Ultrafast nonlinear optical response of excitons in ${\mathrm{Zn}}_{\mathrm{x}}$${\mathrm{Cd}}_{1\mathrm{-}\mathrm{x}}$Se/${\mathrm{ZnS}}_{\mathrm{y}}$${\mathrm{Se}}_{1\mathrm{-}\mathrm{y}}$ multiple-quantum-well structures studied by femtosecond pump and probe spectroscopy

Femtosecond pump and probe spectroscopy has been applied to II-VI semiconductor multiple-quantum-well structures. The nonlinear optical response of excitons has been investigated by measuring differential absorption spectra with various time delays for different pumping intensities. The spectral shape analysis has shown that the spectral change induced by the pumping pulses is composed of the broadening, the peak shift, and the decrease of the absorption area of the exciton band. From the intensity dependence of the absorption area under the low pumping intensity limit, the saturation exciton density has been determined to be 5×${10}^{11}$ ${\mathrm{cm}}^{\mathrm{-}2}$, which agrees with the theoretical value taking into account the phase-space filling and the exchange effect due to the excitons generated from free electron-hole pairs excited by the pumping light. The intensity dependences of the peak shift and the absorption area are also discussed. The peak shift and the absorption area exhibit the same dependence, and the origin of both spectral changes is found to be the exciton-exciton exchange effect. The saturation density and the peak shift depending on the width of the well layers are also discussed. The dependences of their parameters are explained by the confinement effect of excitons in thin well layers.