Ultrafast extreme-ultraviolet reflection spectroscopy of electro-phonon dynamics in Germanium

Ultrafast transient reflection spectroscopy in the extreme UV (XUV) is developed as a new technique for following bandgap dynamics after photoexcitation. Signatures of carrier and lattice dynamics are obsd. through the measurement of reflectivity changes in the XUV of single-cryst. germanium after exciting the sample with a sub-5 fs visible-to-IR pulse. In the expt., a 5 fs near IR (NIR) pulse excites carriers across the band gap. The subsequent dynamics are probed by measuring the transient reflectivity changes at the Ge M_(4,5) edge around 30 eV, corresponding to transitions from the 3d core levels to the unoccupied conduction and valence band states. The energetically sepd. spectral signatures of photoexcited electrons and holes in the XUV enable to measure the thermalization dynamics of the carriers. Importantly, shifts in photon energy of the electron and hole features allow real time tracking of the energy sepn. of valence and conduction bands change over time. The dynamics obsd. in the expts. suggest dynamics on different time scales. Within 500fs, decays and energetic shifts of the obsd. reflectivity changes occur, which are assigned to electron-electron and electron-optical phonon scattering thermalizing the photoexcited carrier distribution. Within 2 ps, a blue shift of the valence band features is obsd., whereas a red shift of the conduction band features is present. This striking observation is assigned to a renormalization of the band gap due to the lattice expansion of the crystal. This band-gap renormalization sets in once momentum of the excited carriers has been transferred to phonons by electron-phonon and phonon-phonon scattering. Transient reflectivity is shown to overcome the thin-film and x-ray membrane related issues of transmission geometries. It will be shown how transient reflectivity can further be employed to ext. the time-dependent complex dielec. function over the timescales governing carrier-carrier, carrier-phonon, and phonon-phonon interactions.