A theory of beam coupling in a hybrid photorefractive liquid crystal cell is developed. The hybrid cell consists of layers of undoped nematic liquid crystal contained within wafers of photorefractive crystal or photoconductors. Reorientation of the liquid crystal is induced by the evanescent component of the space charge field propagating from the photorefractive layers, leading to a modulation of the liquid crystal refractive index. Using coupled wave theory, an analytic solution is obtained for the small signal gain in good agreement with previously published experimental work.
"In particular hybrid optical devices composed of photorefractive (PR) substrates with an organic LC layer are capable of overcoming several of the disadvantages present in pure inorganic or pure organic PR materials . PR hybrid devices take advantage of the high effective trap density present in inorganic PR materials to generate a large space-charge field capable of controlling an organic a liquid crystal layer  . The low birefringence present in PR materials is compensated by the large birefringence possible in LCs. "
[Show abstract][Hide abstract] ABSTRACT: Photorefractive (PR) hybrid liquid crystal (LC) cells have combined the space-charge field generated in either a
polymer (using e.g. PVK;C60) with the large birefringence from a LC layer to generate PR grating for beam
coupling applications. The efficiency of PR beam coupling in hybrid devices is dependent on the amplitude of
the space-charge field, as well as the ability of the LC molecules to align with the corresponding field. In this
paper the time dynamics of the formation of the PR gratings are measured in LC hybrid systems and are used to
explain the large variation of gain coefficients found in the literature.
[Show abstract][Hide abstract] ABSTRACT: Holographic gratings were recorded in hybrid cells of the nematic liquid crystal E7 doped with multi-walled carbon nanotubes in a degenerate two-wave-mixing experiment. The hybrid structure comprises doped nematic contained between photoconducting polymeric layers of poly(N-vinylcarbazole) spin-coated on two indium-tin-oxide-covered glass plates. Multiorder diffraction was observed and the first-order self-diffraction efficiency was obtained in order to investigate the properties of the gratings. The hybrid cells showed quite high diffraction efficiency up to 47%, which is far beyond the theoretical limit of 34% predicted by the Raman-Nath theory. In addition, such a structure was shown to exhibit the photorefractive effect due to the presence of two-beam-coupling gain. Under the condition for a fixed total intensity of the two interfering beams, the gain was found to increase with increasing pump-to-probe beam-intensity ratio.
Proceedings of SPIE - The International Society for Optical Engineering 01/2005; DOI:10.1117/12.620854 · 0.20 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: The photorefractive effect in a photoconducting polymer containing small liquid crystal droplets is studied. Two beam coupling experiments indicate a high optical gain at moderate field strength ( ≫50 cm <sup>-1</sup> at 10 V/μm). The writing of photorefractive gratings is reversible. The dynamic behavior can be described by a two step process of the grating formation, the faster time constants being of the order of 100 ms. Modulation of the external voltage indicates that the reorientation of the director in the liquid crystals droplets is faster than the rise and decay times of the space charge field.
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