High-speed complex-amplitude spatial light modulators (SLMs) containing liquid crystals with pixel electronic circuitry on single-crystal-silicon backplanes are undergoing development. The basic approach taken in this project is to use fast-switching liquid-crystal materials and modulation-enhancing device geometries that have not been used in prior display systems.

The modulator materials selected for this project are chiral smectic liquid-crystal (CSLC) materials of the high-tilt type. These materials are capable of modulating light with switching times

  • Rotative switching (rather than variable retardance) occurs and results in bipolar amplitude modulation in the typical crossed-polarizer arrangement. A variable-birefringence device produces coupled phase and amplitude modulation when operated in the same fashion.
  • Depths of modulation are inadequate; CSLC materials act to produce either continuous operation over small ranges or else two-state (binary) switching.
  • Relatively strong electric fields and thus high drive voltages are needed for full switching in analog CSLCs.

These device characteristics as well as other pertinent issues have been addressed during the project. Accomplishments have included the following:

  • Alignment techniques and drive schemes for high-tilt materials to obtain true gray-scale modulation were developed. These CSLC materials directly provide bipolar amplitude modulation (real-axis coverage) and are essential for nondispersive phase modulation.
  • A polymer cholesteric liquid crystal has been verified to function as a handedness-preserving mirror, which enhances the modulation depth of a resonated or a two-pass non-resonated phase-only CSLC modulator.
  • A nonresonated 2π-radian analog phase modulator containing a cholesteric-liquid-crystal (CLC) mirror and a high-tilt analog half-wave element has been demonstrated.
  • A thin-film CLC has been investigated for use as a low-stress planarization layer for a very-large-scale integrated (VLSI)- circuit backplane. The CLC film acts as a phase-flat mirror inasmuch as the top surface is formed against an optical flat. The top surface is reflective and masks the underlying VLSI structures.
  • A mathematical-modeling program that utilizes a 4 × 4-matrix technique was developed for use in analyzing the geometries of modulators that exploit rotative switching in CSLCs or variable retardance in nematic liquid crystals.
  • Closed-form solutions were derived for the modulators for general input polarizations, giving a variety of available operating curves not obtainable when using nematic liquid crystals. This analysis pertains to the arrangement of a reflection-mode SLM with a polarizing beam splitter as an analyzer and beam-routing device.
  • The use of a high-voltage backplane was found to improve optical-correlation performance.
  • A high-performance 128 ¥ 128 analog SLM with a VLSI backplane was demonstrated (at the time of reporting the information for this article, there were plans to develop subsequent devices with higher pixel densities). This VLSI backplane operates at a potential of 12 volts and at a load rate of 10,000 frames/second, with full analog signal storage at each pixel and a pixel pitch of 40 µm. This VLSI backplane supports a variety of liquid-crystal modulators in addition to those based on high-tilt CSLCs.
  • SLM drive circuitry and control software were developed and tested. The SLM drive system is controlled by a personal computer.
  • A postprocessing technique for planarizing a VLSI backplane was investigated. The technique involves deposition of a thick oxide layer on the VLSI silicon wafer, followed by chemical-mechanical polishing. After polishing, optical-quality pixels are deposited and connected to the underlying VLSI circuits.

Overall, the project has been instrumental in increasing understanding of the principles of operation and of manufacturing liquid-crystal-on-silicon SLM products.

This work was done by Gary Sharp and Steve Serati of Boulder Nonlinear Systems, Inc., for Johnson Space Center. For further information, access the Technical Support Package (TSP) free on-line at www.nasatech.com/tsp  under the Electronics & Computers category. MSC-22840

NASA Tech Briefs Magazine

This article first appeared in the May, 2000 issue of NASA Tech Briefs Magazine.

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