Nanosecond response and "coasting" in photorefractive barium titanate

A variety of nonlinear optical devices and effects have been demonstrated in photorefractive crystals of barium titanate, whose speed is known to increase with optical intensity. We present results of experiments to measure the scaling of the response time of barium titanate with optical intensity, with the time response varying over the range of seconds to nanoseconds. We measured the erasure rate of a photorefractive grating in two BaTiO3 samples using either a continuous-wave dye laser or 20-nanosecond pulses from an excimer-pumped dye laser, both at λ= 579 nm. The erasure efficiency was found to vary linearly with intensity I in one sample and as I0.86 in the other, using optical intensities as small as 10-4 Watts/cm2 and as large as 10+5 Watts/cm2. An unexpected result is that in both BaTiO3 samples the erasure efficiency with the pulsed laser decreased by ∼30% as the optical pulse rate was increased from 0.1 Hz to 15 Hz, and remained constant for higher repetition rates, up to the maximum of 50 Hz. We also observe that, after partially erasing a grating with a millisecond duration pulse from a low-power continuous-wave argon laser, erasure of the photorefractive grating continued for ∼100 msec after the optical pulse ended. We believe that this "coasting" of grating erasure is responsible for the dependence of the grating erasing efficiency on the repetition rate of the high-intensity pulsed laser, as described above. We present a simple band model to explain our experimental results. This model includes shallow traps and also the contributions of both electrons and holes to the formation of the photorefractive grating. We also discuss how "coasting" affects grating formation when using nanosecond optical pulses.