Moving speckles in the Rayleigh scattering of excitons in potential gradients

A moving speckle pattern in time-momentum space of the resonantly excited Rayleigh scattering from excitons subjected to a spatial potential gradient is observed. The spatial gradient of the exciton energy was created by an uniaxial mechanical stress applied in the plane of a semiconductor quantum well. The measured energy shift and the increase in the heavy-light hole splitting is well described by $\mathbf{k}\cdot \mathbf{p}$ theory with a strain field calculated by linear continuum elasticity theory. A model based on localized excitons in a center-of-mass potential with a systematic spatial variation explains the observed properties of the speckle pattern, showing in particular that the velocity of the speckles in momentum space is given just by the local gradient of the potential.