Coherent dynamics of continuum and exciton states studied by spectrally resolved fs four-wave mixing

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Abstract
The coherent dynamics of continuum states and their interplay with excitons at the direct gap of germanium (T=1.8 K) is studied by femtosecond four-wave mixing. In order to unambiguously distinguish between the contribution of continuum states and excitons, we spectrally resolve the diffracted wave as a function of time delay for different excitation photon energies. For excitation many binding energies above the direct gap, the spectra exhibit two clearly separated maxima, one narrow line at the exciton and a much broader contribution of the continuum states. Surprisingly, the continuum contribution persists to longer time delays than the exciton. The experimental results are explained in the framework of the semiconductor Bloch equations.