical detector, the system can transform the slight vibration into light intensity signal without a power supply. The system can reach the sensitivity of 165 V/(m/s) and the linearity <1% full range output. The proposed system will be combined with optical fibers to play an essential role in various vibration‐sensing array applications.

Phase-only modulation of light.

We disclose the method to obtain polarization insensitive phase-only modulation that preserves both the state and the degree of polarization of light modulated using a medium with controlled birefringence. We find that, in the double-pass configuration involving reflection from the Faraday rotator mirror, such a medium acts as the phase-only modulator. The experimental data measured in the Michelson-interferometer-based setup for deformed-helix ferroelectric liquid crystal cells are found to be in good agreement with the theoretical results. For such cells, we experimentally demonstrate high-frequency (4 kHz modulation rate) 2π phase-only light modulation governed by the average phase shift and solve the problem of optical axes switching.

Interferometric and Uhlmann phases of mixed polarization states

In our investigation into the effects of the degree of polarization in modulation of partially polarized light we assume general settings of the interferometry of partially polarized lightwaves and perform theoretical analysis of the Uhlmann and the interferometric phases. We introduce the relative Uhlmann phase determined by the Uhlmann holonomies of interfering beams and show that the interferometric phase generalized to the case of nonunitary evolution can, similar to the Uhlmann phase, be cast into the holonomy defined form. By using the technique based on a two-arm Mach-Zehnder interferometer, two different dynamical regimes of light modulation are experimentally studied: (a) modulation of the input light by the rotating quarter-wave plate (QWP); and (b) modulation of the testing beam by a birefringent plate with electrically controlled anisotropy represented by the deformed-helix ferroelectric liquid crystal (DHFLC) cell. In the setup with the rotating QWP, the interferometric phase is found to be equal to the relative Uhlmann phase. Experimental and theoretical results being in excellent agreement both show that this phase is an oscillating function of the QWP angle and increases with the degree of polarization. For modulation by the DHFLC cell, the data derived from our electro-optic measurements are fitted using the theory of the orientational Kerr effect in FLCs. This theory in combination with the results of fitting is used to evaluate electric field dependencies of the interferometric and the Uhlmann phases.

Frequency voltage controlled light transmittance in ferroelectric liquid crystal cells.

Light transmittance of a short-pitch deformed-helix ferroelectric liquid crystal cell was numerically studied when low and high voltage frequencies ([Formula: see text] kHz and [Formula: see text] kHz, respectively) are applied to the cell. The reported finding in the decay of light transmittance at increasing frequency is obtained due to reasonable simplifications of the temporal dependence of director's azimuthal angle. By taking experimentally known data, we numerically demonstrate that the increase in temperature of ferroelectric liquid crystals yields an insignificant change of light transmittance within all visible spectra.

Light modulation in planar aligned short-pitch deformed-helix ferroelectric liquid crystals.

We study both experimentally and theoretically modulation of light in a planar aligned deformed-helix ferroelectric liquid crystal (DHFLC) cell with subwavelength helix pitch, which is also known as a short-pitch DHFLC. In our experiments, the azimuthal angle of the in-plane optical axis and electrically controlled parts of the principal in-plane refractive indices are measured as a function of voltage applied across the cell. Theoretical results giving the effective optical tensor of a short-pitch DHFLC expressed in terms of the smectic tilt angle and the refractive indices of the ferroelectric liquid crystal (FLC) are used to fit the experimental data. The optical anisotropy of the FLC material is found to be weakly biaxial. For both the transmissive and reflective modes, the results of fitting are applied to model the phase and amplitude modulation of light in the DHFLC cell. We demonstrate that if the thickness of the DHFLC layer is about 50μm, the detrimental effect of field-induced rotation of the in-plane optical axes on the characteristics of an axicon designed using the DHFLC spatial light modulator in the reflective mode is negligible.

V‐shaped electro‐optical mode based on deformed‐helix ferroelectric liquid crystal with subwavelength pitch

Abstract— V‐shaped electro‐optical response is shown, both theoretically and experimentally, to be an inherent property of a deformed‐helix ferroelectric liquid‐crystal cell (DHFLC) under a special choice of the applied rectangular alternating‐electric‐field waveform, frequency, and cell geometry. In contrast to other known V‐shaped ferroelectric liquid‐crystal (FLC) modes, the discovered V‐shaped switching is observed in a broadband frequency range including 1 kHz, and not at a certain characteristic frequency. This type of V‐shaped switching allows for a drastic increase in the operating frequency of field‐sequential‐color (FSC) LCD cells in comparison with fast nematic liquid‐crystal (NLC) modes.

Polarization-gratings approach to deformed-helix ferroelectric liquid crystals with subwavelength pitch

Electro-optical properties of deformed helix ferroelectric liquid crystal (DHFLC) cells are studied by using a general theoretical approach to polarization gratings in which the transmission and reflection matrices of diffraction orders are explicitly related to the evolution operator of equations for the Floquet harmonics. In the short-pitch approximation, a DHFLC cell is shown to be optically equivalent to a uniformly anisotropic biaxial layer where one of the optical axes is normal to the bounding surfaces. For in-plane anisotropy, orientation of the optical axes and birefringence are both determined by the voltage applied across the cell and represent the parameters that govern the transmittance of normally incident light passing through crossed polarizers. We calculate the transmittance as a function of the electric field and compare the computed curves with the experimental data. The theoretical and experimental results are found to be in good agreement.

Effect of Surface Anchoring on Optical Bistability in Deformed Helix Ferroelectric Liquid Crystal

The effect of surface anchoring on the optical bistability in deformed helix ferroelectric liquid crystal (DHFLC) has been investigated by electro-optical and textural methods. The threshold voltage studies have been carried out in strongly and weakly treated DHFLC cells. The long lasting memory effect has been observed in weakly treated DHFLC cells, which is due to minimization of depolarizing and ionic charges. Moreover, the response time has been shown to exhibit the dependence on the surface anchoring and the cell thickness. The informations obtained could be helpful for optimizing the parameters for memory devices based on DHFLC materials.