Abhishek Kumar Srivastava

Fast switchable grating based on orthogonal photo alignments of ferroelectric liquid crystals

We demonstrate a fast switchable grating based on ferroelectric liquid crystals and orthogonal planar alignment by means of photo alignments. Both 1D and 2D gratings have been constructed. The proposed diffracting element provides fast response time of around 20 μs, contrast of 7000:1 and high diffraction efficiency, at the electric field of 6 V/μm. The saturated electro-optical (EO) states up to very high frequency (≈5 kHz) are the real advantage of the proposed switchable grating, which opens several opportunities to improve the quality of existing devices and to find new applications.

Polarization independent liquid crystal gratings based on orthogonal photoalignments

The polarization independence of liquid crystal gratings with alternate orthogonal aligned regions is theoritically studied and demonstrated by means of photoalignment technique. The different alignments in adjecent regions are achieved by two-step photo exposure to guide orientations of sulfonic azo dye layers and further align the liquid crystal molecules. Both one-dimensional and two-dimensional switchable phase gratings have been demonstrated. Such polarizer-free gratings show very high transmittance (∼92%), diffraction efficiency (over 31%), and optical contrast (over 150) including low power consumption.

Liquid crystal gratings based on alternate TN and PA photoalignment

A diffraction grating is proposed by periodically defining the liquid-crystal director distribution to form alternate parallel aligned and twist nematic regions in a cell placed between two crossed polarizers. Based on the combined phase and amplitude modulation, both 1D and 2D tunable gratings are demonstrated. Low voltage ON/OFF switching of 1st order diffracted light with extinction ratio over 80 is achieved within a small voltage interval of 0.15 Vrms. Unique four-state feature of the cell is obtained and their applications in optical logic devices are discussed.

Zinc Oxide (1% Cu) Nanoparticle in Nematic Liquid Crystal: Dielectric and Electro-Optical Study

The present study reports the dielectric and electro-optical study of inorganic ZnO (Cu 1%) nanoparticles doped nematic liquid crystal (5CB). The dielectric permittivity is found to decrease in the nano doped sample. Nanoparticle doped liquid crystal has good response under various conditions to evaluate their potential as electro-optical materials for various purposes. From electro-optical study we have found Freedricksz transition voltage Vth and response time τ0 and hence K11 and rotational viscosity γ are also calculated. It is found that K11 and Vth increases for nano-doped sample. Rotational viscosity γ also increases for the nanoparticle doped sample. These changes have been explained in this article according to Osipov and Ternentjev theory of rotational viscosity of nematic liquid crystal.

Polymer and azo-dye composite: a photo-alignment layer for liquid crystals

In this article, we disclose a method to fabricate polymer-stabilised azo-dye photo alignment layers for liquid crystal. The idea includes the introduction of polymer network in the alignment layer, in optimal concentration, followed by two-step irradiation. The stabilised photo-alignment layer has been explored for different aspects of the display-related parameters, viz. anchoring energy, stability for various display-related environments as a function of concentration of monomer and irradiation, residual DC and voltage holding ratio. The composite photo-alignment layer offers well-suited parameters for the liquid crystal alignment and therefore could find application in a variety of modern photonic and display devices.

Liquid Crystal Active Glasses for 3D Cinema

Liquid crystals have been extensively studied and are massively used in display technology. Their recent use to provide optical shutters in active glasses for 3D cinema has focused the attention on new specific requirements. Recent improvements in the quality of 3D movies production and projection (e.g., involving triple flash projectors) resulted in the need for high quality glasses with no ghosting, no color banding, large viewing angle and good residual light. We present here the main relevant parameters to assess the quality required today by the studios and we compare the main liquid crystal options which can be used with their respective advantages.

Single walled carbon nano-tube, ferroelectric liquid crystal composites: Excellent diffractive tool

We present a switchable grating based on chiral single walled carbon nano-tube (SWCNT) doped ferroelectric liquid crystals (FLCs). The presence of SWCNTs improves the diffraction profile of the pure FLC. The diffraction efficiency, i.e., the ratio of intensities of first order and zero order maxima is more than 100% for the higher concentration of SWCNTs in pure FLC. This phenomenon has been explained by the decrease in ferroelectric domain periodicity, due to the doping of SWCNTs in pure FLC, and optical activity of the chiral SWCNTs. These gratings with very high diffraction efficiency may find application in many devices.

Switchable liquid crystal grating with sub millisecond response

In present article, we disclose a fast switchable grating based on nematic liquid crystals. A fine striped electrode has been exploited to generate the grating profile and a driving scheme to drive the grating cell has also been proposed, which enable us to drive the cell up to very high frequency (∼2 kHz) with evident saturated optical states. The response time for the first order diffracted beam, at the electric field of 10 V/μm, is less than 100 μs and optical contrast is higher than 400:1. Thus, the proposed LC grating could find application in several advance electro-optical devices.

Optimization of alignment quality of ferroelectric liquid crystals by controlling anchoring energy

In this article, a method of measuring the anchoring energy by evaluating the relaxation time is proposed. The method is valid for both helical and helix-free structures. The results obtained using the proposed method are compared with published results 22) for the same photosensitive alignment material. The limitations of the proposed method are also discussed.

Optically tunable and rewritable diffraction grating with photoaligned liquid crystals

An optically tunable and rewritable liquid crystal (LC) diffraction grating cell has been revealed that consists of an optically active and an optically passive alignment layer. The grating profile is created by confining the LC director distribution in alternate planar and twisted alignment domains by means of photoalignment of the LCs. The proposed grating is optically tunable for diffractive and nondiffractive states with a small response time that depends on the exposure energy and LC parameters. In addition, the grating can be erased and rewritten for different diffracting characteristics. These optically tunable diffractive elements could find application in various photonic devices.