Abstract
We have measured the temporal evolution oil the absorption coefficient around the band edge on a thin GaAs film at room temperature by femtosecond pump-probe techniques. For earner densities larger 3×1015 cm−3 injected at 2 eV, optical gain is observed in a wide spectral region (from 850 to 900 ran) on a subpicosecond time scale (Fig. 1). We have compared the measured time delays for gain to occur with a kinetic model1 and concluded that the origin of the subpicosecond gain is (i) the nearly instantaneous equilibration of the carrier distributions near the Γ point of the Brillouin stone,2 and (ii) the very efficient cooling mechanism for Γ-valley electrons by preferential scattering of high-energy electrons to the X and L valleys.
© 1992 Optical Society of America
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