Abstract
We have studied the time varying macroscopic polarisation associated with charge oscillations in GaAs/AlGaAs asymmetric double quantum wells by coherent detection of the emitted THz radiation. The charge oscillations are optically excited with femtosecond near infrared radiation and the radiated field is detected by an optically gated photoconducting antenna. We discuss the source of the emission, considering the dependence on pump fluence and excitation wavelength. Magnetic fields ≈︂1 T suppress the emission. For fields parallel to the quantum well plane the main result is that the electric dipole dephasing rate increases with increasing field as expected theoretically. In perpendicular fields the dephasing rate remains constant but there is an overall reduction in the oscillation amplitude which is not presently understood.
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