Bloch oscillations of excitonic wave packets in semiconductor superlattices

Abstract

Vfe present a detailed investigation of the coherent dynamics of excitonic wave packets composed of heavy/light-hole, electron miniband, and Wannier-Stark states in GaAs/Al Gai As superlattices. Using transient degenerate four-wave mixing, we study the dependence of Bloch oscillations and heavy/light-hole beats on the applied field, miniband width, lattice temperature, and excitation conditions. Bloch oscillations are observed in samples with minibandwidths varying from 13 to 46 meV and at lattice temperatures up to 200 K. Under certain excitation conditions, we observe higher harmonics of the Bloch oscillation frequency. Spectrally resolved transient four-wave mixing experiments show unambiguously that quantum-mechanical interference is the origin of the oscillations. The experimental four-wave mixing traces are compared with a theoretical model based on many-level third-order density-matrix theory.

Dephasing effects induced by weak disorder in superlattices

“…This can be understood by noting that the amplitude of BO's is proportional to the minibandwidth: Thus, the coherent motion of electrons takes place in a smaller region when decreasing the mini band-width and, therefore, carriers are less influence by disorder. This result is in very good agreement with the experimental observations [12].…”

“…This fact can be explained by the absence of translational invariance at flatban and, consequently, by scattering of electrons with the random potential. Similar results are obtained with the SL's reported by Leisching et al [12]. We have to note in this respect that for those samples we have observed a slower decay of BO's, the slowest evolution corresponding to the SL with the narrowest miniband.…”

“…They explain this dependence by assuming that LO phonon emission LO is excluded when D , E . On the contrary, Leisching et al [12] detected up to six BO's but they did LO not observe any sign of a phonon threshold in SL's with D ranging 13-46 meV. These authors argued that the reduced sample quality of Ref.…”

“…We have considered the same parameters as those of the SL's used in previous experiments [11,12]. In particular, we present here results for the firs one of these SL's, i.e., 100 periods of 30 GaAs and 30 Ga Al As [11] In this case, the conduction-band offset is D E 50.25 eV, and the effective mass , although we do not present here these results for brevity.…”

“…This periodic motion persists until the Bloch electron loses energy gained from the fiel through scattering processes. Reports of unambiguous experimental evidences for BO's in GaAsGa Al As are presently available [8][9][10][11][12]. Inelastic scattering by phonons, deviations from SL's perfect periodicity due to unintentional imperfections, intraband scattering, interminiband transitions, and scattering by impurities severely reduce the quantum coherence required for the observation of BO's.…”