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Infrared absorption spectra of photoexcited holes in undoped GaAs.

The infrared absorption of free holes in undoped GaAs is studied. An electron-hole plasma generated by picosecond two-photon excitation gives rise to strong absorption bands in the wavelength range from 4 to 12 μm, which are monitored by delayed picosecond probe pulses. The spectra are dominated by direct and indirect inter-valence-band transitions of holes whereas the contribution of electrons to the overall absorption is small