Deformed Helix Ferroelectric Liquid Crystals with Large Tilt Angles in Optically Addressed Spatial Light Modulators for
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hotoconductive films, based on amorphous silicon carbide (a-SiC:H) as well as on photoconductive polyimide films, were measured. A comparable high diffraction efficiency on the order of 20% was obtained. The operation of the developed OASLMs for the holographic correction of images is described. EXPERIMENT Deformed helix ferroelectric (DHF) materials The DHF effect is considered as the lowest voltage effect in liquid crystals [9]. Appreciable electrooptical response takes place for voltages on the order of tenths of a volt. A deviation of the averaged optical indicatrix on the angle of ±22.50 is usually obtained applying less than ± 5 Volt, and it allows for a gray scale modulation. The response time of DHF material depends on the pitch of the helix and not on the voltage. For applications in OASLMs with high DE we developed DHF materials with a response time on the order of 200.ts and a spontaneous polarization of about 100nC/cm 2 . The molecular tilt angle E in the totally untwisted state is 39400 at room temperature. The pitch of the helix Po is 0.2pm or less. In Table 1 the temperature intervals of the SmC* phase and other characteristic data are listed for some developed FLC materials. Table 1. Parametersof DHFLC materials. FLC material FLC-459 Parameter Interval of SmC* phase, 'C -10...+65 410 Tilt angle 0 (25 0 C) Spontaneous polarization, nC/cm 2 (25 0 C) 18 Pitch of helix po, [im (25 0 C) 0.35
FLC-461
FLC-464
FLC-471
0...63.5 41.50 93 0.22
+5...+62 400 120 0.18
+2...+62.5 39.50 115 0.19
Photoelectric characteristics of photoconductive films We developed photoconductive films on the basis of photodiode a-SiC:H thin layers in p-i-n configuration [10,11] and of polymer (polyimide) films [12]. These films are transparent for red light and the radiation of He-Ne lasers (633nm) can be used for read-out without an induction of spurious photocurrents in the photosensors. Hence, a transmissive operation can be simply realized. As it was shown [13] the sensitivity and the response time of silicon carbide photoconductors are of the same order as of the well known amorphous silicon a-Si:H films, but the spatial resolution of the a-SiC:H film can be improved owing to the decrease of the dark conductivity of the a-SiC:H layer with the increase of carbide content. The a-Sil-xCx:H p-i-n structure was deposited on glass or quartz substrates with a transparent ITO (indium-tin oxide) electrode by the magnetron sputtering method. The individual layers were deposited by high frequency decomposition of a mixture of gases CH 4 and SiH 4 in a glow-discharge plasma. Doping was carried out by adding a small amount 0.1-1% of phosphine PH3 (n-type layer) or diborane B 2 H 6 (p-type layer). All layers were grown in a single cycle without exposure to air between intermediate procedures. The thickness of the ptype layer was 150A, of the i-type layer (with intrinsic conductivity) 5000A, and of the n-type layer less than 300A. Though the photosensitivity of silicon-carbide films decreases with the
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increase of carbide concentra
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