Femtosecond tracking of carrier relaxation in germanium with extreme ultraviolet transient reflectivity

Publication date
DOI http://dx.doi.org/10.1103/physrevb.97.205202
Reference C.J. Kaplan, P.M. Kraus, A.D. Ross, M. Zürch, S.K. Cushing, M.F. Jager, H.-T. Chang, E.M. Gullikson, D.M. Neumark and S.R. Leone, Femtosecond tracking of carrier relaxation in germanium with extreme ultraviolet transient reflectivity, Phys. Rev. B 97, (20), 205202: 1-9 (2018)
Group High-Harmonic generation and EUV science

Extreme ultraviolet (XUV) transient reflectivity around the germanium M4,5 edge (3d core-level to valence transition) at 30 eV is advanced to obtain the transient dielectric function of crystalline germanium [100] on femtosecond to picosecond time scales following photoexcitation by broadband visible-to-infrared (VIS/NIR) pulses. By fitting the transient dielectric function, carrier-phonon induced relaxations are extracted for the excited carrier distribution. The measurements reveal a hot electron relaxation rate of 3.2±0.2ps attributed to the X−L intervalley scattering and a hot hole relaxation rate of 600±300fs ascribed to intravalley scattering within the heavy hole (HH) band, both in good agreement with previous work. An overall energy shift of the XUV dielectric function is assigned to a thermally induced band gap shrinkage by formation of acoustic phonons, which is observed to be on a timescale of 4–5 ps, in agreement with previously measured optical phonon lifetimes. The results reveal that the transient reflectivity signal at an angle of 66∘ with respect to the surface normal is dominated by changes to the real part of the dielectric function, due to the near critical angle of incidence of the experiment (66∘–70∘) for the range of XUV energies used. This work provides a methodology for interpreting XUV transient reflectivity near core-level transitions, and it demonstrates the power of the XUV spectral region for measuring ultrafast excitation dynamics in solids.