Liquid Crystal Light Valve Research Articles

Opto-Electronic Properties of MBE-Grown ZnSSe Thin Films on ITO Substrates

The opto-electronic properties of molecular-beam-epitaxy (MBE)-grown ZnSSe thin films on indium-tin-oxide (ITO) glass substrates were investigated in this work. Ultraviolet (UV) photoresponsivity as high as 0.01 A/W and three orders of visible rejection power were demonstrated. The results of d.c. resistivity measurements revealed that the resistivity of the ZnSSe thin films decreased as the crystal size increases and reaches a value of4.3 × 1011 Ω cm for a thin film grown at the optimized substrate temperature of 290°C. The results of a.c. impedance measurements performed in the frequency range of 40 to 4000 Hz further indicated that the impedance of this alloy thin film can provide a good match with the liquid crystal layer of a liquid crystal light valve for UV imaging applications.

Real-time all-optical three-dimensional integral imaging projector.

We present an all-optical three-dimensional integral imaging projector. An optically addressed spatial-light modulator is used, which potentially provides better image resolution than the conventional CCD and liquid-crystal display pair. We present experimental results using a liquid-crystal light valve.

L‐3: Late‐News Paper: A 0.9″ XGA Temperature Poly‐Si TFT LCLV with Stacked Storage Capacitor

AbstractA 0.9” XGA liquid crystal light valve with low temperature Poly‐Si TFT has been developed by using a stacked capacitor structure. The stacked capacitor structure could reduce storage capacitor area because most of the capacitor area is located above the pixel components, such as bus‐lines and TFTs. Therefore, this structure could obtain the required storage capacitance without decreasing aperture ratio. A high aperture ratio of 65% and a high contrast ratio of over 500:1 have been achieved with the fabricated light valve.

31.3: A New Type of Liquid Crystal Diffractive Light Valve with Very Small Pixel Size

AbstractPolymer wall confined switchable diffractive liquid crystal light valve is investigated by two‐dimensional finite‐difference time‐domain (FDTD) optical simulations and liquid crystal director calculations. The calculated brightness vs. device pixel size and the number of turn “ON” pixels are provided. The results indicate that this kind of device is very promising candidate for high resolution and high brightness projection displays.

Spatial mode control of a diode-pumped Nd:YAG laser by an intracavity liquid-crystal light valve

We present a new beam-shaping technique with an intracavity optically addressed liquid-crystal spatial light modulator. The Nd:YAG resonator is able to deliver beams with various spatial profiles such as flat-topped super-Gaussian and square-shaped beams.

MBE-grown ZnSSe thin films on ITO substrates for liquid-crystal light valve applications

The MBE growth of ZnSSe alloy thin films on ITO substrates using ZnS and Se sources was studied and various structural and opto-electronic properties of the as-grown thin films were characterised. The XRD rocking curves resulting from these films indicate that the as-grown polycrystalline ZnSSe thin films have a preferred orientation along (1 1 1) direction. The evaluated crystal sizes as deduced from the FWHM of the XRD layer peaks were found to show a strong growth temperature dependence with the optimised temperature at about 290°C. TEM measurements done on these thin films also indicate a similar growth temperature dependence. The TEM cross-sectional micrograph of the sample grown at the optimised temperature shows a well-defined columnar structure whose nucleation seems to be highly correlated with the ITO grains. UV responsivity as high as 0.01 A/W and more than three orders of magnitude in rejection power for wavelengths longer than 450 nm have been achieved. It was also found that the sample grown at the optimised temperature has the lowest resistivity of 4.3×10 11 Ω cm, which provides a good match with that of a liquid-crystal layer. These results indicate that MBE-grown ZnSSe thin film is a promising candidate as the photoconductive material of liquid-crystal light valves for UV imaging applications.

Constrained order in frustrated 2D optical patterns

In 2D optical patterns obtained in a Liquid Crystal Light Valve with optical feedback, we show a new kind of geometrical frustration which comes from the imposed form of the boundaries. The circular section of the incoming laser beam presents a symmetry which belongs to the O(2) group, whereas the optical feedback selects patterns with a symmetry restrained to a dihedral subgroup of O(2). By imposing boundaries which respect the symmetry of the dihedral group, we lift the frustration and obtain perfectly ordered patterns.

Image differentiation with the aid of a phase knife

A scheme for optical differentiation of images based on spatial filtering is proposed. A phase knife—a glass plate consisting of two halves, whose thicknesses are chosen so that the optical path difference between the light waves passed through them makes up half the wavelength—serves as a filter. The linearity of the transformation and the sensitivity of the system to noise are investigated theoretically. The scheme was experimentally implemented on the basis of a liquid-crystal light valve. The experimental results confirm that such a setup can be efficiently used for processing images with a narrow-band spatial spectrum and for visualization of phase objects.

