All-optical generation of coherent in-plane charge oscillations in GaAs quantum wells

We have induced ultrafast charge oscillations in the plane of an unbiased and undoped (110)-oriented GaAs quantum well by coherent optical excitation of heavy-hole and light-hole exciton transitions. The oscillations arise from an in-plane charge displacement between heavy-hole and light-hole states, which results from the periodic parts of the Bloch wave functions and the (110) orientation. The observations are evidence for the existence of a substantial far-infrared transition-dipole moment between heavy- and light-hole subbands for in-plane wave vectors, which we estimate to be ∼0.5 e for the quantum well under study. Our findings might prove important for designs of far-infrared detectors and emitters. © 2011 American Physical Society.