Dynamic director grating and two beam energy exchange in photorefractive hybrid cholesteric cell

Abstract

We develop a theoretical model to describe two-beam energy exchange in a hybrid photorefractive inorganic-cholesteric cell. Weak and strong light beams are incident on the cell and induce a periodic space-charge field in the photorefractive windows. This field penetrates into the LC, inducing a diffraction grating written on the LC director. Flexoelectricity is the principal physical mechanism governing the magnitude of the director grating. We calculated the energy gain of the weak beam as a result of its interaction with the pump beam within the diffraction grating. The calculation includes rotation of the light beam polarization in the cholesteric LC and the spectral position of its gap. Theoretical results for the exponential gain coefficient have been compared with experiment for hybrid cells filled with cholesteric LC BL038/CB15 in the single photorefractive window geometry. A good fit between experiment and theory can be achieved, providing good evidence that the key physics of the system has been correctly identified.