Sub-millisecond photorefractive two-wave coupling in Bi12TiO20 at 633 nm

The unusually fast response of the two-wave mixing process in fibre-like Bi 12TiO20 crystals on transient change of the external electric field was exp erimentally observed. Pulses of the amplification of the signal wave with a rise time of 0.2ms and a width of 0.5ms at a total input intensity of 6mWm m(-2) are obtained when applying a rising or decreasing front of an externa l electric field to the crystal. The two-wave coupling without external fie ld has a response time of 200 ms. A simple calculation based on the band-tr ansport model qualitatively explains the fast transient amplification but f ails to explain the slow response without the electric field.