Stressed-liquid-crystal (SLC) light-modulating devices suitable for use as liquidcrystal- on-silicon (LCOS) spatial light modulators (SLMs) that could operate in nearand mid-infrared wavelength ranges have been demonstrated. These SLC devices were conceived to exploit the SLC electrooptical effect, which makes it possible to obtain response times shorter than those of prior infrared LCOS SLMs.

An LCOS SLM includes a light-modulating layer of a liquid crystal material placed directly on a complementary metal oxide/semiconductor (CMOS) integrated circuit. A typical LCOS SLM is compact, inexpensive, and easy to use, and contains millions of gray-scale pixels electrically addressable at a rate of thousands of frames per second. LCOS SLMs are used as imagegenerating devices in viewfinders of electronic cameras and rear-projection highdefinition television receivers, but have found only limited use in applications involving infrared light. The principal barrier to widespread acceptance in infrared applications has been long response times.

Turn-On and Turn-Off Responses of SLC devices having three different thicknesses were measured in terms of transmitted-light intensities and converted to retardances. In each case, the response time was calculated as the time interval between 10 percent and 90 percent of the retardance range covered.
For most liquid-crystal optical-modulation modes, the thickness of the liquidcrystal layer needed to modulate light of a given wavelength is proportional to the wavelength, and the response varies quadratically with the thickness. Hence, in effect, the response time varies quadratically with wavelength. For example, if the response time of a liquid-crystal SLM were 1 ms at the visible wavelength of 0.5 μm, the response time of a comparable device designed for and operating at the mid-infrared wavelength of 5 μm would be 100 ms.

In an SLC device of the present type, the liquid-crystal material is, more specifically, a liquid-crystal/polymer composite aligned by shear stress. The device functions as a variable retarder to modulate light. The device is designed and constructed to exploit the facts that (1) when a sheared liquid- crystal polymer composite is placed between crossed polarizers with its shearing axis aligned at 45° to the polarizer axes, efficient intensity modulation can be obtained; and (2) the response time of an SLC device is shorter than that of a comparable unstressed-liquid-crystal device and is nearly independent of the thickness of the liquid- crystal layer.

The SLC devices that were demonstrated were designed and fabricated for operation in three wavelength bands: a near infrared band (1.8 to 2.5 μm), a midinfrared band (3 to 5.5 μm), and a far infrared band (8 to 14 μm). The devices for these three bands had thicknesses of 5, 10, and 20 μm, respectively. The devices were operated at drive potentials of 25, 50, and 125 V, respectively. As thus designed, built, and operated, the devices imposed half-wave modulation at response times ranging from 1.3 to and 1.6 ms (see figure). Although the drive potentials needed for near- and mid-infrared devices are high relative to potentials used in modern signal- and data-processing semiconductor circuits, they are low enough to enable the design and operation of stressed-liquid-crystal- on-silicon (SLCOS) SLMs into which the needed high-voltage transistors could be incorporated by use of standard CMOS fabrication processes.

This work was done by John R. McNeil, Michael J. O'Callaghan, and Mark A. Handschy of Displaytech, Inc.; Guoqiang Zhang, Anatoliy Glushchenko, and John L. West of Kent State University; and Kerry Lane and Stephen D. Gaalema of Black Forest Engineering for the Air Force Research Laboratory. For further information, download the free white paper at under the Photonics category. AFRL-0008

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Fast Liquid-Crystal-on-Silicon Spatial Light Modulators

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This article first appeared in the February, 2007 issue of Defense Tech Briefs Magazine.

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