The anchoring strength, anchoring angle and molecular order at the droplet wall are measurable parameters which affect the nematic director configuration and hence the electro- optic response of PDLC display devices. Technologies to measure these quantities and the authors' current understanding of their role in droplet morphology are presented. The cylindrical geometries of Nuclepore filters are shown to be useful in studying the competing effects of anchoring energy and cavity curvature for different surface treatments. Nuclear magnetic resonance methods are used to determine director configurations of submicron cavity sizes while optical microscope textures are preferable in supramicron-size droplets.

Generally one type of polymer is used in the construction of polymer dispersed liquid crystal (PDLC) films. The authors tested the effect of incorporating an additional fluoropolymer in varying percentages on the electro-optic performance of PDLC films based on poly(methyl methacrylate). Compared to samples made from a single polymer, several correlations are noted, including a decrease in liquid crystal fall-time, an increase in rise-time, and an increase in the threshold voltage (Vt).

The dielelectric response of three types of PDLC films were measured as a function of frequency (1 mHz to 10 kHz) at room temperature. PDLC films containing nematic liquid crystal droplets in different polymer binders -- epoxy (Epon 828), thermoplastic (PMMA), and a UV curable adhesive (Norland 65) -- were studied for a range of droplet sizes. Transmission at normal incidence as a function of the frequency (0.1 Hz to 200 Hz) of the applied field (0 to 120 Vrms) was also measured. The dielectric data show that charge is built up at both the electrode (epoxy films) and droplet/polymer interface (UV, PMMA films). This charge buildup causes a shielding of the field in the droplets, resulting in a much higher switching voltage as shown by the optical transmission. In the PMMA-based film the extent of the shielding is strongly dependent on droplet size.

Suitable addressing methods for polymer dispersed liquid crystal (PDLC) displays are dependent upon both the type of display and the PDLC properties. Video displays require fast frame rates, high resolution, and gray scale. The addressing method depends upon PDLC properties such as threshold voltage, operating voltages, response times, resistivity, and hysteresis effects. the properties of PDLC films which are formed by ultraviolet (UV) polymerization of LC/monomer solutions depend upon their formation conditions. The threshold and operating voltages, contrast ratio and response times are affected by formation factors including the UV intensity, temperature, LC concentration, LC composition, field pre- alignment conditions, cell thickness, and droplet shape. The PDLC resistivity and hysteresis properties are more difficult to control, and can strongly affect the light throughput, response times, and gray scale of video displays. Apparent limitations from these properties can be overcome by utilizing pulse mode addressing factors pertinent for active matrix and photoactivated light valve PDLC displays.

The on and off switching speeds of nematic droplet-polymer films (NCAP) are shown to depend on the collection angle (f/#) and the wavelength of the light used in the measurement. Conventional twisted nematic liquid crystal displays have switching speeds which depend little on these factors. The switching speed dependence on collection angle (f/#) and wavelength for nematic droplet-polymer films is inherent to the mechanism by which light is modulated in these films. This mechanism is the scattering of light by the nematic droplets. The on times become faster and the off times become slower as the collection angle of detection is increased. The overall change in switching speed can be large. Greater than 100X changes in off time have been observed. As the wavelength of the light used to interrogate the sample is increased (blue yields green yields red) the on times become faster and the off times become slower. This dependence of switching speed on wavelength is apparent at all collection angles. An awareness of these effects is necessary when developing nematic droplet-polymer films for display applications and when comparing switching speed data from different sources.

The effects of droplet size on light scattering by polymer-dispersed liquid crystal (PDLC) films have been studied theoretically and experimentally. Rayleigh-Gans calculations for a collection of droplets which scatter independently predict that backscattering at wavelength (lambda) will be maximized when the droplet radius is about (lambda) /7. These calculations are consistent with hemispherical transmittance and reflectance measurements on PDLC films with controlled droplet sizes in which multiple scattering is known to be important. These measurements indicate that solar attenuation by PDLC films will probably be greatest when the droplet size is chosen to maximize backscattering of visible radiation. This choice also increases solar attenuation due to absorption of radiation trapped inside a PDLC film by total internal reflection.

