Momentum-dependent intraband high harmonic generation in a photodoped indirect semiconductor

Abstract

Nonlinear optical response of solid-state materials exposed to strong non-resonant light fields leads to the generation of harmonic frequencies as a consequence of interband polarization and coherent intraband dynamics of the electrons. The efficient production of a macroscopic wave requires the preservation of the mutual phase between the driving wave and the individual microscopic sources of radiation. Here, we experimentally and theoretically show that the yield of high harmonic generation in a photodoped silicon crystal is enhanced by the nonlinear intraband current whose amplitude depends not only on the volume density of the photogenerated carriers but also on their momentum distributions within the bands. The strongest enhancement is reached when the carrier system is relaxed to the band minima before interacting with the strong nonresonant wave, which drives the high harmonic generation. These results extend the possibilities of high harmonic spectroscopy towards the investigation of ultrafast carrier relaxation in condensed matter. High Harmonic Generation in solids differs with respect to its atomic counterpart due to additional intraband harmonic emissions and to the presence of momentum scattering. The authors demonstrate that the additional intraband contribution depends on the momentum distribution of photodoped carriers when the driving pulse arrives to the sample.