Excitons and trions confined on CdTe nano-islands: Optical tuning of the dielectric response

We present optical nonlinearities due to neutral and charged excitons trapp ed on monomolecular CdTe islands embedded in wide ZnTe-(Zn, Mg)Te quantum w ells. These nonlinearities are demonstrated by low-temperature reflectivity and transmission experiments with additional continuous-wave excitation of the sample by different lines of an Ar2+ laser, at various intensities. Su ch "pumped" transmission measurements show the bleaching and blue-shift of the ground-state heavy-hole exciton, simultaneous to the appearance and str engthening of a lower energy contribution, assigned to a negative trion (X- ). These phenomena are explained in terms of (i) space-charge regions in th e heterostructure inducing a built-in electric field and (ii) a photo-induc ed excess of electrons which alters the probability of exciton creation. Th e experimental results are in good agreement with a simple modeling of the neutralization, by injected carriers, of the initial Stark shift of electro nic levels.