Liquid Crystal Light Research Articles

Simulation of coherent backscattering of light in nematic liquid crystals

Multiple scattering of light by the fluctuations of the director in a nematic liquid crystal (NLC) aligned by a magnetic field is considered. A peak of coherent backscattering is calculated by numerical simulation. Since the indicatrix of single scattering for a liquid crystal (LC) is known exactly, the calculations are carried out without any simplifying assumptions on the parameters of the liquid crystal. Multiple scattering is simulated as a random walk of photons in the medium. A peak of coherent backscattering in such a medium is very narrow; therefore, the so-called semianalytical method is applied. The parameters of the backscattering peak obtained by numerical simulation are compared with the available experimental data and with the results of analytical approximations. It turns out that the experimental data are in good agreement with the results of simulation. The results of numerical simulation adequately describe the anisotropy and the width of the backscattering peak.

Self-induced nonlinear spin–orbit interaction of light in liquid crystals

We report on the observation of self-induced nonlinear spin-orbit interaction of light driven by Kerr orientational optical nonlinearities in liquid crystals. It consists of the self-induced spin-to-orbital nonlinear conversion for the angular momentum of light. The optical angular momentum conversion is driven by the creation of a topological liquid crystal defect by the light itself. Moreover, we show that such a nonlinear process can be significantly enhanced by using additional electric fields.

Temperature as a Control Parameter of the Light Trajectories in Nematics with Topological Defects

The influence of controlable parameters like the temperature on the trajectories of light in a nematic liquid crystal with topological defects is studied through a geometric model. The model incorporates phenomenological details as how the refractive indices depend on such parameters. The deflection of light by the topological defect is then shown to be greater at lower temperatures.

Leakage of light in liquid crystal displays and birefringent thin film compensators

This paper elucidates the physical origin of leakage of light in the dark state of liquid crystal displays (LCDs) and describes birefringent thin film compensators to mitigate the problems. There are two fundamental sources of leakage of light in LCDs. They are the leakage of light through a pair of ideal crossed polarizers, and the leakage of light due to elliptical state of polarization after transmitting through the liquid crystal (LC) cell. The leakage of light in the dark state leads to poor contrast ratios and color instability, particularly at large viewing angles. The paper then describes the employment of sixth-wave (λ/6) plates as well as various wave plates that function as birefringent compensators for eliminating the leakage of light in LCDs.

Coherent backscattering of light in nematic liquid crystals

Multiple light scattering by director fluctuations in nematic liquid crystals is considered. A uniform director orientation is assumed to be specified by an applied magnetic field. The coherent backscattering effect, which consists in the presence of a sharp light backscattering peak, is studied. The Bethe-Salpeter equation is used to calculate the multiple scattering intensity taking into account the contributions of ladder and cyclic diagrams. An analytical expression for the angular and polarization dependences of the coherent backscattering intensity is obtained in terms of the diffusion approximation. The calculation and experimental results are compared. The developed theory is shown to qualitatively describe the elliptical shape of the backscattering cone, to explain the absence of a coherent contribution for crossed polarizations, and to calculate the relative peak height.

Geometric Phase for Light Propagating in Nematics with Disclinations

A geometric model is used to investigate the propagation of light in nematic liquid crystals with topological defects. Spinors, representing polarization of the light, are parallelly transported around the defects. It is shown that they acquire a phase which depends on the ratio between the extraordinary and ordinary refractive indices and on the defect winding number. This result can be interpreted as a manifestation of a geometric phase which may be detected by suitable interference experiments.

Propagation and Scattering of Light in Helical Liquid Crystals with Large Pitch

The problems of light propagation and scattering in twisted nematic and cholesteric liquid crystals are considered. We study the correlation function of the director fluctuations, the eigenwaves, and the Green's function for helical liquid crystals with the pitch, which significantly exceeds the wavelength of light. The obtained results make possible to describe angular and polarization dependencies in scattered light, the wave-guide channel formation, the turning points and the effect of percolation through the forbidden zone.

