The term "light" is employed in accordance with its accepted definition encompassing electromagnetic radiation in the ultraviolet, visible, infrared and X-radiation wavelength ranges.
With increasing interest in employing light for information transfer theoretically useful models of spatial light modulators (commonly referred to as SLM's) have been proposed.
It has been observed that light being transmitted through a medium can be modulated by spatially intersecting light from a second source when the medium exhibits a refractive index that can be varied in response to light transmission. By "spatially intersecting" it is meant that light from the separate sources traverse intersecting paths, but not necessarily within overlapping time periods. The effect, referred to as a photorefractive effect, was first observed in working with lithium niobate (LN), lithium tantalum niobate (LTN), and potassium tantalum niobate (KTN) crystals intended for second harmonic generation (SHG) applications. Observations of photorefractive effects allowing holographic images to be stored in LN and KTN are reported in Chapter 11, Optical Phase Conjugation in Photorefractive Materials, Optical Phase Conjugation, Academic Press, 1983, pp. 417-425.
G. Moddel, K. M. Johnson, W. Li, and R. A. Rice, "High Speed Binary Optically Addressed Spatial Light Modulator", Appl. Phys. Lett., Vol. 55, No. 6, Aug. 7, 1989, pp. 537-539, illustrate a photorefractive light modulating device which employs liquid crystals as a photorefractive material. Although Moddel suggests that the ferromagnetic liquid crystals employed represent an improvement in terms of switching speeds over nematic liquid crystals, the fact is that all liquid crystal photorefractive devices are inherently limited in their frequency response, since the entire liquid crystal molecule must change its orientation to effect switching.
Another significant disadvantage of liquid crystals employed to provide photorefractive effects is that separate aligning layers must be provided above and below the liquid crystal layer to achieve the best attainable response. This involves constructing three separate layers and is consequently a fabrication disadvantage. The use of a liquid crystal layer between alignment layers is illustrated by E. M. Yeatman and M. E. Caldwell, "Spatial Light Modulation Using Surface Plasmon Resonance", Appl. Phys. Lett., Vol. 55, No. 7, Aug. 14, 1989, pp. 613-615.
R. Lytel, F. G. Lipscomb, J. Thackara, J. Altman, P. Elizondo, M. Stiller and B. Sullivan, "Nonlinear and Electro-Optic Organic Devices", pp. 415-426, Nonlinear Optical and Electroactive Polymers (P. N. Prasad and D. R. Ulrich, editors), Plenum Press, N.Y., 1988, disclose in FIG. 1 at page 419 a spatial light modulator comprised of a photodiode for receiving modulating light, a light blocking layer, a dielectric mirror, an electro-optic crystal and a transparent electrode for external circuit connection to the photodiode. While it is speculated that organic electro-optic materials might be substituted for the electro-optic crystal, the device even when so modified remains quite complicated to construct and limited in potential configurations because of the photodiode addressing required.
D. J. Williams, "Organic Polymeric and Non-Polymeric Materials with Large Optical Nonlinearities", Angew. Chem. Int. Ed. Eng. 23, (1984) 690-703, illustrates the known relationships between polarization properties and organic molecular dipoles.
An electrooptic modulator based on electrically varying the efficiency of coupling a collimated monochromatic beam into surface plasmons is described in the article by G. T. Sincerbox et al in Applied Optics, Vol. 20, No. 8, Apr. 15, 1981, pages 1491-1494. Alternatively, an electrooptic modulator based on electrically varying the degree of coupling of a light beam to a long-range surface plasmon (LRSP) is described in the article by J. S. Schildkraut in Applied Optics, Vol. 27, No. 21, Nov. 1, 1988, pages 4587-4590.
Latent images stored in, for example, silver halide film or selenium xeroradiographic plates can be converted into real (human-readable) images by using known chemical or powder cloud developing processes. More particularly, the surface charges representing the latent image on xeroradiographic plates are used to attract and hold powder particles prior to their transfer to a receptor sheet and fusing. However, such conversion of an electrophotographic latent image to a real image requires physical contact between the exposed electrophotographic plate and the powder particles (or a liquid containing them) and rollers, skivving knives, etc., which eventually may degrade the plate surface.
More recently, the surface charges on electron-radiographic plates, representative of a latent radiographic image, have been read directly from the charged plate by scanning a beam of light over the charged plate and reading the representative charge emanating from the plate at the point of scan by an external electronic device. Such light scanning systems require high surface charge densities and are especially prepared (patterned or multilayer) electrophotographic plates as described above.
A typical scanning light-beam electron-radiographic arrangement is described in Fenn U.S. Pat. No. 3,970,844 wherein parallel strips of electrically conductive, optically transparent, material are disposed on a charged electrode. The parallel strips are charged and then exposed to X-rays to produce static surface charges thereon representative of the resultant latent image. The change of resistance of a photoconductor between its dark and light stages is used to store the generated charge. Additionally, the change of resistance of the photoconductor is also used to conduct the stored charge from the strip(s) to an external electronic readout device as various points on the parallel strips are subsequently scanned with the beam of light from, for example, a laser. The light beam from the laser can be used to raster-scan the individual strips with a single beam of light or concurrently linearly to scan multiple corresponding points on the parallel strips via a linear array of lightguides coupled to the laser which move across the electrode orthogonal to the linear array and parallel to the strips.
Another light-scanning electron-radiographic imaging arrangement is described in Korn et al U.S. Pat. No. 4,176,275 in which a multilayered device having a photoconductive insulating layer is utilized to provide an electrostatic charge image at a first or second conductive layer of the device in response to imaging radiation directed at the device. A scanner for scanning the device with readout radiation is used with readout electronics for converting the electrostatic charge image into electrical signals. A D.C. voltage source is used both during the imaging step to impress an electric field across the device and to provide an electric field across the device and support the charge flow to the readout electronics as initiated by the readout radiation during the readout step.