MAGNETIC-FIELD SUPPRESSION OF THZ CHARGE OSCILLATIONS IN A DOUBLE-QUANTUM-WELL

We have observed a suppression of the THz radiation emitted by charge oscillations photoexcited in the conduction band of GaAs/AlxGa1-xAs as ymmetric double quantum wells in magnetic fields of order 1 T. Coheren t charge oscillations are created by femtosecond excitation of a super position of two states between which there is an electric dipole momen t which oscillates in time as the wave packet evolves. The electric fi eld radiated by this intraband polarization is coherently detected usi ng an optically gated antenna The nature of the suppression is qualita tively different for the two cases of magnetic field B parallel and pe rpendicular to the plane of the quantum wells. For B parallel the pola rization dephasing rate increases with increasing field whilst the ini tial amplitude remains approximately constant. For B perpendicular the dephasing rate does not change significantly with increasing field bu t there is a reduction in the initial amplitude. The behavior in paral lel field can be understood in a semiclassical picture in which electr ons are deflected as they tunnel, thus destroying the phase coherence. The observation of amplitude suppression in a moderate perpendicular field may be partly associated with magnetic confinement but the effec t is much larger than expected. The effect of magnetic field on the ch arge oscillation frequency is also discussed. [S0163-1829(98)52216-3].