Dephasing Time Measurements in Quantum Dots by Non-Degenerate Four Wave Mixing

Abstract

Studies on the relaxation process of coherently excited polarizations associated with excited states are interesting to reveal the mechanisms of dephasing in quantum confined semiconductor systems. In two-dimensional systems like quantum wells (QW) or superlattices (SL), the most important mechanisms considered are the scattering of the excited electron-hole pairs with phonons (LO-phonons or confined acoustic phonons), with interface potential fluctuations or localized states or, at higher excitation densities, the scattering by carrier-carrier interaction. In QD’s, at present, the relative contribution of these mechanisms to the dephasing process is an open question and one is trying to identify the different dephasing processes from the experiment. To determine ultrafast relaxation times in the range below 100 fs, various nonlinear spectroscopic methods in the frequency domain have been applied, like hole-burning spectroscopy [1–3] or coherent transient spectroscopy using broadband incoherent light [4,5]. In the present paper the method of nondegenerate four-wave mixing (NDFWM) has been used to derive the dephasing time T ₂. The samples investigated are CdS and CdS_0.7Se_0.3 quantum dots in glass with radii 2 to 3 times larger than the Bohr radius. The large sizes were chosen to minimize the interface influence and by this to investigate the electron-hole pair states before trapping. The minor importance of trap states have been deduced from the dominance of a sharp band-edge related luminescence for these large dot sizes [6]. The dephasing times determined are consistent with results from differential absorption spectroscopy [7,8] at comparable excitation densities. As a new phenomenon we discuss the influence of carrier-carrier scattering in the weak confinement range and determine the dephasing time of this process by increasing gradually the excitation intensity.