P. Leisching

Detection of Bloch oscillations in a semiconductor superlattice by time-resolved terahertz spectroscopy and degenerate four-wave mixing

Abstract We have studied the spatial dynamics of coherently excited wave packets of charge carriers in a semiconductor superlattice structure by both time-resolved THz spectroscopy and degenerate four-wave mixing. In the Wannier-Stark bias regime, we observe electron Bloch oscillations with a frequency that depends linearly on the electric bias field and can be tuned between a few hundred GHz and 5 THz.

Terahertz Bloch oscillations in semiconductor superlattices

The coherent excitation of excitonic wavepackets in AlGaAs/GaAs superlattices is investigated in a comparative study employing three different time-resolved optical techniques. The experimental results obtained in time-resolved four-wave mixing (PWM), THz emission spectroscopy and transmissive electro-optic sampling (TEOS) provide the final proof of the existence of Bloch oscillations in the THz range. Each of these measurements provides complementary information on these oscillations. As a result, Bloch oscillations appear as a special case of quantum interference involving the coherent superposition of Wannier-Stark states.

Investigation of Bloch oscillations in a GaAs/AlGaAs superlattice by spectrally resolved four-wave mixing

We study Bloch oscillations (BO) in GaAs/AlGaAs superlattices with miniband widths of 19 and 38 meV at various lattice temperatures. In the time domain, we use transient degenerate four-wave mixing (FWM), while simultaneously monitoring the Wannier-Stark ladder (WSL) in the frequency domain by transmission spectroscopy. Spectrally resolved FWM measurements prove unambiguously that the oscillations in the decay of the FWM signal are due to quantum interference as expected for a wavepacket performing the BO.

Dephasing and selection rules of interband and intraband polarizations in superlattices

We study the coherent dynamics of interband and intraband polarizations of Bloch oscillations in a GaAs/Al0.7Ga0.3 As semiconductor superlattice. Time-resolved measurements of both polarizations are simultaneously performed by combination of four-wave mixing and an electro-optic detection scheme in one experimental setup. This combination allows us to gain a much more comprehensive picture of the dynamics of a multilevel system. Distinctive differences in the oscillation frequencies and the dephasing of both polarizations are found. They arise from the fact that excitonic quantum beats are observed in four-wave mixing, whereas a quantum interference of electrons in continuum states is detected in the electro-optic signal. As a consequence, the polarizations are governed by different selection rules that explain the pronounced differences in the oscillation frequencies found for specific experimental conditions.