Subterahertz self-oscillations in ultrafast self-modulation of optical absorption in GaAs

Ultrafast self-modulation of the optical absorption spectrum occurs during intense picosecond optical pumping of GaAs. It is found that the ultrafast self-modulation of the absorption of light with a fixed photon energy ħω manifests itself as self-oscillations consisting of two subterahertz harmonics (of which the second one is at the doubled frequency) or a single subterahertz harmonic. The dependences of the parameters of selfoscillations on ħω at different energies of the pumping pulse are determined. An explanation is given for the previously observed cyclic replication of the shape of ultrafast self-modulation of the absorption spectra. It is suggested that, first, self-modulation of optical absorption is due to changes in the populations of energy levels with charge carriers and, second, the self-modulation is associated with the self-modulation of the intrinsic stimulated picosecond emission from GaAs. On these assumptions, the above-mentioned dependences can serve to gain an insight into the nature of subterahertz self-oscillations for the population depletion and the emission.