A. B. Davey

The Silicon Backplane Design for an LCOS Polarization-Insensitive Phase Hologram SLM

Polarization-insensitivity is achieved in a reflective spatial light modulator by laying a quarter-wave plate (QWP) at the incident wavelength directly over the mirror pixels of a silicon backplane, and forming a nematic Freedrickcz cell over the QWP to modulate the reflected phase. To achieve the highest drive voltage from the available silicon process, a switched voltage common front electrode design is described, with variable amplitude square wave drive to the pixels to maintain constant root-mean-square drive and minimize phase fluctuations during the dc balance refresh cycle. The silicon has been fabricated and liquid-crystal-on-silicon cells both with and without the QWP assembled; applications include optically transparent switches for optical networks, beam steering for add-drop multiplexers for wavelength-division-multiplexing telecommunications, television multicast, and holographic projection.

Liquid crystal over silicon device characteristics for holographic projection of high-definition television images

We discuss some fundamental characteristics of a phase-modulating device suitable to holographically project a monochrome video frame with 1280 x 720 resolution. The phase-modulating device is expected to be a liquid crystal over silicon chip with silicon area similar to that of commercial devices. Its basic characteristics, such as number of pixels, bits per pixel, and pixel dimensions, are optimized in terms of image quality and optical efficiency. Estimates of the image quality are made from the noise levels and contrast, while efficiency is calculated by considering the beam apodization, device dead space, diffraction losses, and the sinc envelope.

Telecommunications applications of ferroelectric liquid-crystal smart pixels

Ferroelectric liquid crystal over silicon smart pixels offers potential advantages over conventional electronic and waveguide approaches to telecommunications switching. The role of such smart-pixel architectures in space/wavelength optical interconnect and in high-performance ATM switches based on interconnection of optically accessed memory is discussed.

Binary phase modulation using electrically addressed transmissive and silicon backplane spatial light modulators

The operation and performance of a 128-by-128 matrix electrically addressed ferroelectric liquid crystal (FLC) multiplexed transmissive spatial light modulator (SLM) and a 320-by-240 reflective silicon- backplane SLM are described. The two-dimensional multiplexed FLC SLM is probably the only device that exists to date optimized for ? = 1.55-?m operation. The spatial and temporal measurements of the binary phase modulation of the two devices are discussed. For the spatial measurements, a novel idea regarding the effects of the LC modulation characteristics in the interpixel deadspace of the silicon backplane on the quality of the binary phase modulation is described. For the temporal measurements, two addressing schemes for holographic beamsteering applications are described and compared experimentally with the silicon-backplane SLM. The limitation in performance for these two devices is briefly stated, and an indication given of future devices that will be possible using this rapidly advancing technology.

Applications of Ferroelectric Liquid Crystal LCOS Devices

Ferroelectric liquid crystal (FLC) liquid crystal over silicon (LCOS) devices can address large arrays of pixels at very high frame rates. They can also allow the phase of the light to be modulated in a manner that is independent of the state of polarisation of the incoming light. The optical versatility of the liquid crystal can now be matched by the electronic versatility of CMOS (complimentary metal oxide semiconductor) silicon backplanes as developments in very small feature sizes (sub micron) allows more and more complex electronic structures to be integrated into the device. In this paper, we briefly review some possible future applications of this technology in the area of displays, image processing and optical switching. In order to create these new device structures, we need suitable FLC materials.

Optically enhancing the small electro-optical effect of a fast-switching liquid-crystal mixture

The method of optical enhancement by means of intersurface-reflection-excited resonance was applied to a single-pixel device filled with a fast-switching electroclinic mixture M68. A contrast ratio of 6.5: 1, which is sufficient for a spatial light modulator (SLM), has been obtained for an electric-field-induced twist angle of 2.97 deg with an electro-optical response time of 162 ns. Success in a single-pixel device should allow very fast multipixel active-silicon-backplane SLMs to be made in the near future. Using this method, we have observed contrast ratios>5 with a field-induced twist angle>2.5 deg. However, better contrast can be achieved with improved alignment of the material used and device fabrication. The thickness effects of the light modulating layer on the resonant behavior were computer-modeled and are discussed in detail. It was found that considerable change occurs even for a thickness variation of 0.1 ?m. However, the effect is much less for a variation of 0.01 ?m. If SLM fabrication cannot control the thickness better than 0.1 ?m, either computer simulation for known thickness or preliminary measurements are needed.

