Ultrafast Response Induced by Interference Effects between Weakly Confined Exciton States

Abstract

We report an ultrafast response of weakly confined excitons in GaAs thin films observed by a degenerate four-wave-mixing (DFWM) technique. The time-domain signals excited by broadband pulses show an oscillatory structure with a period that varies according to the pulse spectral width and an ultrafast response comparable to the pulse width; on the other hand, the exciton dephasing time is hardly changed by the excitation spectral width. This ultrafast response is much faster than the radiative lifetime expected on the basis of the nonlocal response theory. The DFWM spectra measured at various time delays clearly indicate the contribution of weakly confined excitons to the ultrafast response. Moreover, in the measurement of excitation-energy dependence, the time-domain signals excited by a controlled spectral width exhibit the variation of the oscillation period and response time. These characteristics are similar to those of the excitation spectral width dependence. From the analysis of oscillatory structu...