Excitons and Trions Confined on CdTe Nano-Islands: Optical Tuning of the Dielectric Response.
Résumé
We present optical nonlinearities due to neutral and charged excitons trapped on monomolecular CdTe islands embedded in wide ZnTe–(Zn, Mg)Te quantum wells. 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. Such “pumped” transmission measurements show the bleaching and blue-shift of the ground-state heavy-hole exciton, simultaneous to the appearance and strengthening of a lower energy contribution, assigned to a negative trion (X–). These phenomena are explained in terms of (i) space-charge regions in the heterostructure inducing a built-in electric field and (ii) a photo-induced excess of electrons which alters the probability of exciton creation. The experimental results are in good agreement with a simple modeling of the neutralization, by injected carriers, of the initial Stark shift of electronic levels.