High resolution displays using ferroelectric liquid crystals (FLCs) driven by a single crystal silicon backplane are a powerful option for many current and future applications. The FLC light-modulating layer is placed on top of a reflective CMOS backplane. This geometry effectively takes advantage of the fast switching speed and high resolution possible in a sub-micron thick layer of FLC and fine design rules available in standard CMOS fabs. The microsecond FLC switching speeds allow both gray scale and color to be generated with time sequential addressing. In addition, low temperature performance is remarkable ad showing switching in the sub-50 millisecond regime down to -40 degrees C. Current FLC materials operate well with mainstream 5 volt CMOS as well as with newer 3.3V processes. These lower voltages result in lower power consumption and finer design rules allowing for higher density of on-chip electronics such as MPEG2. This option is difficult to achieve in higher voltage processes typical of nematic on VLSI devices. The reflective quarter wave plate design of the display affords a large angular acceptance resulting in flexibility of optical design as well as excellent contrast and throughput vs. viewing angle. We will describe 24 bit color displays and the FLC mixtures optimized for their particular application.
Chiral smectic A electroclinic liquid crystalline materials are prime candidates for electrooptic applications because of their gray scale capability and fast dynamic response. The evolution of this technology greatly depends on the development of chiral smectic A materials with large induced tilt angles, field-independent fast switching times and a broad operating temperature range. We report in this paper the physical properties of a series of liquid crystal materials. Features of this series of materials are broad smectic A phase range and large induced tilt angles. Such large electroclinic coefficients make these materials good candidates of the development of silicon based reflective displays. Comparison of the electrooptic performance of the materials in SmA mesophase will be discussed.
Liquid crystals have been shown to align on polarized UV (PUV) exposed polyimide films with the liquid crystal director oriented perpendicular to the polarization direction. Uniform pretilt is produced by a second oblique PUV exposure. We report here the factors controlling liquid crystal alignment and pretilt on photo-exposed polyimide films. We show that the chemistry of the polyimide controls the liquid crystal aligns either parallel or perpendicular to the PUV. Also, the tilt angle of the liquid crystal can be varied continuously between 0 degrees and 90 degrees by adjusting the polyimide chemistry. Our results indicate that the liquid crystal has uniform pretilt with no preferred alignment direction on unexposed polyimide. Normal exposure aligns the director but the pretilt is degenerate, pointing in either the plus or minus direction. Oblique exposure breaks this degeneracy, producing a unique pretilt direction. We show that uniform alignment and pretilt can be produced with a single oblique exposure if the liquid crystal aligns parallel to the polarization direction.
A photo-dimerized monolayer (PDML) has been recently developed by us for liquid crystal alignment. Electro-optic properties of a single pixel prototype device with the PDML alignment layer have been measured. Data on the contrast, threshold voltage and slope of the response curve are presented which demonstrate the efficacy of the PDML layer for LCD applications. In addition, the polar anchoring energy was measured for PDML layers differing in the chemical composition. The ability to tune this anchoring energy through chemical modification is demonstrated. It is shown that the anchoring strength is comparable to that obtained for liquid crystal cells using the mechanically rubbed polyimide alignment layer.
Non-contact optical alignment technology has been identified by the liquid crystal display (LCD) industry as one of the key technologies for manufacturing next generation wide viewing angle, high resolution LCDs. In this paper we review the field of optical alignment technology. We will report our progress in optical alignment materials and equipment development. Data on optically aligned twisted nematic, super-twisted nematic, vertical alignment, and in-plane switching test cells will be presented.
In this paper, a range of polymethacrylate derivatives of hydroxycoumarin are investigated as photoinduced alignment layers for liquid crystals, and their performance is compared with polyvinyl cinnamate. For al the coumarin- containing polymers, the liquid crystal alignment direction is parallel to the polarization direction of the incident UV light at low fluences. At a critical fluence threshold, a sharp change to perpendicular alignment is found. Molecular models and spectroscopy are used to explain this phenomenon as well as the fact that only perpendicular alignment is observed for polyvinyl cinnamate. The azimuthal anchoring energies of the alignment layers are measured and values greater than 6 by 10-5 J m-2 are found. The incorporation of a flexible spacer into the coumarin sidechain results in stronger anchoring at low fluences. Exposure of the coumarin-containing alignment layers at oblique incidence gives pretilted alignment with pretilt angles up to 5.1 degrees.