Polymer-dispersed liquid crystals (PDLCs) have been developed for modulation of infrared radiation in the 2-5 and 8-14 micrometers wavelength regions. The electro-optic performance of an IR PDLC shutter depends on film thickness, liquid crystal droplet size, and the transparency of the substrates and the PDLC components. The effect of each of these factors on IR electro-optic performance of PDLC films was investigated using double modulation experiments and infrared spectroscopy. The authors have also compared the sensitivity of a pyroelectric vidicon infrared camera using PDLC shutter and a mechanical shutter.

The systematic substitution of fluorine for hydrogen in mesogenic materials has proved to be of great value for the modification of a wide range of physical properties. This is especially true in the case of ferroelectric liquid crystals in which the fluorine has been strategically introduced into the aromatic core (k), at or near the chiral center (Z) or in the achiral tail (n) in the generic liquid crystal (with X equals O, S and Z equals Me, halogen): H/F(CF2)n(CH2)mOC6H(4 - k)FkCOXPhOCOC*HZR. In this system the fluorine can be introduced in any of these three specific locations, or any combinations thereof, with the intent of enhancing performance such as broadening of the ferroelectric Sc* phase temperature range, enhancement of the magnitude of spontaneous polarization Ps and a variety of more subtle yet important modifications such as viscosity and dielectric and optical anisotropy. The ramifications of the incorporation of fluorine on these physical properties are discussed with a variety of examples.

Ferroelectric (chiral) smectic C* liquid crystals have been shown to exhibit high speed, multistate electro-optic, display device and opto-electronic computing applications, particularly when incorporated into the surface stabilized ferroelectric liquid crystal (SSFLC) light valve. High polarizations and low viscosities are necessary to achieve the speeds desired in opto- electronic computing, waveguide, and fiber-optic applications. Use of the Boulder Model allows one to examine the most probable conformations of an FLC molecule in a hypothetical smectic C 'binding site' and thereby predict the sign and order of magnitude of the spontaneous polarization based on electronic and steric arguments for the particular chemical structure. By combining these high polarization materials with a low viscosity smectic C host, fast FLC mixtures can be formulated.

The interface stabilized director n- polarization P orientation states near a ferroelectric liquid crystal (FLC)-solid interface are investigated using the total internal reflection technique. Experiments are carried out with the solid surface coated with a film of rubbed polymer (nylon 6/6), commonly used to align FLCs in the surface stabilized FLC (SSFLC) electro-optic devices. Direct measurements of the effects of the external electric field on surface molecular orientation under the influence of the rubbed polymer are reported in this paper.

Polymer-dispersed liquid crystal (PDLC) films are useful in light control applications because they can be switched electrically between light scattering and transparent states. The light scattering effectiveness of a film is intimately related to the size-distribution of the liquid crystal droplets and varies with the wavelength of light. The authors have prepared PDLC films with several droplet sizes and size-distributions, some of which exhibit a polydispersity of droplet sizes. Compared with films characterized by a standard or monodisperse droplet size distribution, polydisperse films exhibit enhanced light scattering and appear well suited for a variety of applications where extended or tuned-wavelength-range light-scattering properties are important.

At present there is considerable interest in fixed binary optic systems made by micro- fabrication techniques to produce arrays of miniature optical components for the control of light. These devices essentially replicate the phase curvature of their macroscopic counterparts by a 2n level substrate etched using n binary masks, the approximation being tolerated because of the multiplicity achieved. There exists a corresponding set of active binary devices where instead of the phase modulation being produced by substrate depth changes, micro electrodes modulate the orientation of birefringent molecules with applied binary voltages. This paper reviews some of the experimental devices which have been made using this approach such as phase gratings, prisms and lenses. It is shown how the analog dependence of molecular orientation on voltage actually improves the efficiency of the devices in addition to enabling an adaptive switching capability. An example is given of how a smoothly varying phase profile in a small region can be produced by allowing one of the electrodes to conduct above the ground plane and thereby drop volts and induce a continuous phase gradient. Furthermore electrostatic edge effects at the finely spaced electrodes can be exploited at the sharp binary edges produced by the lithographic mask to suppress the higher order diffraction orders which can limit the performance of these systems.

An adaptive optical neural network (ONN) using inexpensive pocket-size liquid crystal televisions (LCTVs) and containing 8 X 8 equals 64 neurons was constructed by a group of graduate students in the Electro-Optics Laboratory at The Pennsylvania State University. By the limited resolution of the LCTV, the current optical architecture can be easily extended to 16 X 20 equals 320 neurons. The major advantages of this LCTV architecture as compared with other reported ONNs are low cost and the flexibility to operate. To test the performance, several neural network models are implemented, in which are interpattern association, hetero- association, and unsupervised learning algorithm. The system design considerations and experimental demonstrations of the neural network models are given.

