Abstract

In recent years a great deal of interest has been addressed to the study of space-charge induced effects because of the foreseen possibility of enhancing the nonlinear optical response of liquid crystals (LCs). In fact, photorefractive and photovoltaic effects play a crucial role in determining the nonlinear refractive index due to director reorientation. The photovoltaic effect in LCs was reported for the first time several years ago [1]. However, no further investigation was carried out since then on this subject except for the recent contribution of Khoo et al. [2] who reported generation of photovoltage across a homeotropically LC cell doped with Methyl Red (MR) showing an extraordinarily large optical nonlinearity. In the present contribution we report a detailed study of the transient behavior of the photovoltaic effect detected in the same kind of cell, using both c.w. (the λ =488 nm line of an Ar ion laser ) and pulsed light (the second harmonic λ =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 (1% in weight) of MR. The most relevant features experimentally observed are: (a) the change of polarity of the photoinduced electromotive force while reaching the steady-state value under c.w. 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.

© 2000 IEEE