Observation of high gain in a liquid-crystal panel with photoconducting polymeric layers

Abstract

A novel, to our knowledge, liquid-crystal panel suitable for real-time holographic purposes has been prepared. A nematic liquid-crystal layer sandwiched between photoconducting polymeric layers, when exposed to a sinusoidal light-intensity pattern, shows efficient formation of refractive-index gratings. The unique feature of the presented panel is its ability to switch energy from beam to beam in a manner similar to the charge-diffusion-controlled photorefractive effect. In a two-wave-mixing experiment multiple orders of diffraction are present, and a very high two-beam coupling-gain ratio (2.5) and a net exponential gain coefficient of ? = 931 cm(-1) have been measured. This gain was achieved in samples biased by a dc external electric field and tilted with respect to the beam-incidence bisector at 45 degrees . The time constants for grating formation and erasure in the studied system are functions of the applied voltage and can be made as short as a few milliseconds under favorable conditions. The mechanism of beam coupling is linked with an electric-field-driven reorientation of the nematic director as a result of a spatially modulated space-charge field created by light in a photoconducting poly(3-octyl)thiophene polymeric layer.

The role of surface charge field in two-beam coupling in liquid crystal cells with photoconducting polymer layers

“…17 Finally, we have observed that cells with just one photoconductive layer were as efficient, in terms of two-beam coupling, as cells with two photoconductive layers, the design adopted in all earlier publications. 3,11,12 As expected, our experimental data on two-beam coupling, which we will present in detail in the following section, are in good agreement with previously published results. 3,11,12 Our data are also consistent with the results obtained in cells with nonphotoconducting polymer layers.…”

“…3,11,12 As expected, our experimental data on two-beam coupling, which we will present in detail in the following section, are in good agreement with previously published results. 3,11,12 Our data are also consistent with the results obtained in cells with nonphotoconducting polymer layers. 13 …”

“…[4][5][6] In this work, we focus our attention on structures containing liquid crystals and photoconducting polymer layers. The aim of our work was neither optimization nor achieving higher coupling coefficients than those reported earlier, 2,3 but to study the role of surface effects and their contribution to reorienation in the two-beam coupling process.…”

“…[1][2][3] The electro-optic response and the buildup of a refractive index grating in liquid crystals' cells arise from the reorientation of molecules due to an induced space charge field 4 and this effect has been named the "orientational photorefractive effect" (or "photorefractivelike effect"). [4][5][6] In this work, we focus our attention on structures containing liquid crystals and photoconducting polymer layers.…”

“…3,7 The role of such a polymer is not only to impose order on liquid crystal molecules, but also to significantly contribute to the nonlinear response of the whole structure. Dopants such as TNF, pure C 60 , or mixtures 8,9 of C 60 and C 70 make PVK photosensitive to visible light.…”