Abstract
We study, both theoretically and experimentally, the variations of the absorption coefficient of bulk GaAs in the presence of a nonthermalized electron-hole plasma. We investigate the changes of the Coulomb enhancement factor (CEF) due to nonthermal carrier distributions. For comparison with experimental data, time-dependent nonlinear absorption spectra are calculated using carrier distributions obtained from the numerical solution of Boltzmann's equations. Experimental absorption spectra are derived from differential transmission and reflectivity measurements of GaAs thin films in the subpicosecond regime. Both experimental and theoretical spectra show a redshift of the transient-hole-burning peak that can be attributed to the spectral dependence of the CEF changes. An induced absorption above the spectral hole is predicted for narrow distributions. This effect, which critically depends on the width of the carrier distributions, is strongly suppressed by fast collisional broadening in accordance with the experimental results.
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