The authors present a general methodology for achieving any amount of pure phase modulation between 0 and 180 degrees by controlling the polarization of light used to address a given ferro-electric liquid crystal (FLC) material. Using this technique, an eight-state phase modulator made by cascading three FLC layers is demonstrated. Motivated by the need for predictable behavior within spatial light modulators (SLMs), experimental data characterizing the point by point spatial uniformity of an FLC device used for 0/90 degree phase modulation also is presented.

Several possibilities based on the principles of the scattering and polarization of light in liquid crystals and PLZT ceramics were studied for the replacement of mechanical shutters and attenuators in optical instruments. A scatter mode liquid crystal was selected as the active material, and two modules were developed for fiber optic attenuator applications. In the first, the light transmitted through an optical fiber (input fiber, (phi) 100/140 micrometers ) is collimated to a beam using a Selfoc microlens (0.25 P, (phi) 1.8 mm). This beam penetrates two LC cells (7 X 12 mm2) and is received by a Selfoc microlens at the output fiber. The second, for large-diameter fibers ((phi) 400 micrometers ), employed conventional aspherical lenses to optimize the fiber-to-fiber coupling instead of microlenses. Components were built for both (phi) 100 micrometers and (phi) 400 micrometers fibers. The maximum transmission is up to 70% and maximum contrasts 10,000:1 (632.8 nm), 3000:1 (830 nm) and 1200:1 (905 nm). The rise time is typically under 1 ms and the decay time under 5 ms, given a temperature above +20 degree(s)C. The contrast remains high from 0 degree(s)C to +50 degree(s)C. Both components function with +/- 100 VAC.

This paper describes an optical holographic interconnection method for neural network implementations in which each connection weight is distributed among many angularly and spatially multiplexed photorefractive gratings. This approach greatly reduces crosstalk caused by the conical Bragg degeneracy associated with a single grating and permits the entire input plane to be used. Experimental results for an optical Perceptron network implemented using this method are presented.

The continuous gray scale operating characteristics of a ferroelectric liquid crystal spatial light modulator (FLC-SLM) are described. Then an FLC-SLM having a dielectric mirror between an a-Si:H photoconductor and an FLC light modulatory material is presented, and its operating characteristics are reported. The FLC-SLM having a dielectric mirror allows for continuous gray scale applications. It exhibits a resolution of 80 to 100 lp/mm and a higher optical gain than the device without a dielectric mirror.

Operation of generally twisted nematic liquid crystal displays (LCDs) in circularly instead of in conventional, linearly polarized operating modes is shown to allow the design of reflective supertwisted nematic (STN)-LCDs with 2-3 times improved multiplexibility and/or shorter response times. One single circular or linear polarizer and in-situ reflectors simplify and improve LCD projection systems. The optical thresholds of low bias tilt, generally twisted nematic LC configurations are shown to agree with their mechanical thresholds.

The use of liquid crystal displays in combination with projection is not new. However, this type of display is becoming even more prevalent than it has been in the past. A variety of such displays are currently available for commercial or even consumer use. In this paper the authors review a number of liquid crystal devices, also known as light valves, developed specifically for use in projection systems. These include not only analog-addressed devices such as the photoactivated liquid crystal light valve, but matrix-addressed displays as well.

The status of the Hughes CCD-addressed liquid crystal light valve is reported. 256 X 256 array devices with good output uniformity and full video operations are demonstrated.

Active matrix LCDs are now penetrating the market for avionic and computer displays after initial success in pocket TVs. AMLCDs require the deposition and patterning of metal, insulator and semiconductor films, comparable to the processing for integrated circuits, but on large area substrates. The thin film switch at each picture element can be a three-terminal device (thin film transistor or TFT) or a two-terminal device (diode). Both technologies have been successful and are being scaled up for volume production. As three-terminal switches, amorphous silicon and ploy-Si TFTs are mostly used. Two-terminal devices include metal- insulator-metal diodes and a-Si pin diodes. Each technology has its pros and cons. TFT LCDs are generally more difficult to fabricate and have crossing lines on the active array substrate, which can reduce yield. Diode LCDs are usually simpler to manufacture and do not have crossing lines on one substrate. Auxiliary capacitors at each pixel, which improve gray scale performance, can be incorporated in TFT LCDs without extra processing, whereas in diode LCDs they complicate processing significantly. These and other merits and drawbacks of the two technologies are presented.

