Electron transfer and relaxation dynamics in heterovalent ZnSe/GaAs quantum well structures

We investigate the electron transfer and relaxation dynamics in heterovalent ZnMgSe/ZnSe quantum wells (QW's) grown on GaAs using the nonlinear optical method of three-beam degenerate four-wave-mixing (FWM). We use ultra-short (90 fs) laser pulses with non-collinear wave-vectors \textbf{k}$_{1}$, \textbf{k}$_{2}$ and \textbf{k}$_{3}$ at a center wavelength of 441 nm ($\sim $2.81 eV) which is resonantly tuned to the heavy hole exciton transition energy at 25 K. In the experiment the time coincident strong pump pulses \textbf{k}$_{1}$ and \textbf{k}$_{2}$ creates both an exciton density grating in the QW and an electron-hole pair grating in the GaAs while the delayed weak pulse \textbf{k}$_{3}$ simultaneously probes the exciton lifetime T$_{1}$ as well as the electron grating injection time T$_{t}$ from the substrate into the QW. Intensity dependent experiments reveal that the diffraction efficiency due to the electron grating increases faster with increasing \textbf{k}$_{1}$ and \textbf{k}$_{2}$ pulse intensities than the FWM efficiency due to the generated exciton density grating. This behavior which is attributed to exciton bleaching at high intensities enables the discrimination of times T$_{1}$ and T$_{t}$, both being in the order of a few tens of picoseconds.