First-order Fréedericksz transition in the presence of light-driven feedback in nematic liquid crystals.

We show that feedback, which introduces a dependence of the electric field on the liquid-crystal director, renders the Fréedericksz transition first order. Experimentally, the feedback is introduced as a light loop in a liquid-crystal light valve. Theoretically, we include the feedback term into the Frank free energy and we derive an amplitude equation that is valid close to the transition. The theoretical description is in qualitative agreement with the experimental observations. Depending on the values of the feedback parameters, both theory and experiment exhibit bistability, propagation of fronts, and a Maxwell point.

Optically Addressed Liquid Crystal Light Valves and their Applications

The beams delivered by high energy laser chains suffer from spatial and temporal aberrations which degrade the performances of the source. These aberrations araise from thermal lensing and group delay dispersion effects which reduce the available peak power density when focusing the beam on a target. To increase the laser brightness to a value close to the diffraction limit in the space and time domains we propose to use a liquid crystal spatial phase modulator. It consists of an optically addressed light valve which permits to control the phase of a wavefront and the phase of each spectral component of an ultrashort femtoseconde pulse. We present the technology and the characteristics of the device and demonstrate the following applications: wavefront correction, compensation of group delay dispersion and dynamic holography with gain.

An optical feedback nonlinear system with a Takens–Bogdanov point: experimental investigation

A two-component nonlinear optical system based on a liquid crystal light valve (LCLV) with external feedback loops is experimentally investigated. A degenerate codimension-2 bifurcation point in the parameter space is experimentally located, and the nonlinear behavior of steady and drifting roll patterns in the vicinity of this point is investigated. The experimental results are found to correlate well with the nonlinear theory developed recently. The results reported represent, to our knowledge, the first experimental observation of such nonlinear effects in optics.

Nonlinear effects of phase blurring on Fourier transform holograms

Liquid-crystal light valves can have intensity-dependent resolution. We find for a nematic liquid-crystal light valve that this effect is well modeled as a phase that has been blurred by a linear space-invariant filter. The phase point-spread function is measured and is used in simulations to demonstrate that it introduces intermodulation products to the diffraction patterns of computer-generated Fourier transform holograms. Also, the influence of phase blurring on a pseudorandom-encoding algorithm is evaluated in closed form. This analysis applied to a spot array generator design indicates that nonlinear effects are negligible only if the diameter of the point-spread function is a small fraction of the pixel spacing.

Localized versus delocalized patterns in a nonlinear optical interferometer

We compare the morphology of the spatial structures displayed by an optical system consisting of a liquid crystal light valve (LCLV) in a feedback configuration, as the feedback is gradually tuned from purely diffractive to mixed interferential and diffractive. Different kinds of spatially coherent structures (e.g. hexagons, rolls), as well as localized structures and space-time turbulent patterns are observed. The features of the localized structures change with the parameter setting, and certain regions of the parameter space provide stable clusters of isolated spots (`molecules'). Numerical simulations based on a Kerr-like model of the LCLV are in agreement with the experimental observations. We analyse the links between the observed behaviours and the results of a linear-stability analysis of the underlying homogeneous stationary states.

Complex behaviour in optical systems and applications

This issue contains a collection of papers devoted to dynamics and pattern formation in nonlinear optical systems. The articles originate from contributions to a conference on control of complex behaviour in optical systems and applications (COCOS) held in Munster, Germany, in October 1999. It was the second of a series of Euroconferences on Trends in Optical Nonlinear Dynamics. Physical Problems and Applications funded by the European Union through its TMR Programme and co-sponsored by various institutions. The topics of the conference covered all kinds of complex self-organization phenomena in nonlinear optics and this diversity is also reflected in the papers. The contributions cover a great variety of material systems (atomic vapours, nonlinear crystals, photorefractives, liquid crystal light valves, fibres, semiconductors, solid-state lasers, ...) and interaction schemes (nonlinear resonators, single-mirror feedback schemes, nonlinear beam propagation, four-wave mixing). Though there is emphasis on the characterization and control of spatial structures including extended patterns as well as localized states, domain walls and solitons, several papers also discuss temporal instabilities and develop strategies for stabilization or optimal steering. In both fields there are a number of contributions considering semiconductor systems and their control. This is a strong indication that nonlinear dynamics in optics is on its way to making more and more useful contributions to applications. Some papers are also providing connections between two fields, either between patterns and localized structures or between purely temporal and spatial phenomena. They analyse e.g. the dynamics of coupled lasers, pulse trains in nonlinear fibres or spatiotemporal chaos. Last but not least, papers investigating vectorial effects or coherence phenomena (e.g. in inversionless amplification) demonstrate that there are features of optical systems that have no obvious counterpart in nonlinear systems of other kinds. The diversity of the papers is even more remarkable, since the collection of papers presented here does not form `proceedings' in a comprehensive sense, but covers about twenty percent of the contributions to COCOS only. Nevertheless, it is our opinion that they are somehow representative of the European activities in the field of nonlinear dynamics in optics. It is our hope that the papers give an idea of how nonlinear optics can help in understanding general features of self-organization common to all kinds of complex systems on the one hand, and on the other hand how it can provide well-controlled, high performance devices for the photonic applications of tomorrow. Finally, we thank the Editors of the journal for providing the forum for this presentation and the Editorial Office for all the work done.

