Christian Waschke

Radiative decay of optically excited coherent plasmons in a two-dimensional electron gas

We report on the observation of coherent submillimeter-wave emission from optically excited plasmons in a two-dimensional electron gas. Phase-synchronous plasma oscillations are induced by femtosecond optical pulses generating electron–hole pairs in the accumulation channel of an AlGaAs/GaAs heterostructure. The radiative decay of the grating-coupled plasmons is traced in time by terahertz-emission spectroscopy. An analysis of the data suggests that ultrafast thermalization and current surges are the plasmon-driving mechanism.

Efficiency of submillimeter-wave generation and amplification by coherent wave-packet oscillations in semiconductor structures

We discuss the emission of submillimeter waves from optically excited coherent wave-packet oscillations in quantum-well structures in terms of the emitted power. The material dynamics are treated in a two-band real-space density-matrix approach; the radiation process, by conventional antenna theory. Excitation geometries that assist cooperative radiation from all points of the excited spot are proposed. We consider amplification by phase-locked feedback of the terahertz wave to the emitter in a resonator. Our results lead to concepts for optimized power extraction from coherent terahertz sources.

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.

THz-wave emission by coherent optical phonons

Abstract Coherent optical phonons are excited in single-crystal Tellurium by above-band-gap excitation with femtosecond laser pulses. Coherent Raman-active modes are resolved in amplitude and phase by monitoring the time-resolved reflectivity changes in a pump-probe experiment. By detecting the far-infrared radiation emitted from the crystal, we observe for the first time coherent THz radiation from infrared-active lattice modes. This technique opens a new way for the spectroscopy of lattice dynamics. We discuss details of the generation and detection mechanism of the coherent lattice modes. We show that the driving force for the coherent infrared-active modes is a diffusive process, i.e. the ultrafast build-up of a photo-Dember field.