Impact of photo-transformed molecules on two-beam energy exchange in hybrid photorefractive cholesteric cells

We develop a theory describing two-beam energy exchange in a hybrid photorefractive cholesteric cell with photosensitive molecules. A cholesteric liquid crystal (LC) layer is placed between two inorganic substrates. One of the substrates is photorefractive. Weak and strong light beams are incident on the hybrid cell. The interfering light beams induce both a periodic space-charge field in the photorefractive substrate and the photo-transformed molecules (PM) in the LC layer. The PM have the helical twisting power different from that of the initial molecules. The space-charge field penetrates into the cholesteric LC slab and modulates the director interacting with the LC flexopolarization. The PM are periodically distributed in the cell and modulate the cholesteric pitch. The periodic director modulation (director grating) arising in the cell is a sum of two in-phase gratings, the flexoelectric effect driven grating and the PM-driven grating. The director grating gives rise to the dielectric permittivity grating. Each light beam diffracts from the induced permittivity grating leading to an energy exchange between the beams. We calculate the signal beam gain coefficient and analyze its dependence on the PM parameters and concentration. We show that doping the LC with photosensitive molecules provides a control of the gain of the two-beam energy exchange in the hybrid cholesteric cell.