Abstract

Following Valley’s work,¹ the theoretical expression for the space-charge field’s (E_sc) buildup is derived for both cw and picosecond-pulsed low-irradiance excitation, I ≫ I_sat, where I_sat is the intensity at which the number of excited charges saturates. The cw and picosecond-pulsed expressions for the time-dependent E_sc both reduce to where m is the modulation index of the grating and τ_di is the dielectric relaxation time. An explanation of how saturation affects the growth of the space-charge field for cw vs picosecond- pulsed excitation is presented. The differential equation for the space-charge field’s time dependence is solved numerically and compared to the quasi-cw result. Theoretically, saturation effects become noticeable for an input intensity of 10⁻³I_sat. For 10 kW/cm² peak intensity, 22.6-ps pulses, no evidence of saturation is observed experimentally for cerium-doped Sr_{0 6}Ba₀4Nb₂O₆ (SBN). The rise times are com parable for pulsed and cw inputs of the same average intensity, over a range of average intensities. This provides a lower limit on the saturation intensity of 10 MW/cm². An SPPC signal is observed in rhodium-doped SBN in the first 150 pulses (3.69-ps pulsewidth) of illumination yielding an early grating formation time of no longer than 554 ps.

© 1991 Optical Society of America