Ultrafast in-well screening of the piezoelectric field in (111) quantum wells

Ultrafast establishment of important exciton blueshifts on the order of the exciton linewidth has been observed in photoexcited (111) $\mathrm{CdTe}/{\mathrm{Cd}}_{1-x}{\mathrm{Mn}}_x\mathrm{Te}$ multiple quantum wells (MQW's). We attribute the rapidity in the appearance of the blueshift to the instantaneous nature of screening due to the spatial separation of the electron and hole wave functions (in-well screening). These findings are in contrast with recent results where the slow establishment of out-of-well screening of a ${\mathrm{Ga}}_{1-x}{\mathrm{In}}_x\mathrm{As}/\mathrm{GaAs}$ MQW structure as a whole was reported to be dominant. We have developed a theoretical exciton model that reproduces not only the magnitude of the experimental blueshift but also the shape of the transmission spectra and thus confirms our interpretation in terms of in-well screening.