R. Woscholski

Giant dynamical Stark shift in germanium quantum wells

We report a strong dynamical (ac) Stark shift of the direct gap transitions in Ge quantum wells at both cryogenic and room temperature. A blueshift of 67 meV is observed, exceeding values reported for III-V materials by about an order of magnitude. The fast intervalley scattering in the Ge material system leads to short dephasing times which in return causes larger shifts.

Controlling the polarization dynamics by strong THz fields in photoexcited germanium quantum wells

The interaction of strong single-cycle THz pulses with the optically induced polarization in germanium quantum wells is studied. With increasing THz field strength, it is observed that the excitonic resonances shift toward higher energy and broaden before weak signatures of a splitting of the exciton line occur. In comparison with high-quality GaAs-based quantum wells, where a much clearer Autler–Townes splitting is observed, the germanium system response is significantly more broadened and shows signatures of a quasi-steady-state behavior due to the intrinsic fast dephasing times dominated by intervalley scattering.

Influence of growth temperature and disorder on spectral and temporal properties of Ga(NAsP) heterostructures

We have studied the optical properties of Ga(NAsP)-heterostructures, which were systematically grown at different temperatures by means of continuous-wave and time-resolved photoluminescence. We show that both the long ranged and the short ranged disorder scales increase for higher growth temperatures. Furthermore, samples with a higher disorder not only emit less photoluminescence (PL) intensity but also exhibit a longer effective PL decay time.

Relaxation and recombination processes in Ge/SiGe multiple quantum wells

The carrier dynamics that occurs in Ge/SiGe QWs when electrons are excited to confined states at Γ is studied by means of optical spectroscopy at different lattice temperatures. The typical times for the different relaxation and recombination processes are given and discussed.

Ultra-fast inter-subband relaxation and non-thermal carrier distribution in Ge/SiGe quantum wells

A systematic study of carrier relaxation in Ge quantum wells is performed applying photoluminescence and pump-probe spectroscopy. Intersubband-relaxation on a 100 fs time-scale and the presence of a non-thermal carrier distribution are identified.

Giant AC Stark shift in germanium

We observe a strong ultrafast AC Stark shift of the direct band transition in strained germanium quantum wells grown on silicon. At 150 meV, the maximum measured blue shift of the band edge is one order of magnitude larger than typically found in III-V materials. The power dependence shows a linear behavior between the electrical field intensity and the magnitude of the shift which is in good agreement with a dressed-exciton model. The coherent excitation dynamics in germanium are thus mainly governed by the direct transitions.