Large Signal Gain Effects in Photorefractive Bi12TiO20 at 633 nm

Bi12TiO20 (BTO) is a photorefractive material in the same structural class (sillenite) as Bi12SiO20 (BSO) and Bi12GiO20 (BGO). However, BTO offers some unique advantages over BSO and BGO: (1) larger electro-optic coefficient (5.7 pm/V)1, and (2) lower optical activity (6°/mm at 633 nm)1, 2. Previous photorefractive measurements3,4 have shown that gain coefficients on the order of 10-15 cm−1 can be produced through the use of an applied AC field. In this work we show that the largest gain values can only be obtained for large values of the pump/probe intensity ratio β. As β approaches unity (large signal regime), higher spatial order gratings become prominent, and the gain is reduced from its large-β value.5−8 Our results are similar to those obtained by other researchers for BSO5 and GaAs6 with an applied field. We have analyzed this and related phenomena using a finite difference method to model the photorefractive grating formation. This method yields accurate numerical solutions which are valid for for all values of β.