Enhancement of the optical nonlinearity in liquid crystals through the photovoltaic effect

Summary form only given. We report a detailed study of the transient behavior of the photovoltaic effect detected using both CW (/spl lambda/=488 nm line of an Ar ion laser) and pulsed light (second harmonic /spl lambda/=532 nm of a Q-switched Nd:YAG laser) excitation. The experiments were performed on sandwich-like cells consisting of two parallel plane ITO-coated glass substrates, filled with a mixture of the nematic 5CB LC and a small amount of Methyl Red. The most relevant features experimentally observed are: (a) the change of polarity of the photoinduced electromotive force while reaching the steady-state value under CW irradiation; (b) the generation of a transient photovoltage induced by a single 4 ns laser pulse with moderate energy density. The results obtained can be interpreted in the frame of a model based on the coexistence of a fast and a slow process in the origin of the photovoltaic effect. The former includes the exchange of charges between the stimulated electrode and the solution, and the latter includes the diffusion process of photoinduced charges with different mobility between the positive and the negative carriers. The observed transient photovoltage due to pulsed irradiation allows writing transient holographic gratings with a single 4 ns pulse. The energy density required for the grating formation is below the threshold value for the optical Freedericksz transition in the investigated samples, thus indicating that the light-induced photovoltage enhances the nonlinear response of the cells.