Abstract
We report on the simultaneous performance of four-wave-mixing1 (FWM) and transmittive electro-optic-sampling2 (TEOS) experiments to investigate Bloch oscillations in biased GaAs/AlxGa1-xAs superlattices. These techniques allow a comparative study of the coherent dynamics of the inter- and the intraband polarization, because they can be performed in exactly the same experimental arrangement. FWM signals are well known to have their strongest contributions from excitonic resonances at low excitation densities.4 The contribution of continuum states can be neglected because of the strong dephasing of their interband coherence, which is typically below 100 fs in GaAs quantum wells and bulk GaAs, even at 10 K. In contrast to FWM, TEOS enables us to monitor directly the dynamics of the intraband dipole moment induced by the spatial motion of coherent electronic wave packets in amplitude and phase.2 Thus, in TEOS experiments we directly measure the source of coherent terahertz radiation emitted by Bloch oscillations.3 A careful comparison of TEOS and FWM data under exactly the same excitation conditions reveals significant differences in the frequency and dephasing of Bloch oscillations. These specific differences show that Bloch oscillations observed in FWM are based on excitonic quantum beats, whereas Bloch oscillations observed in TEOS are mainly performed by electrons in continuum states. The observation of Bloch oscillations performed by continuum states is of prime importance for the realization of electronically driven Bloch oscillations and of Bloch oscillations at room temperature. The results reported here are supported by recent results in terahertz emission experiments, in which Bloch oscillations were observed for excitation well above the lowest optical transitions in the superlattice.5
© 1995 Optical Society of America
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