We directly detect the coherent electromagnetic radiation originating from Bloch oscillations of charge carriers in an electrically biased semiconductor superlattice structure. The oscillation frequency can be tuned with the applied bias field from 0.5 THz to more than 2 THz, the detection limit of our measurement system

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The concentration p and the mobility mu of holes in metal-organic chemical vapor deposition (MOCVD) GaN:Mg layers were studied by room temperature Hall-effect measurements as a function of the Mg concentration N(A) in the range 3 x 10(exp 18) cm(exp-3) <= N(A) <= 1 x 10(exp 20) cm(exp -3). The hole density first increases with increasing N(A), reaches a maximum value p(max)~6*10(exp 17) cm(exp -3) at N(A)~2*10(exp 19) cm(exp - 3), decreases for larger N(A) values, and drops to very small values at N(A) 1 x 10(exp 20) cm(exp -3). The hole mobility decreases monotonically with increasing N(A) . The p(N(A)) data provide strong evidence for self-compensation, i.e., for a doping driven compensation of the Mg acceptor by intrinsic donor defects. This effect becomes significant when N(A) exceeds a value of 2 x 10(exp 19) cm(exp -3). A semiquantitative self-compensation model involving nitrogen vacancies is developed. It accounts satisfactorily for the measured p(N(A)) dependence and suggests that self-compensation limits the hole conductivity in bulklike MOCVD GaN:Mg layers grown near 1300 K to about 1.2 (omega cm)(exp -1))

Coherent zone-folded acoustic phonons are excited in GaAs͞AlAs superlattices by femtosecond laser pulses via resonant impulsive stimulated Raman scattering in both forward and backward scattering directions. The relative amplitudes of three distinct modes of first and second backfolded order match well with scattering intensities calculated within an elastic continuum model. The detection of the coherent acoustic modes is based on the modulation of the interband transitions via the acoustic deformation potential and exhibits a strong enhancement at interband transitions. [S0031-9007(98)08288-X] PACS numbers: 78.66.Fd, 63.22. + m, 78.47. + p The investigation of low-energy elementary excitations in semiconductor heterostructures is driven by their relevance as the final state in the energy relaxation process. In particular, acoustic phonons play a dominant role for dephasing processes at low lattice temperature and for heat transport in general. Acoustic phonons are commonly investigated by continuous wave (cw) Brillouin scattering. Time resolved experiments based on ultrashort pulse lasers have largely contributed to the understanding of acoustic phonon dynamics. Recently, the generation and propagation of ballistic acoustic phonons in a single semiconductor quantum well was observed by surface deflection spectroscopy [1]. Semiconductor superlattices exhibit zone folding of the acoustic branches within the mini-Brillouinzone (mini-BZ) due to the artificial periodicity of the elastic properties along the growth direction. Here, light can couple to zone-folded acoustic modes of the superlattice at higher frequencies in the 100 GHz to THz range. These modes have been extensively studied in cw Raman spectroscopy [2][3][4]. The folded bulk acoustic branches are optical branches within the superlattice zone scheme; thus light scattering from those modes is referred to as Raman scattering. The coherent excitation of a single first-order zone-folded mode was observed in GaAs͞AlAs superlattices using a time-derivative detection scheme [5]. However, the excitation and detection mechanisms are not yet fully clarified. In this paper we report on the nature of the excitation and detection mechanisms relevant for coherent zone-folded acoustic vibrations of first and second order in GaAs͞AlAs superlattices.Most time resolved experiments on coherent lattice excitations dealt with longitudinal optical phonons [6] or phonon polaritons [7]. While the excitation of phonon polaritons relies on impulsive stimulated Raman scattering (ISRS) [8], in the case of longitudinal optical phonons, a variety of excitation processes have been identified which cannot be explained within the context of stimulated Raman scattering. The most prominent non-Raman type mechanisms are the displacive excitation of coherent phonons relevant for symmetry maintaining optical modes [9] and the generation of coherent LO phonons via rapid surface field screening in polar semiconductors [10]. One important hint towards the determination of the excitation pro...

We directly observe the electromagnetic radiation emitted by electrons coherently oscillating between the two wells of a semiconductor coupled-quantum- well structure. Using time-resolved coherent detection of the submillimeter-wave radiation from these spatial charge oscillations, we trace up to fourteen oscillations at 1.5 THz before phase relaxation destroys the coherence of the oscillating wave packet. In addition to the oscillatory electromagnetic signal, we also observe an instantaneous signal from electricfield-induced optical rectification in the semiconductor structure

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We present the first observation of the spatial dynamics of a wavepacket in a solid. Using an ultrashort laser pulse, we create an excitonic wavepacket in one well of an asymmetric double quantum well structure. The oscillation of this wavepacket from one well to another and back is traced by time- resolved pump-probe spectroscopy as well as time- resolved degenerate fourwave-mixing. We present results for two GaAs/AlGaAs double quantum wells with oscillation periods of about 1.3ps and 800fs, respectively. The experimental observations are compared with a theory that shows that the two experimental techniques give complementary information about the relaxation dynamics of the coupled system. The analysis of the experiment explains the strong damping of the oscillations by the fast thermalization between the delocalized states. We present the first study of the dynamics of an extended electronic wave packet in a solid. The wave packet is created in a GaAs/AlGaAs double-quantum-well stru cture by ultrashort pulse excitation. We observe the oscillatory motion of the wave packet between the two wells by using time-resolved degenerate four-wave-mixing and pump-and-probe spectroscopy