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42 OPTOINFORMATICS’05 STRONGLY NONLINEAR REVERSIBLE HOLOGRAPHIC RECORDING AT THE STRUCTURES Sb 2 S 3 – LC AND As 40 Se 60 -LC L. P. Amosova, A. N. Chaika, N. I. Pletneva All-Russian Research Center S. I. Vavilov State Optical Institute 12, Birgevaya line, St. Petersburg, 199034 Russia FAX: +7-812-3283720, E-mail: l_amosova@mail.ru The task <strong>of</strong> creating a medium for the reversible recording <strong>of</strong> holograms with a nonlinear modulation characteristic is essentially opposite to the traditional trend <strong>of</strong> ensuring maximum linearity <strong>of</strong> a holographic recording medium. The need for a new medium is related to the use <strong>of</strong> a holographic correlator for constructing fuzzy algebra algorithms in artificial intelligence simulators. This requires a medium suitable for writing Fourier holograms and possessing nonlinear properties [1] . Previously [2] it was demonstrated that such media are <strong>of</strong>fered by optically controlled structures <strong>of</strong> photoconductor (PC)- nematic liquid crystal (LC) type with a planar initial orientation. In these structures the modulation characteristic, representing the dependence <strong>of</strong> the diffraction efficiency on the recording light intensity in the first order <strong>of</strong> diffraction, has a substantially nonlinear shape with rising and falling regions if the phase modulation depth is sufficiently large. We have carried out the competitive investigation <strong>of</strong> two types <strong>of</strong> the Optically Addressed Liquid Crystal Spatial Light Modulators (OA LC SLM), optimized for the holographic application: with Sb 2 S 3 and As 40 Se 60 as a photoconductor. The spectral characteristics <strong>of</strong> these semiconductors allow a highly sensitive recording medium to be obtained for writing holograms using He-Ne laser radiation. OASLM consist <strong>of</strong> a number <strong>of</strong> thin layers sandwiched between two glass substrates: a photoconductor (PC), a liquid crystal (LC), alignment layers and transparent electrodes. When DC voltage is applied to the electrodes, it is divided between the photoconductor and liquid crystal according to the exposure. This enables the optical activity <strong>of</strong> the liquid crystal layer to be modulated to produce an image. The OASLM based on nematic LC have perfect characteristics and high optical quality. The thickness <strong>of</strong> the LC layer should be enough to provide the phase shift not less than 2π at the wave length <strong>of</strong> read out light (814 nm in our case). We used a holographic technique for studying the modulation characteristics <strong>of</strong> the OASLM. A He-Ne laser (λ= 633 nm) was used to write in a holographic grating. The writing beams had equal intensity, and their diameter in the plane <strong>of</strong> photo-semiconductor was equal to 10 mm. An interference pattern <strong>of</strong> two plane wave fronts was formed at the photoconductor-LC boundary. The intensities <strong>of</strong> the reference and object beams were equal. The readout was performed by radiation <strong>of</strong> the semiconductor laser <strong>of</strong> the wavelength λ= 814 nm in a transmission mode. We studied the behavior <strong>of</strong> the diffraction efficiency in the first order <strong>of</strong> diffraction depending on the write light intensity. The value <strong>of</strong> diffraction efficiency was determined as the ratio <strong>of</strong> the intensity <strong>of</strong> read-out radiation transmitted in the first diffraction order to the corresponding value <strong>of</strong> the transmitted from OASLM radiation in the absence <strong>of</strong> the holographic grating. These light intensities were measured with a help <strong>of</strong> photomultiplier, which was placed in the Fourier-lens focal plane. The sample structure was arranged in the optical scheme so that the LC director orientation would coincide with the polarization vector direction in the readout beam. The curves <strong>of</strong> diffraction efficiency as a function <strong>of</strong> the writing light intensity for different values <strong>of</strong> applied voltage exhibit a rising region and a falling region, in which the
SAINT-PETERSBURG, October 17 – 20, 2005 43 diffraction efficiency drops by more than 100 times in comparison with the maximum value. The diffraction efficiency is highly dependent on the voltage applied to the modulator. The angle between the liner segment <strong>of</strong> our curve and the abscess axis increases with the increase <strong>of</strong> applied voltage. Correspondingly the maximum <strong>of</strong> diffraction efficiency removes to the side <strong>of</strong> the writing light low intensities. The slope <strong>of</strong> the falling part <strong>of</strong> the curves depends on applied voltage according the same law: the larger bias – the steeper curve. This fact means that the applied voltage and the writing light intensity compensate each other in some reasonable limits. The total acting voltage is summing up from the applied voltage and from the generated by writing light charge in the photoconductor, witch can be comparable with the external bias in the case <strong>of</strong> high sensitivity <strong>of</strong> PC. For the present instance we can observe the same picture as if we applied the bias larger than operating: the diffraction efficiency and the contrast decreases rapidly. The maximum diffraction efficiency achieved with As 40 Se 60 PC was equal to 42% and with Sb 2 S 3 PC –up to 40%. Such high value <strong>of</strong> diffraction efficiency is connected with the diffraction orders asymmetry. In our case the recording medium corresponds to the criterion for thin holograms. The diffraction efficiency higher than the theoretical limit for thing holograms is achieved owing to the deviation from symmetry <strong>of</strong> the holographic grating strokes formed in the LC. In the case <strong>of</strong> the structure with Sb 2 S 3 PC we are able to increase the maximum diffraction efficiency in the “first plus” order (η max+1 ) utilizing the writing beams modulation in the time. The writing radiation pulses were 5 sec long and their repetition period consists 10-15 sec. The resolution <strong>of</strong> stibnite structures grows substantially in the whole range <strong>of</strong> the spatial frequencies. As about the structures with the As 40 Se 60 PC, the writing beam modulation gives no result: the diffraction efficiency and resolution remain the same as in the case <strong>of</strong> the constant writing beam. We have investigated the influence <strong>of</strong> the LC layer thickness on the OASLM characteristics. The minimum thickness was chosen in such a way that a calculated phase shift consists no less than 2π for reading out radiation. In our case we have manufactured and measured the samples with 6,5 µm, 10µm and 15µm LC layers thick. The structure resolution decreases with the increase <strong>of</strong> LC layer thickness, but the behavior <strong>of</strong> sensitivity and diffraction efficiency is not so simple: the structures with 10µm LC layer poses the best characteristics. 1. A.V. Pavlov “On Algebraic Foundations <strong>of</strong> Fourier Holography”// Opt. Spectrrosc. 90 (2001), p.452. 2. N. Chaika and F.L. Vladimirov, "Nonlinear hologram recording on an optically controlled transparency <strong>of</strong> the glassy chalcogenide semiconductor-liquid crystal tipe," Tech. Phys., Let..,. 26, No 2, p. 139, 2000.
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