N. S. Köster

Clustering effects in Ga(AsBi)

The photoluminescence from a Ga(AsBi) sample is investigated as a function of pump power and lattice temperature. The disorder-related features are analyzed using a Monte Carlo simulation technique. A two-scale approach is introduced to separately account for cluster localization and alloy disorder effects. The corresponding characteristic energy scales of 11 and 45 meV are deduced from the detailed comparison between experiment and simulation.

Luminescence dynamics in Ga(AsBi)

The temporal evolution of the spectrally resolved luminescence is measured for a Ga(AsBi) sample at low temperatures. The results are analyzed with the help of kinetic Monte Carlo simulations incorporating two disorder scales attributed to alloy disorder and Bi- clustering. An average time of 5 ps is identified as the upper limit for carrier capture into the Bi clusters whereas the extracted hopping rate associated with alloy fluctuations is much faster than the transitions between the individual cluster sites.

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.

Transient optical gain and carrier dynamics in Ge/SiGe quantum wells

We perform ultrafast pump-probe experiments on a 50 period Ge/SiGe multiple-quantum-well structure held at room temperature. Tunable 80 fs pulses emitted by an opto-parametric amplifier are used as a pump and a white-light supercontinuum generated directly from a 1 kHz Ti:sapphire regenerative amplifier system is used as a probe. The resulting spectro-temporal response shows three distinct temporal regimes. Coherent oscillations dominate at negative times yielding a well-defined time zero across the whole detected spectral range. Dynamics are observed within the direct conduction band valley during and shortly after the excitation while the electrons are also scattered towards the indirect minima. After several hundreds of fs to a few ps almost all electrons populate the L-valley states. These carriers decay out of the L-valleys on a timescale longer than several ns. During the first ps, carrier inversion is obtained for strong enough pumping due to faster intra-valley than intervalley scattering. The obtained gain values are similar in magnitude to those observed in typical III-V compound semiconductors.

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.

Ultrafast electron-hole scattering monitored by hole cooling in optically excited Germanium quantum wells

The scattering and cooling dynamics in Ge quantum wells are investigated on a picosecond time scale. Time-resolved pump-probe experiments reveal an efficient scattering process between electrons in the L-valley and holes in the Γ-valley.

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.

Carrier thermalization in Ge quantum wells

Ultrafast carrier relaxation of Ge/SiGe quantum wells grown on a Si substrate are investigated using pump-probe spectroscopy. A nonthermal carrier distribution is observed. The corresponding inter- and intra-valley scattering times are deduced.