Attosecond quantum kinetics of photoexcited Germanium

Abstract

The electronic motion in a semiconductor after light absorption is the central aspect of modern opto-electronics. However, a real-time observation of the initial electronic response following single-photon excitation has so far defied exptl. approaches due to its extreme time and energy scales. Here, attosecond transient reflectivity is developed to measure the attosecond time-resolved dielec. function in the extreme UV following carrier promotion by visible-to-IR sub- 5 fs (fs) pulses from the valence into the conduction band in germanium. The buildup of holes and electrons in the valance and conduction band is monitored on attosecond timescales by the change in the reflection at the M_(4,5) edge (30 eV). The electron and hole features are found to exhibit a 1.4 fs oscillation, which is indicative of a field-induced polarization of the bands. The measurement of the attosecond dielec. function further enables the buildup of screening of the core-hole potential due to the collective electronic motion in the valence and conduction band to be tracked. It is exptl. obsd. through the real part of the dielec. function that a bare, unscreened Coulomb potential is formed instantaneously after photoexcitation. A subsequent broadening of the real part of the dielec. function over a few fs is attributed to the screening of the Coulomb potential due to the collective electronic motion in the valence and conduction bands. Simultaneously, the imaginary part of the dielec. function tracks the buildup of new absorption channels. The expt. shows that two sharp features appear instantaneously due to the change in state-filling in the valence and conduction band, with an addnl. broadening occurring on a few fs time scale. This broadening is attributed to a response time of the collective electronic motion, which screens the Coulomb potential and changes the absorption due to a carrier redistribution. The time scale of the screening clocks in well with the inverse plasma frequency of the electron-hole plasma created by photoexcitation. Similar electronic responses after photoexcitation are anticipated to occur in all materials. The results presented here provide an exptl. measurement of the fundamental timescales of charged particle interactions in solids.