Picosecond dynamics of high-density laser-induced transient plasma gratings in germanium

Abstract

A variation of the excite-and-probe technique is used to measure the picosecond evolution of laser-induced transient gratings that are produced in germanium by the direct absorption of 35-psec optical pulses at 1.06 \ensuremath{\mu}m. Grating lifetimes are determined for peak optical excitation levels between 1.8 mJ/${\mathrm{cm}}^{2}$ (\ensuremath{\sim} 50 MW/${\mathrm{cm}}^{2}$) and 36 mJ/${\mathrm{cm}}^{2}$ (\ensuremath{\sim} 1 GW/${\mathrm{cm}}^{2}$), for four grating spacings, and for two sample temperatures, 295 and 135 K. For the lower fluence, a linear diffusion-recombination model for the grating decay provides a good fit to the experimental data and allows the extraction of the diffusion coefficient and an estimation of the linear recombination lifetime. For the higher excitation level, the usual degenerate (and therefore density-dependent) expression for the diffusion coefficient and nonlinear (Auger) recombination processes are included. Comparison of the numerical solutions to the resulting nonlinear partial differential equation to experiment indicates that the commonly used degenerate version of the diffusion coefficient is not sufficient to account for the density dependence of the grating decay and that other nonlinear diffusion mechanisms must be considered.