Anisotropic optical response of InP self-assembled quantum dots studied by pump-probe spectroscopy

Transient anisotropic reflectivity change spectra of InP quantum dots have been observed by means of two-color pump-probe spectroscopy. The results show a fast decay component with a lifetime of 100–200 ps which depends on the probe energy, followed by the slow decay component of ~1 ns. The reflectivity change spectra have a dispersive shape having a maximum on the higher energy side of the photoluminescence (PL) band by 80 meV, and a dip located at the maximum of the PL band. Interestingly, the reflectivity change signals observed for the [1[overline 1]0] and [110] polarizations have the opposite sign when the probe energy is set between the first and second exciton states. The temporal change of spectra is simulated by means of a Monte Carlo method, and the model is found to well reproduce the experimental result. Further, the model enables us to evaluate the microscopic exciton parameters of single quantum dots by macroscopic observations. The oscillator strengths along the [110] and [1[overline 1]0] directions at the PL peak energy are evaluated to be fx=0.37 and fy=0.71, respectively. The oscillator strength is about five times smaller than simple theoretical estimates. This suggests a small overlap of the envelope functions which is consistent with the existence of a permanent dipole moment observed in these QDs