M. A. Fedorov

Determining the director tilt and phase lag of liquid-crystal cells by optical methods

This paper presents the results of calculations and optically determined experimental values of the initial director tilt, the layer thickness, and the phase lag for cells with a nematic liquid-crystal (LC). Layers of germanium monoxide (GeO), amorphous hydrogenated carbon (a-C:H), and a combination of them were used in the LC cells to vary the director tilt. Known methods for determining the director tilt are compared with a method proposed by the authors. The proposed method has an advantage in the case of oblique orientation of the slope angle of the LC for angles greater than 15° and is of interest for studying LC devices of adaptive optics, since, along with the main parameter—the maximum phase increment—it makes it possible to estimate the director tilt.

Optical modulators based on a dual-frequency nematic liquid crystal

This paper experimentally investigates the dynamic characteristics of electrically controlled modulators based on liquid crystals (LCs) in the visible and near-IR regions and the effect of the electric-field parameters on them, along with the variation of the conditions of the interphase interaction of the LCs with the orienting surface. It is shown that it is effective to use a dual-frequency liquid crystal for phase and amplitude modulation of radiation with wavelength 1.55μm. In an LC modulator operating on the S effect, a 2π phase lag is obtained in a time of 2ms. The switching times can be reduced to the microsecond range when the twist effect is used and the LC layer is about 7μm thick by increasing the voltage from 30to50V.

Optical transmission decay dynamics in dual-frequency nematic liquid crystal cells

We have experimentally studied the S-effect dynamics in a dual-frequency nematic liquid crystal (NLC) cell. It is demonstrated that the optical transmission rise and decay times depend on the mode of control over the NLC director orientation in an applied electric field, including the rectangular (square-wave) dc voltage pulses and sinusoidal low-and high-frequency addressing schemes. It is established that the presence of a thin dielectric layer of amorphous hydrogenated carbon (a-C:H) at the NLC boundary can decrease by an order of magnitude the transmission decay time under the action of a high-frequency voltage as compared to the case of natural elastic relaxation in a cell where only the rise time is controlled.

Effect of surface on phase modulation of light in a nematic layer

The nanorelief of orienting surfaces in a nematic layer is studied experimentally. The initial inclination angle of the director and the phase retardation of light in the crystal are determined, and the director reorientation dynamics in the crystal under SB deformation in an electric field is analyzed. It is shown that a thin layer of amorphous hydrogenated carbon (a-C: H) deposited on a GeO monoxide layer with an anisotropic nanorelief produced by the inclined deposition method smoothens the surface topography without changing the surface structure. Modification of the structure and physicochemical properties of the GeO surface alters the conditions of the anisotropic-elastic interaction at the interface with the liquid crystal, as evidenced by an increase in the S-effect threshold and a decrease in the initial inclination of the director from 22° (on the GeO surface) to 0–6°. Strong influence of the surface nanostructure on the dynamics of the director reorientation in the electric field and on the phase modulation of light is experimentally demonstrated. It is shown that the phase retardation of light in the GeO layer covered by an a-C: H film is twice as large as in the layer of the same thickness with a virgin surface.

Dynamic characteristics of dual-frequency nematic liquid crystal with quasi-homeotropic twist structure

Dynamic characteristics of a liquid crystal (LC) cell with a quasi-homeotropic twist structure formed in a dual-frequency nematic liquid crystal (DFNLC) layer with the director pretilt angle increased to 60° have been experimentally studied. The cell was switched from the off to on state using a 30-kHz electric field, while the reverse (off/on) switching was effected by a 1-kHz field. An increase in the director pretilt angle allowed the switch-on time of a 6.4-μm-thick DFNLC cell to be reduced to 1 ms and the relaxation (switch-off) time, to 0.5 ms.

Reorientation dynamics of a dual-frequency nematic single crystal with quasi-homeotropic structure

This paper discusses the features of the switching dynamics of the phase lag of light by 2π and 4π in cells with a dual-frequency nematic liquid crystal as a result of bend and splay deformation of layers with oblique homeotropic orientation by means of an electric field with frequency 30kHz and as a result of relaxation when a low-frequency field of 1kHz is applied. The slope angle of the director was varied from 36° to 84° by means of cerium oxide layers obliquely deposited in vacuum. Studies of the surface texture of the orienting layers by means of atomic-force microscopy showed that this is achieved as a result of variation of the surface nanotexture.

Two-coordinate electrically controlled liquid-crystal optical phase wedge

Based on the capacity of a parallel-oriented layer of nematic liquid crystal to alter its birefringence under the influence of an applied electric field, a two-coordinate electrically controlled optical phase wedge has been developed.

Influence of the alignment layer and the liquid crystal layer thickness on the characteristics of electrically controlled optical modulators

The screening effect of the amorphous hydrogenated carbon (a-C:H) alignment layer and its dependence on the thickness of a dual-frequency nematic liquid crystal (NLC) layer have been studied. Optimization of the a-C:H layer thickness allows a threshold voltage for the optical S-effect to be reduced and the characteristic switching time and relaxation time of 0.5 and 2.5 ms, respectively, to be obtained for a phase retardation of 2π at a wavelength of 0.86 μm.

Dynamics of twist effect in a dual-frequency nematic liquid crystal

The dynamics of the electrooptical 90° twist effect in a dual-frequency nematic liquid crystal is investigated for wavelengths of 0.65 and 1.55μ m. It is shown that the boundary conditions of the interaction between the phases affect the optical threshold of the twist effect, the contrast, and the working voltage range. The switching time of the twist effect from the off to the on state upon a variation of the amplitude of a rectangular dc voltage pulse from 15 to 50 V changes from 1.5 to 0.3 ms for a thickness of the nematic crystal layer of about 7 μ m. The minimal time of switching from the “on” to the “off” state was 3 ms in the case when relaxation of molecules in a cell with asymmetric boundary conditions was controlled electrically. The dynamic range of transmittance variation at a wavelength of 1.55 μm extended to 30 dB.

Electrically controlled relaxation at twist deformation of a dual-frequency nematic liquid crystal

The relaxation of a dual-frequency liquid crystal at the twist effect and the influence of the external electrical circuit parameters on the relaxation process in the case of a large initial inclination angle (44) of the director are studied. It is found that oscillation arising at the trailing edge of the modulator’s electro-optic response considerably increases the time of relaxation due to the action of a high-frequency electric field. The influence of the electric field on the relaxation time is stronger, the thinner the liquid crystal layer. It is experimentally shown that the duration of the interval between the removal of low-frequency voltage from and the application of high-frequency voltage to the modulator affects the relaxation time.