Non‐classical excitonic transport in quantum wells

We report investigations on excitonic transport in ZnSe quantum wells using far‐field nano‐photoluminescence enhanced by a solid immersion lens. The 250‐nm spatial resolution and 5‐ps temporal resolution allow the access to non‐classical transport regimes. From zero‐phonon‐line spectroscopy we deduce the spatial distribution of the PL intensity under different cw excitation condition and its temporal evolution after a pulsed excitation, respectively. The periodic quenching of the transport length as function of excitation excess energy and the nonlinear expansion of the PL spot observed in time‐resolved experiments reveal the dominance of hot‐exciton effects. The experimental data are well modelled by a Monte Carlo simulation. Spatially resolved phonon‐sideband spectroscopy is used to observe directly the coherent transport and the energy relaxation of hot excitons during transport. The coherence length is measured to be 300–400 nm at low temperatures and the excitons are found to remain hot during transport on a length scale of several micrometers.