Liquid Crystal Gratings Research Articles

High resolution polarization gratings in liquid crystals

Efficient recording of polarization gratings in dye-doped liquid crystals is reported. By exploiting the effect of light-induced anchoring of the molecular director, it has been possible to write stable holographic gratings with a diffraction efficiency of 8% at a resolution of 1000 lines/mm. The required surface energy density of 0.3 J/cm2 sets these materials among the most sensitive for optical storage.

Efficient, polarization-independent, reflective liquid crystal phase grating

A reflective liquid crystal diffraction grating is proposed which makes use of the maximum available surface area. The structure consists of alternating stripes, each a twisted-nematic domain with surface orientation perpendicular to that of its neighbors. All domains have the same twist sense. This device yields, in principle, 100% diffraction efficiency independent of incident polarization.

Effects of Material Systems on the Polarization Behavior of Holographic Polymer Dispersed Liquid Crystal Gratings

This study has investigated the effects of the material systems involved in the fabrication processes on the polarization behavior of a volume holographic grating. The diffraction efficiency of the gratings fabricated using prepolymer/liquid crystal mixtures shows strong dependence on the polarization of incoming light. Depending on the materials used in the formation of a grating, the diffraction properties are such that either p- or s-polarized light is strongly diffracted while the light with the other polarization is very weakly diffracted. The magnitude of the dependence on the polarization is greatly affected by the type of monomers, liquid crystals and substrates. The comparison of various types of monomers added to the base prepolymer mixtures, two distinctly different types of liquid crystals and glass slides and indium-tin oxide (ITO) coated glass as substrates was carried out using polyester-based and urethane-based oligomers.

Monte Carlo simulation of electric-field-induced spatial gratings in nematic liquid crystals

In this paper we present results of the Monte Carlo simulation of a two-dimensional nematic liquid crystal model in the presence of a spatially modulated externally applied electric field E(x). It is a situation encountered in optically addressed spatial light modulators based on liquid crystals. The boundary conditions employed in the simulations corresponded to an experimental configuration used frequently for light-induced grating recording in two-beam coupling experiments. The inter-molecular interactions in a liquid crystal model were described by the Lebwohl - Lasher Hamiltonian. Refractive index profiles were calculated for chosen values of offset electric field and as a function of spatial frequency . The dependences of amplitude of the refractive index modulation on and were determined.

Writing and Erasure of Holographic Gratings in Dye-Doped Polymer Dispersed Liquid Crystals

We Demonstrate that is possible to write and erase holographic gratings in dye-doped polymer dispersed liquid crystals (PDLC). PDLC samples were irradiated after complete curing using two beams in an interferometric geometry. In this way permanent holographic gratings were written with a resolution up to 100 lines/mm. The erasure was obtained by a single beam, while successive writing were possible again by two beams interference, without any degradation after many cycles. The origin of the effect can be the onset of periodic distribution of droplets of different shape and spatial diffusion of the dye molecules.

Dye-doped liquid crystals as high-resolution recording media

Light-induced anchoring of the molecular director is reported to be an efficient method for writing permanent holographic gratings in dye-doped liquid crystals. We have achieved higher sensitivity and spatial resolution in these materials with other methods. An energy density as low as 10(-1) J/cm(2) was sufficient to write gratings with a resolution higher than 100 lines/mm.

Storage of laser-induced holographic gratings in discotic liquid crystals

We demonstrate that low molar mass discotic liquid crystals can be used for holographic storage. Grating structures are optically induced and stored in the glassy state of the investigated liquid crystal. The material is suitable for multiple storage processes since the stored information can be erased by heating the sample above the glass transition temperature.

Storage of laser-induced holographic gratings in discotic liquid crystals

Storage of laser-induced holographic gratings in discotic liquid crystals

Spatially resolved phase imaging of a programmable liquid-crystal grating

Phase imaging is used to compare near-field measurements with the corresponding far-field intensity distribution. A liquid-crystal device serves as a phase object that can be programmed as a variable grating. Real-time phase visualization then provides an avenue for direct optimization of complex phase gratings.

Kinetic theory of phase separation induced by nonuniform photopolymerization

A kinetic model is developed to study the process of phase separation that occurs when a polymer-dispersed liquid-crystal diffraction grating is produced by a process of photopolymerization. We find that the type of system produced depends sensitively on the rate of polymerization and on whether the spatially periodic illumination used to produce the grating is followed by a period of uniform illumination. While the average liquid-crystal concentration is always increased in the region of least irradiation, in some cases small inclusions of liquid-crystal form by spinodal decomposition at the minima in the irradiating intensity, while in others they form at the maxima. The simplest version of the theory combines the Flory-Huggins and Hillert models of phase separation. In some circumstances it must be augmented by including the effects of interdiffusion of polymer molecules having different degrees of polymerization.

Diffraction and Polarization Properties of a Liquid Crystal Grating

Diffraction properties of an amplitude liquid crystal grating with a periodic electrode structure are studied under different polarizer configurations. Based on these experimental results, polarization characteristics of light diffracted from this grating are discussed. Both diffraction light intensities and polarization states of this liquid crystal grating can be controlled electrically. Symmetrical diffraction light intensities but antisymmetrical polarization states in corresponding negative and positive diffraction orders are demonstrated.

