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 ${\mathrm{G}\mathrm{a}\mathrm{A}\mathrm{s}/\mathrm{A}\mathrm{l}}_x{\mathrm{Ga}}_{1-x}\mathrm{As}$ asymmetric double quantum wells in magnetic fields of order 1 T. Coherent charge oscillations are created by femtosecond excitation of a superposition of two states between which there is an electric dipole moment which oscillates in time as the wave packet evolves. The electric field radiated by this intraband polarization is coherently detected using an optically gated antenna. The nature of the suppression is qualitatively different for the two cases of magnetic field $B$ parallel and perpendicular to the plane of the quantum wells. For $B$ parallel the polarization dephasing rate increases with increasing field whilst the initial amplitude remains approximately constant. For $B$ perpendicular the dephasing rate does not change significantly with increasing field but there is a reduction in the initial amplitude. The behavior in parallel field can be understood in a semiclassical picture in which electrons 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 effect is much larger than expected. The effect of magnetic field on the charge oscillation frequency is also discussed.