The authors report the operation of the Hughes Schottky diode-based silicon liquid crystal light valve (SLV) using readout light in the visible region. Limiting resolutions of 28 lp/mm limited by the Schottky diode periodicity, contrast ratios of >100:1, visible input light sensitivities of better than 50 (mu) W/cm2, and response times as fast as 5 ms have been measured. Both standard twisted nematic and homeotropically-aligned liquid crystal configurations have been utilized. The main parameter of this device is the leakage current of the Schottky diodes.

A detailed light budget is performed on reflective displays made from nematic droplet/polymer (NCAP) films incorporating dichroic dyes. It is shown that the radiance of these displays can be modeled using a simple scheme involving the convolution of the transmission spectra of the various components of the display. Comparisons are made between the relative importance of these optical elements in the display. It is shown that first-surface reflection (glare) is an important factor in reducing the optical performance of these displays, but that the effect of glare can be minimized through the proper choice of dye concentration.

The rearrangements in dispersed nematic drops with normal boundary conditions in an external ac electric field have been closely investigated with a polarizing microscope. It is shown both experimentally and theoretically that the dependence of the critical field Ec, which causes the replacement of the radial-like structure by an axial one, on the drop diameter d can be nonmonotonic. For small drops (d < de) Ec decreases as d increases; for large drops (d >> de), Ec increases with d. Here de is the thickness of the domain wall associated with strong director deformations localized at the drop equator, which may be roughly estimated as electric coherence length. The elastic deformations associated mainly with this wall are shown to be responsible for the above mentioned peculiarities of Ec(d).

A cholesteric liquid crystal (CLC) is considered in a cell with planar boundary conditions, in which long-lived metastable states of molecules with conformation different from that of initial molecules are formed under the action of light. It was suggested that the phenomenon of molecular phototransformation be used for varying the cholesteric twisting ability and thus for controlling the CLC pitch and the wavelength of light selectively reflected from the CLC cell. The dependence of the pitch on both the molecular twisting ability and the anchoring energy of CLC with oriented cell planes was investigated. The expression is derived for the anchoring energy which makes it possible to determine its value through the experimentally measured data. It is shown that with discontinuous change of a pitch there may occur a jump of mean values of components of the CLC dielectric susceptibility tensor. The CLC pitch relaxation appearing on switching off the light was considered.

Measurements of second harmonic generation (SHG) by polymer dispersed liquid crystal (PDLC) films have been carried out using nanosecond pulses from a Q-switched Nd:YAG laser. The PDLC films consist of micron-sized droplets of liquid crystal dispersed in a polymer matrix. The liquid crystal-polymer interface breaks inversion symmetry, allowing SHG, and the large surface to volume ratio may enhance the signal intensity. The intensity of the signal depends on the dielectric permittivity difference across the interface, and thus changes in orientational order or configuration of the liquid crystal in the inclusions give rise to changes in the second harmonic intensity. The authors have measured the temperature dependence of the SH signal from PDLC films at temperatures both below and above the nematic-isotropic transition temperature of the liquid crystal in the film. An externally applied electric field can affect the orientation of the liquid crystal, and hence affect the SHG. The modulation of the intensity of the second harmonic signal by a low frequency voltage applied across the film, as well as the effects of droplet size on SHG in PDLC films, have been studied. Possible mechanisms responsible for the measured results are discussed.

A transmissive, analogue, active backplane electrically addressed SLM using chiral smectic liquid crystals is described with CdSe TFT's to switch the pixels. The device consisted of a matrix of 200 im x 200 im pixels forming 128 rows by 190 columns. The liquid crystal used was 854E (BDH Ltd. England) which allowed observation of the device behaviour in both the smectic C* (ferroelectric switching) and smectic A (electrocinic effect switching) liquid crystal phases. Some observations are made concerning each of these.

Compact optical processing systems are being achieved by the assembly of miniature components. This paper considers the development of integrated 2-D optical processing systems based on planar optics structures and free space communication. Active liquid-crystal optical elements in planar optics form are discussed. Attention is drawn to the use of optically addressed liquid crystal spatial light modulators (OASLM) as demonstration vehicles for planar optics structures. An exclusive NOR optical gate is demonstrated in planar optics form using a nematic liquid-crystal OASLM.