Director sliding induced by a circularly polarised light in dye-doped liquid crystals

We report the first detailed experimental study of the transient effects of a circularly polarised beam on dye-doped liquid crystal cells. Experiments show that, as linearly polarized light does, light with circular polarization induces quasi-free sliding of the molecular director on the irradiated surface. The behaviour of the sliding angle vs. the incident intensity, its dependence on the exposure time and its independence on the sign of the light ellipticity, suggest that the phenomenon is connected to surface effects instead of being directly due to the transfer of intrinsic angular momentum from light to the LC molecules.

Propagation of light through a forbidden zone in chiral media

The particular features of the propagation of light in uniaxial chiral liquid crystals with a large pitch of the spiral are considered. There exist forbidden zones in these systems for fairly large angles of incidence of an extraordinary ray. On the one hand, this results in an efficient reflection of the wave from the zone boundary, and, on the other hand, this causes the wave to decay inside the zone. A case of narrow forbidden zones is studied, and it is shown that optical effects that arise upon propagation of rays near turning points are equivalent to the tunnel and over-barrier reflection effects. The angular dependences of the intensities of rays that were refracted in a forbidden zone and transmitted through it are calculated. The percolation effect is experimentally studied in a mixture of a nematic liquid crystal with a chiral addition. The intensity of a transmitted extraordinary ray is studied as a function of the angle of incidence, which determines the width of the forbidden zone. Both the over-barrier reflection and the percolation effects are observed. The calculation results are shown to agree with experiment.

Radiation detector based on liquid crystal light valve for large-area imaging applications

Currently most large-area radiation detectors rely on the use of a phosphor screen and an optical sensor. In such conversion process, there will be signal loss due to inefficiencies in coupling between phosphor screen and later light-detection components. This paper presents a radiation detector based on X-ray induced light transmittance of liquid crystal (LC) as a function of radiation. Our detector incorporates a europium-doped gadolinium oxide (Gd 2O 3:Eu) phosphor and an amorphous selenium (a-Se)-coupled LC light valve. The Gd 2O 3:Eu is synthesized using a low-temperature solution–combustion method, and the optical properties of the Gd 2O 3:Eu are investigated as a function of a sintering temperature. The transmission–voltage ( T– V) curve and the optical response of the detector are also investigated. Our experimental results show good T– V characteristics of the LC, switching between 12% and 86% transmission. The radiation dose is also presented in this work. The results show that our detector can operate at low X-ray dose and, therefore, a promising application is the medical-imaging detector.

Liquid crystal diffractive phase grating as light modulator for projection display

The widely used liquid crystal (LC) projection display based on rotation of polarization has some disadvantages, such as significant (about 50%) loss of the unpolarized light on an input polarizer and a limited contrast ratio. The drivable LC phase diffraction grating, embedded in the Schlieren system as a spatial light modulator, is introduced as the alternative to a conventional LC light valve. The phase grating produced by a pattern alignment of the LC enables the construction of an LC diffractive modulator, which is independent from the polarization of the incident light. The LC phase diffraction grating redirects light passing through rather than absorbing light. The LC phase diffractive grating is potentially able to transform all incident light, at the ON state of the modulator, into diffractive maximums with no light in the zero-diffraction order. This enables the construction of an LC diffractive modulator with more modulation efficiency than the conventional LC light valve. Therefore, a model of the nematic LC diffractive phase grating is proposed. This model is based on an approximation for a retarded electrical field of diffracted radiation. The presented model is developed to optimize parameters of the LC light diffractive modulator to achieve maximum efficiency.

Effect of boundary conditions on phase modulation of light in a nematic liquid crystal featuring the S-effect

Phase modulation of light has been studied under the S-effect conditions in a nematic liquid crystal (NLC) with a planar director alignment set by a layer of amorphous hydrogenated carbon (a-C:H). It is demonstrated for the first time that the formation of an anisotropic surface relief on the electrode surface favors a more homogeneous alignment of liquid crystal molecules by the a-C:H layer and allows the maximum phase shift to be obtained at a certain thickness of the NLC layer.

Depolarization of partially coherent light in liquid crystals

AbstractIn the paper we present results of analysis of partially coherent light depolarization in two types of liquid crystals possessing linear birefringence controlled by temperature and external electric field changes. Some experimental results of degree of polarization measurements for different light sources as a superluminescent diode and a laser diode are also presented.

