TRANSIENT 4-WAVE-MIXING ON (INGA)AS INP MULTIPLE-QUANTUM WELLS USING A FEMTOSECOND OPTICAL PARAMETRIC OSCILLATOR/

We present femtosecond four-wave mixing (FWM) experiments on unstraine d, thin (InGa)As/InP multiple quantum wells in the spectral region aro und 1175 nm. These experiments became feasible by employing a stabiliz ed femtosecond optical parametric oscillator which generates tunable 1 60 fs pulses in the near-infrared. The dephasing time of the heavy-hol e exciton is determined to be 1.2 ps at low temperature, corresponding to a homogeneous linewidth of 1.1 meV. In addition, a beating of the FWM signal with a time period of 1.3 ps corresponding to an energy spa cing of 3.2 meV is observed. We attribute this to the simultaneous exc itation of localized excitons and excitons bound to an impurity or def ect, which are not spectrally resolved in the linear optical spectra d ue to considerable inhomogeneous broadening.