Ellipsometers have long been recognized as the best instrument for measuring the refractive index and thickness of thin films. Two aspects of ellipsometry have important implications. First, the refractive index and thickness of the thin film can be retrieved by using the proper layer model and by adopting an appropriate numerical-inversion technique. Secondly, the accuracy of the incident light beam angle is important in retrieving the correct answer. The main focus of this paper is to discuss the effects of the incident angle correction algorithm on the accuracy of the parameters retrieved, and then try to compensate for the inaccuracy resulting from the angle-varying mechanism of the ellipsometer. Detailed simulations have been performed to examine the accuracy of the optical indices and thin film thickness under various measurement conditions. From the simulation result, it has been shown that adopting the incident angle correction algorithm can be improve the accuracy of the ellipsometer. This leads us to believe that the accuracy of an ellipsometry system can be further improved if the incident angle correction algorithm is included in the data-retrieving algorithm.
A new approach to underwater ultrasonic imaging is described and demonstrated which directly converts a 2D acoustic pressure image formed from an acoustic lens into an intensity-mapped visual image. There are no computers nor electronic requirements, nor piezo arrays necessary. The imager relies on the acoustic coupling, which occurs between directed acoustic energy and aligned nematic liquid crystals. The aligned liquid crystal, being optically birefringent, thereby serve as a display when viewed through crossed polarizers. Pressure waves established by acoustic transducers are reflected by the target, focused by an acoustic lens system, and converted into a visible image for target identification in littoral water. Anticipated uses are for searching and identifying underwater mines which are hazardous to military and civilian ships, ferries, and fishing boats. Other uses include search and rescue and inspection of underwater hazards and structures. Acoustic images obtained using only liquid crystal and light are included.
The introduction of the 'hybrid electrode' has proven to successfully simplify both the electrode design and the requirements on the electrical driver of liquid crystal optical devices. In this paper, we present the design and summarize some experimental results with respect to liquid crystal lenses and lens arrays employing the hybrid electrode structure. We further introduce the design of the circular hybrid electrode structure that brought forth, to our knowledge, the first single-layer liquid crystal spherical lens array with quadratic lens apertures.
THe ability to attain the highest possible resolution with any optical imaging system depends on maintaining the quality of the wavefront entering and passing through the systems. Wavefront degradation can arise from errors in optical design and manufacture, random aberrators like the Earth's atmosphere, and the structure supporting the imaging system. In the last two decades technology has been developed and tested to remove, in real-time, the effect of aberrators. This technology is commonly known as Adaptive Optics. However, AO is expensive and very complicated. In an effort to reduce complexity and costs, alternative technologies are under study at various institutions.In this paper we will present the status of our effort in using liquid crystal devices as corrective elements for AO applications.
We developed a bistable cholesteric reflective display using a dual frequency cholesteric liquid crystal. When a high frequency voltage is applied to the matrices, it exhibits a negative dielectric anisotropy and is switched to the planar texture which is Bragg reflecting. When a low frequency voltage is applied to the materials, it exhibits a positive dielectric anisotropy and is switched to the focal conic texture which is weakly scattering. Both the planar texture and the focal conic texture are stable at zero field, and therefore the material can be used to make multiplexed displays with passive matrix. This new display can be directly switched between the planar texture and the focal conic texture, and therefore is suitable for displaying dynamic images.
Crystalline silicon-chip-based reflective light valves are suitable for realizing high definition and bright liquid crystal projectors. We have developed an XGA silicon-chip- based light valve with a diagonal display area of 1 inch. A normally white reflective twisted nematic mode is selected for the valve. An optimum condition of the mode is analytically solved by the Jones matrix method. This mode is suitable for a narrow cell gap and a fast response time can be expected. This mode also has a stable contrast ratio even with temperature and/or cell gap fluctuation. In addition, the driving voltage of this mode is low and it has good chromaticity with small retardation. The cell gap of the light valve is 2 micrometers . The cell gap support is made using spacer posts formed on the silicon chip with a photo- definable resin. The response time is 12 ms including both rise and fall times. The contrast ratio is more than 400 at 5 Vrms.
Recently LCOS microdisplays are becoming available for personal IT applications, despite some problems which are less critical in poly silicon or amorphous silicon based displays. The most common problems which must be encountered are the polarization of the pixels and the light shielding of the silicon substrate. In this paper a methods proposed which solves the light shielding and pixel flatness problem. A non-critical back-end processing which can be applied outside the silicon foundry has been developed. The effectiveness of the light shielding on a working demonstrator display is shown. To avoid light losses caused by a polarization filter, a polymer dispersed LC has been chosen. By decreasing the cell gap we made the PDLC voltage compatible with a standard 3 micrometers CMOS process and its response fast enough to be used for video applications. It is shown that this choice is very suited in direct view and portable applications. The realized prototype has 3 bit grey levels and is video compatible and can be used in a number of applications, such as personal viewers, PDAs and data displays.
