Direct monitoring of the excited state population in biased SiGe valence band quantum wells by femtosecond resolved photocurrent experiments

The authors report a direct measurement of the optical phonon intersubband hole relaxation time in a SiGe heterostructure and a quantitative determination of hole relaxation under electrically active conditions. The results were obtained by femtosecond resolved pump-pump photocurrent experiments using a free electron laser (wavelength 7.9 μm). Additionally, the intensity dependence of the nonlinear photocurrent response was measured. Both types of experiments were simulated using a density matrix description. With one parameter set, a consistent modeling was achieved confirming the significance of the extracted heavy hole relaxation times. For an intersublevel spacing of 160 meV, a value of 550 fs was obtained.