Ultrafast exciton dynamics in ternary II-VI semiconductor quantum wells

Ultrafast exciton dynamics is investigated in a high-quality ${\mathrm{Zn}}_x{\mathrm{Cd}}_{1-x}\mathrm{S}\mathrm{e}-\mathrm{Z}\mathrm{n}\mathrm{S}\mathrm{e}$ multiple quantum well at low photoexcited carrier density by means of a two-color femtosecond absorption saturation technique. Broadening of the exciton lines due to carrier photocreation is found to dominate the observed nonlinear response. Measurements performed as a function of the initial energy of the photoexcited carriers show that the dynamics of the induced broadening strongly depends on the carrier distribution, suggesting that Pauli exclusion principle effects significantly influence exciton broadening in ternary quantum-well systems. Localized excitons are shown to modify the picosecond nonlinear response, inducing a frequency shift of the absorption change peak as the system approaches quasiequilibrium.