Unstable periodic orbits in the presence of spatio-temporal chaos

In this paper we investigate spatio-temporal disorder in a liquid crystal light valve with two-dimensional feedback and modulated input. We focus on the specific case of an optical feedback with a 180° rotation. By modulating the input field with appropriate amplitude and frequency, chaotic domains are formed in different locations of the transverse space [1, 2]. Because of diffusion, a global description of the spatio-temporal dynamics requires a partial differential equation. However, the local dynamics of the chaotic domains can be embedded in low-dimensional manifolds. The detection of a variety of different unstable periodic orbits (UPOs) at different spatial locations and the presence of UPOs for different detector sizes are strong indications that the dimensionality of the model of spatio-temporal chaos in our system can be drastically reduced.

THE LIQUID CRYSTAL LIGHT VALVE WITH OPTICAL FEEDBACK: A CASE STUDY IN PATTERN FORMATION

A nonlinear optical system, consisting of a liquid crystal light valve with feedback, can display an enormous variety of spatial patterns. Starting from the basic principles of operation, we review several physical configurations enlighting the specificity of each one and the mechanism by which it leads to certain rules of pattern selection.

35.2: Direct Observation of Disclination Evolution in Vertically Aligned Liquid Crystal Light Valves

AbstractThe dynamic evolution processes of fringe‐field induced disclinations of vertically aligned LC light valves were directly imaged for the first time by using a CCD camera with 1 ms time resolution. Various switching patterns corresponding to different fringe fields, full off (dark) to full on (bright), interlaced column on and off to full on, checkerboard on to full on, have been studied. Experimental results show that, due to the elastic coupling, the transformation speed of LC directors at the location of disclination between two topologically in‐equivalent states depends strongly on display patterns. The observed transformation speed varies from milliseconds to about one second, depending on the alignment conditions.

Design and fabrication of a projection display using optically addressed polymer-dispersed liquid crystal light valves

A prototype projection display consisting of optically addressed polymer-dispersed liquid crystal (PDLC) light valves and a Schlieren optics is designed, built, and evaluated. This projection display, which controls light by a scattering effect in the PDLC film, can be configured without requiring a polarizer and can create a brighter high- definition picture display with which high optical efficiency is possible. The relationship between the contrast ratio and the optical efficiency of a Schlieren projection system is discussed. To design a projection display with both high brightness and a high contrast ratio, the scattering characteristics of a reflection-mode PDLC cell is evaluated and the optical efficiency of the total projection system is studied using a simple model with perfect diffusion of a light source. Using these results, we design and prototype PDLC light valves for a full-color display and construct a projection display. We succeed in displaying pictures with a contrast ratio of 80:1 and a brightness greater than 1800 ANSI lumens.

Trichroic prism assembly for separating and recombining colors in a compact projection display

A trichroic prism assembly design, believed to be new, is proposed and demonstrated. This new design has the advantages of low s- and p-polarization dependence in the reflectance spectra of the optical coatings. Hence it can be used for both color separation and color recombination with polarization change. This new trichroic prism assembly is especially useful in a compact color projector employing reflective liquid-crystal light valves.

Pixelated liquid-crystal light valve for neural network application

The liquid-crystal light valve (LCLV) is a useful component for performing integration, thresholding, and gain functions in optical neural networks. Integration of the neural activation channels is implemented by pixelation of the LCLV, with use of a structured metallic layer between the photoconductor and the liquid-crystal layer. Measurements are presented for this type of valve, examples of which were prepared for two specific neural network implementations. The valve fabrication and measurement were carried out at the State Optical Institute, St. Petersburg, Russia, and the modeling and system applications were investigated at the Institute of Microtechnology, Neuchâtel, Switzerland.