An electro-optically controlled liquid crystal diffraction grating

A structure for an electro-optically controlled liquid crystal diffraction grating is proposed, which can dramatically simplify the fabrication process of liquid crystal optical gratings. The structure consists of two alternating stripes. Each stripe is a hybrid liquid crystal cell with adjacent stripes oriented perpendicularly. This kind of electro-optically controlled diffraction grating in principle gives 100% diffraction efficiency and no polarization direction dependence. The detailed fabrication process is presented.

Self-induced optical gratings in nematic liquid crystals with a feedback mirror

Spontaneous transverse one-dimensional optical grating formation has been observed in slightly asymmetric single-feedback mirror experiments using nematic liquid crystals as nonlinear optical media. It is shown that weak asymmetries can be compensated by increasing the intensity and transition from one-dimensional gratings to two-dimensional patterns.

Pulse-width modulation drive technique for high-resolution liquid-crystal gratings

We report the use of a pulse-width modulation drive technique for high-resolution liquid-crystal gratings and show how the grating diffraction efficiency depends on the duty cycle of the drive signal. We identify a regime in which the diffraction efficiency is linearly proportional to the duty cycle, thereby providing a linear drive characteristic.

Partial pixels: a three-dimensional diffractive display architecture

We describe in detail the partial pixel architecture that permits the realization of three-dimensional (3-D) displays that are functionally equivalent to a real-time holographic stereogram. This architecture permits the simultaneous presentation of multiple stereoscopic images so that motion parallax is discernible in the resultant 3-D scene. The key innovation of the architecture is that each pixel is subdivided into partial pixels, which in turn can be implemented as individual diffraction gratings. We describe a static display that exhibits a 3-D image with one-dimensional motion parallax, thereby demonstrating key features of the architecture. A variety of partial pixel implementations are discussed that can operate at video frame rates. These include voltage-controlled liquid crystal gratings and binary optic gratings integrated with conventional liquid crystal amplitude modulators. In addition, we describe how the partial pixel architecture can be generalized for the implementation of full-color displays and displays having two-dimensional motion parallax.

Optically generated liquid crystal gratings

Optically controlled planar orientation of liquid crystal molecules with polarized light is used to make phase gratings in liquid crystal media. The electro-optical properties of liquid crystal linear and chirped gratings are discussed.

Diffraction Characteristics of Ferroelectric Liquid Crystal Grating

A ferroelectric liquid crystal grating is investigated. Diffraction behavior is analyzed on the basis of a model where the birefringent grating generated in the ferroelectric liquid crystal layer acts as an absorption/phase hybrid grating. Light diffraction is observed by using an optically addressed spatial light modulator that contains a photoconductive film and a ferroelectric liquid crystal. We find some disagreement between the simulation and experimental results. The errors are found to be caused by the incomplete bistability of the ferroelectric liquid crystal.

Reversible Storage of Light-Induced Gratings in Low Molar Mass Liquid Crystals

Abstract Low molar mass liquid crystals which exhibit an anisotropic glassy state are investigated for erasable recording processes. Optical gratings were written with intensities less than 1000 W/cm2 obtained from an argon-ion laser and read out by diffraction of a weak He-Ne laser. Grating build-up times of less than 5 ms have been observed. Diffraction efficiencies of several percents were measured as a function of intensity and grating periods down to 5μm. The stored gratings are erased by homogeneous laser heating. It is shown that the observed recording phenomena is connected with optical heating and light-induced transitions from the glassy to the liquid crystalline state of the material.

Thermal grating diffusion in rather thin liquid crystal films

The decay of picosecond-laser-induced thermal gratings in rather thin liquid crystal films is investigated. It is shown that, in contrast to the case samples several tens to hundreds of micrometres thick, the relaxation of thermal gratings in liquid crystals of about 2 mu m thickness, stacked between two semi-infinite glass substrates, is mainly determined by transverse heat transport processes through the cell surfaces instead of longitudinal diffusion in the direction of the grating wavevector. Experimental observations are compared with theoretical considerations and numerical solutions of the heat flow equation.

Applications look at the use of liquid crystal writable gratings for steering passive radiation

Liquid crystal writable grating technology is being developed for beam steering in laser radar systems. We consider the ability of writable gratings to steer broad-spectral-band radiation for use in passive sensors. We find that there is potential for these devices in microscan systems because there is little or no dispersion for the small scan angles required in microscanning. The dispersion that is present is less than the resolution of the sensor considered here. For large angle steering we find that dispersion correction or a narrowing of the spectral bandwidth is required. The degradation in sensitivity resulting from narrowing the spectral bandwidth is considered. We find that a high-quantum-efficiency step-stare sensor with a two-dimensional focal plane array responsive over a narrow spectral width can achieve the same sensitivity as current linear scanning sensors while being able to steer the field of view (FOV) over a larger field of regard with no moving parts. Approaches for dispersion correction and postdetection correction are discussed. A promising approach for steering a narrow FOV with broad spectral content and good resolution is described.