The backplanes of Liquid-Crystal-on-Silicon microdisplays are derived from a VLSI silicon chip that includes the active matrix as well as row and column drivers. One away to convert this silicon chip into a functional backplane is to planarize the silicon chip, then etch vias through the planarization layer and finally to pattern an array of flat, highly reflective electrodes, each of which is electrically connected to a corresponding cell of the active matrix underneath. Such a post-processing sequence can be carried out in different ways, using either Chemical-Mechanical Polishing or spin-on planarization. We have chosen spin-on planarization with Dow Chemical's Cyclotene resin followed by reactive ion etching of the vias. Finally, electrodes are patterned by aluminum sputtering and lift-off. This step also establishing the electrical connection to the underlying metalization. To demonstrate this sequence we have fabricated a two-level passive silicon backplane with aluminum stripe electrodes. We describe in detail the processing steps involved and report on the achieved degree of planarization, polymer and aluminum roughness.
Cholesteric liquid crystal display (Ch-LCD) are lightweight, low power, sunlight readable displays. In addition, they can serve a dual function as pen-input device switch no additional hardware. Because of the unique properties of this technology, Ch-LCDs can be made with plastic substrates thus making the displayed extremely lightweight, compact and unbreakable. We discuss in this paper cent advances in merging Ch-LCD technology with conducting polymer electrodes. Conducting polymer provides potential benefits over the use of the standard display electrode materials, indium tin oxide, by improving the reliability of the display. Furthermore, the potential to print the conducting polymer electrodes could significantly increase manufacturing volume and decrease display cost. We report on scaling display size and resolution by demonstrating a 1/8 VGA Ch-LCD using polypyrrole as the conducting polymer. We fabricated these displays using either a vacuum fill or polymer wall/lamination approach and we discus subsequent failure analysis to determine the cause for the line-outs observed on these displays. We present initial results in determining the suitability for using Ch-LCD technology as a pen-input device. Finally, we discuss initial work towards printing the conducting polymer electrodes to determine the feasibility of printing electrodes on plastic substrates in a roll-to-roll, high volume, low cost process.
We report a full color 1/4 VGA reflective cholesteric display with 4096 colors. The display can deliver a brightness approaching 40 percent reflected luminance, far exceeding all other reflective technologies. With its zero voltage bistability, images can be stored for days and months without ny power consumption. This property can significantly extend the battery life. The capability of displaying full color complex graphics and images is a must in order to establish a market position in this multimedia age. Color is achieved by stacking RGB cells. The top layer is blue with right chirality, the middle layer is green with left chirality, and the bottom layer is red with right chirality. The choice of opposite chirality prevents the loss in the green and red spectra from the blue layer on the top. We also adjusted the thickness of each layer to achieve color balance. We implement gray scale in each layer with pulse width modulation. This modulation method is the best choice consideration of lower driver cost, simpler structure with fewer cross talk problems. Various drive schemes and modulation methods will be discussed in the conference.
An important problem associated with multiple pixel liquid crystal (LC) modulators is the incidental diffraction due to amplitude and phase gratings formed by the improperly modulated regions between electrodes. This problem becomes more of an issue as the resolution of the SLM increases and the size of the pixels begins to approach the size of the inter-pixel spacing. We perform 2D director profile modeling in LC diffraction gratings to take into account the electrode structure and fringing electrostatic fields. The electrooptical properties of the grating are simulated and compared with experimental data. This information can be used to design addressing structures with enhanced fill factor. The results obtained could be beneficial for applications in image processing, laser beam control, steering and adaptive optics.
The key point to widen the veiwing angle of a TN-LCD is to realize a birefringent film with its optical axis tiled with respect to the display plane. We performed a negative tilted uniaxial film with holographically recorded volume index gratings in the form birefringence regime and studied the characteristics of such compensator in terms of viewing angle of a TN-LCD.
H-PDLC materials are introduced with sensitivity in the 800- 850 nm region. Materials presented are based on acrylate monomers mixed with liquid crystalline (LC) compounds and sensitized to near IR. This allows highly efficient in-situ recording of holographic optical elements by diode lasers. Various LC compounds have been tested, in particular: E7, 5CB, MBBA. The following parameters of the H-PDLC materials and the recorded gratings were investigated: sensitivity of the materials in the range 800-850 nm, diffraction efficiency in a wide range of wavelengths and photochemical stability of the recorded gratings. The sensitivity of the materials with LCs E7 and 5CB was found to be ca. 300-500 mJ/cm2 to wavelengths 834 and 850 nm. Employment of MBBA led to drop in sensitivity of about an order of magnitude. High diffraction efficiency was demonstrated. Particularly interesting are the following properties of our materials: (1) Control of the dispersion of the diffraction efficiency for p-polarization is shown to be possible by changing the monomer content of the formulations. (2) Recorded gratings exhibit excellent photochemical stability. (3) Switching of the gratings is in principle possible and currently worked on.