VOLTAGE-CONTROLLED SELF-TRANSPARENCY IN A TWISTED NEMATIC LIQUID-CRYSTAL CELL BOUNDED BY PARALLEL POLARIZERS

We report the study of non linear transparency effect given by the lig ht induced creation of an isotropic hole in a 90 degrees twisted nemat ic liquid crystal cell bounded by two parallel polarizers. The twisted structure acts as a pi/2 polarization rotator so that the device is i nitially opaque, but becomes transparent if the twisted structure is b roken, due to Liquid crystal phase transition (e.g. from nematic to is otropic) or director configuration change(e.g. from twisted to homeotr opic). The first, thermal, effect is driven by the impinging beam prod ucing a nonlinear optical behaviour, while the second effect can be us ed to modify it. The result is a voltage controlled nonlinear thermoop tical effect that presents some interest both from a fundamental and a n applicative point of view. Experimental results are reported and dis cussed with a simple theoretical model.