Electro-Optical Bistability in Ferroelectric Liquid Crystal Switching Devices for Use in Displays and Real-Time Holography

Liquid crystalline (LC) materials which exhibit the ferroelectric Sc* phase showed early promise for device applications because of their fast electro-optic response and potential long-term electro-optical bistability. In combination these features should allow fast line at a time addressing and therefore potentially large passive matrix displays. Bistability should also facilitate DC balancing without the need for inverse frame or blanking procedures used in active matrix addressed liquid crystal over silicon devices. Such a feature is very valuable in the application of these materials to telecom devices where signal interruption for DC balancing purposes is very problematic. Long term bistability is also important in realising active tiling based displays. However the realisation of acceptable bistability has not been straightforward. Partly as a result, commercial passive matrix displays have been dominated by super twisted nematic (STN) devices, which have a limited voltage selection ratio that decreases with the number of pixel rows addressed. High quality flat panel LC displays for laptop computers, computer monitors and television has been delivered using active matrix thin film transistor twisted nematic (TFT TN) technology. The first generation of commercially important ferroelectric liquid crystal displays (for near-eye and projection applications) have in fact utilised active matrix liquid crystal over silicon (LCoS) device structures, where the speed of the FLC electro-optic response permits the use of frame sequential methods for generating colour and greyscales in images. Here we discuss the problems encountered in achieving useful bistability within FLC devices, the new applications that would greatly benefit from true FLC bistability and some interesting new results from some siloxane oligomers which suggest new possibilities in achieving true bistability. We hope to highlight the need for a deeper understanding of this problem.

DYNAMIC HOLOGRAPHIC GRATINGS IN METHYL RED-DOPED NEMATIC LIQUID CRYSTALS

Orientational photorefractivity in liquid crystals doped with azo dye MR crystals is studied. The experiments are carried out with no voltage applied. Dynamic behavior and its dependence on writing beams intensity are presented. When excited by light, the dye tends to bond to the cell surface. Such an effect is irreversible and leads to the permanent component formation in diffracted signal. This is not beneficial for applications where change of written information is required (OASLMs, dynamic holography, all-optical switching). The process was strongly suppressed and was virtually eliminated for the light intensities used, by the choice of materials for alignment surfaces.

Photoluminescent liquid crystal displays and a new approach to large screen video applications

Photoluminescent liquid crystal displays use a phosphor screen to separate internal light traversing a liquid crystal display (LCD) from light seen by the viewer. The internal light is narrow bandwidth UV and can be spatially directed. The diffuse phosphor emissions transform the angular viewing characteristics of conventional LCDs to the almost perfectly wide, uniform characteristics of a cathode ray tube. This type of display has the potential to exceed the power efficiency of current LCDs and can boost electro-optic performance in such a way as to give, for instance, enhanced multiplexing in passive matrix displays. The continuous phosphor screen and separate light paths enable truly seamless tiling for the production of large-screen direct-view displays. Here we briefly review photoluminescent LCD operation and describe the main design criteria. Three product-focused architectures are discussed and the current state of the art described for each.

Control of the electro-optic bistability of some ferroelectric liquid crystals useful for binary phase optical modulators

Abstract We report measurements on the bistability of the electrooptic response of a commercially available material (CS2005) and an experimental liquid crystal material (CDRR8) based on an organosiloxane structure. Both materials have a first order transition to the SmC* phase. CS2005 gave the normal monostable behaviour with layers tilted with respect to the rubbing direction. However, the second material, CDRR8, exhibited true bistability and at the same time its smectic layers were found to lie perpendicularly to the rubbing direction. We were able to control the bistable behaviour of the CDRR8 material by means of layer rotation using asymmetrical triangular pulses. As expected the sample shows a monostable response each time the layers are rotated so that one of the two switched states approaches the rubbing direction.