Dynamics and mechanisms of the nonlinear response of quantum confined CdSe nanoparticles

In the resonant regime, the optical Kerr effect of CdSe-doped glasses is due to photoexcitation of carriers whose presence modifies the optical properties of the CdSe quantum dots. The recovery time which governs the repetition rate at which such devices could be operated is equal to the recombination time of these carriers. At low laser intensity, the recombination time may be shortened by the photodarkening effect. At higher density, nonlinear processes such as Auger recombination may play a significant role as already suggested although it was pointed out that the first data could not clearly make the difference between a cubic (Auger) process or a quadratic one. By plotting 1/N² where N is the number density of free carriers as a function of time, we get (as shown in Fig. 1), for a fresh sample at high laser intensity, a straight line which clearly indicates that the Auger process is the dominant one in this case. The experimental determination of N(t) was made in three different ways. Using a conventional phase conjugation set-up, the backward pump pulse was delayed. We also used nonlinear absorption in a pump-probe arrangement and delayed the probe pulse. We finally time resolved the luminescence excited by a picosecond pulse by using a CS₂ Kerr shutter. All these data lead to an Auger constant on the order of a few 10^-30 cm⁶ s^-1. When the sample has been photodarkened, both the linear non-radiative decay and the Auger process are important. The dynamics of the nonlinear response of these CdSe-doped glasses are thus now well understood. This in turn may help us in understanding the mechanisms of this nonlinear response. It may be due to free carriers in which case it corresponds to saturation of the first electronic transitions (1S_3/2-1S_e and 2S_3/2-1S_e) with a relatively fast response. Alternatively, the photoexcited carriers may be trapped at the surface of the nanoparticles and, through the static electric field they create, they may also modify the optical properties. In this last case, the response is slow. Depending on the origin of the sample, commercial or experimental, and on its past history, fresh or photodarkened, we observe that trapped carriers provide the dominant mechanism in certain cases whereas free carriers dominate under other circumstances. The frequency dependence of the Kerr susceptibility and the intensity dependence of the phase conjugate intensity will also be discussed in conjunction with saturation phenomena.