Proceedings of Topical Meeting on Optoinformatics (pdf-format, 1.21 ...

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