Flat panel display – Impurity doping technology for flat panel displays

Features of the flat panel displays (FPDs) such as liquid crystal display (LCD) and organic light emitting diode (OLED) display, etc. using low temperature poly-Si (LTPS) thin film transistors (TFTs) are briefly reviewed comparing with other FPDs. The requirements for fabricating TFTs used for high performance FPDs and system on glass (SoG) are addressed. This paper focuses on the impurity doping technology, which is one of the key technologies together with crystallization by laser annealing, formation of high quality gate insulator and gate-insulator/poly-Si interface. The issues to be solved in impurity doping technology for state of the art and future TFTs are clarified.

Scattering of light in cholesteric liquid crystals with large pitch

We consider the problem of light scattering in a slab of cholesteric liquid crystal with the pitch which significantly exceeds the wavelength of light. The electromagnetic wave propagation and the Green's function are investigated for this medium basing on geometric optics approximation. The correlation function of the director fluctuations is calculated with the aid of the vector analog of the WKB approximation. A general approach to treatment of single light scattering in a stratified medium with smoothly varying properties based on the Kirchhoff method is developed. Angular and polarization dependencies of the single light scattering intensity as well as extinction of the mean field are analyzed. Unusual dependence of the light scattering intensity on the size of the system is found.

31.5L: Late-News Paper: Development of a Combined Liquid Crystal and Fine Pitch Light Source Display (LFD)

We propose a novel display system that we call an LFD (combined liquid crystal and fine pitch light source display). In an LFD, an auxiliary light source patterned with a fine pitch is attached to a reflective liquid crystal display (LCD), and a light shield is formed on the observer's side of the light source. A vertical alignment LCD (VA-LCD) is attached as the reflective LCD, and an organic light-emitting diode (OLED) is attached as the fine pitch light source. An LFD can produce bright, high-contrast images under any ambient light. We built a test sample and confirmed its display characteristics.

Orientational Dynamics Induced by Circularly Polarized Light in Nematic Liquid Crystals

We study theoretically the transitions induced by a circularly polarized plane light wave perpendicularly incident on a thin layer of homeotropically oriented nematic liquid crystal. There are three dynamical regimes separated by different types of transitions.

High-efficiency energy transfer due to stimulated orientational scattering of light in nematic liquid crystals

The saturation stage of stimulated orientational scattering (SOS) of light in thick planarly aligned nematic-liquid-crystal layers was observed and studied experimentally. The power efficiency of o-wave pump conversion exceeded 90%. Both saturation of transmitted pump power and cascading of SOS were observed. The latter indicated that, for higher than ∼50% conversion efficiencies, inverse conversion of the scattered e-polarized signal into wide-angle speckle, o-polarized background started to take place, leading to partial depletion of the primarily scattered e wave. Results were in good qualitative agreement with conventional SOS theory.

Photoalignment Effect Induced by Angular Momentum of Light in Dye-Doped Liquid Crystals

We studied the effective interaction of the angular momentum of light with a liquid crystal (LC) doped with azo-dye. The experiments were carried out in a combined cell with one substrate providing strong unidirectional planar alignment of LC and the other covered by iso-tropic polymer layer providing a degenerated planar alignment. The effect manifests itself as azimuth reorientation of the liquid crystal director under the action of circularly polarized light in the absorption band of azo-dye. The new orientation of LC was captured by the command surface because of a light-induced alignment memory effect. To describe the experimental dependencies we solved the self-consistent problem of the propagation of a circularly polarized light in the absorbing nonlinear media using the geometric optics approximation. Comparing the experimental data with numerical evaluation we obtain the basic parameters of the system: the orientation nonlinearly coefficient and the value of anchoring of the director with the polymer surface.

Radiative Transfer Theory and Diffusion of Light in Nematic Liquid Crystals

In nematic liquid crystals light is strongly scattered from director fluctuations. We are interested in the limit where the incoming light wave is scattered many times. Then, the light transport can be described by a diffusion equation for the energy density of light with diffusion constants D∥ and D⊥, respectively, parallel and perpendicular to the director. We start from a radiative transfer theory, connect the diffusion constants to the dynamic structure factor of director fluctuations, and shortly discuss our results. Temporal correlations of the diffusing light probe the dynamics of director modes on much shorter time scales than single light scattering experiments. To account for the decaying temporal correlations, one has to add an absorption term to the diffusion equation, which we also link to the dynamic structure factor.