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.

Additional Metadata
Publisher APS
Persistent URL dx.doi.org/10.1103/physrevb.97.205202
Journal Phys.Rev.B
Kaplan, C.J, Kraus, P.M, Ross, A.D, Zürch, M, Cushing, S.K, Jager, M.F, … Leone, S.R. (2018). Femtosecond tracking of carrier relaxation in germanium with extreme ultraviolet transient reflectivity. Phys.Rev.B, 97(20), 205202: 1–205202: 9. doi:10.1103/physrevb.97.205202