Liquid Crystals Light Valve (LCLV)

In liquid crystal light valves, LCLVs, nonlinear holography is possible thanks to the realization of transmissive devices that are composed by a transparent photoconductor associated with a liquid crystal medium. In our laboratory, we build liquid crystal light valves by using a wall made of the photorefractive Bi12SiO20 (BSO) crystal, cut in the form of a thin plate (1-mm thickness, 20×30 mm2 lateral size). The BSO is used for its large photocon- ductivity and transparency in the visible range, whereas the electro-optic effect is provided by the large birefringence of the liquid crystals.

Transparent electrodes are deposited over the outer surface of the BSO and the inner surface of the glass walls containing the liquid crystals. These allow applying an external voltage V0 across the LC layer. The typical voltage applied is a. c., with r. m. s. values from 2 to 20 V and a frequency from 50 Hz to 20 kHz. When a light beam impinges on the LCLV, because of the photoconductive properties of the BSO a photo-generation of charges occurs at its surface; hence, the effective voltage across the LC layer increases locally, according to the local illumination conditions. As a consequence, the LC molecules reorient and, because of their birefringence, at the exit of the LCLV the light beam acquires a phase shift that is a function of the applied voltage and of the beam intensity itself.



In the linear region of its response the LCLV behaves as a Kerr-like nonlinear medium, providing a refractive index change proportional to the input light intensity. The equivalent Kerr-like coefficient n2 is as large as 0.7 cm2/W and large optical nonlinearities are obtained for input laser intensities as low as a few mW/cm2. The response time is dictated by the time required by the collective motion of the reorienting LC molecules to establish over the whole thickness of the nematic layer. For a typical thickness d=14 microns, and for typical values of the LC constants, the LCLV response time is of the order of 100 ms. It can be reduced by decreasing the thickness of the LC layer or by using different types of liquid crystals, such as ferroelectrics or LC dispersed polymers. The spatial resolution is if the order of a few microns.