Noninvasive imaging with thin dye-doped liquid crystal films

Abstract

We discuss several imaging techniques that have been studied for optical imaging through turbid media. The effectiveness of many of these techniques hinges on the nonlinear optical material used in the imaging process. We have used Rhodamine dye-doped K15 liquid crystal films in a degenerate four-wave mixing geometry in order to study the potential of this material for low-coherence imaging applications. First, we observed the dc-field-induced Freedericksz transition, which leads to a reorientational nonlinearity in our nematic liquid crystal film, and is responsible for the large optical nonlinearity that leads to phase conjugation. For our low absorbing films, this threshold voltage was on the order of 400 V/cm. Next, we show that our liquid crystal films exhibited higher phase conjugate reflectivities (0.6% maximum) for smaller beam crossing angles, which is necessary for wider field-of-views in the imaging process. Finally, we studied the time-dependent response of the phase conjugate reflectivity due to electric field assisted nonlinearities in dye-doped liquid crystal films. The motivation of these studies is to improve the imaging resolution demonstrated in our previous work.