Charge carrier dynamics and the development of optical gain in semiconductor quantum dots.

摘要

State-resolved pump/probe measurements with femtosecond temporal resolution were made to examine the size dependent charge carrier dynamics of strongly confined CdSe quantum dots. Following optical excitation, relaxation in the conduction band is shown to be consistent with the expectations of an Auger mediated process. In the valence band, the presence of a previously unobserved relaxation mechanism is detected. This mechanism is associated with non-adiabatic interactions with the surface ligands. The quantitative nature of these measurements, resulting from the explicit investigation of state-to-state transition rates, allowed for the development of a unified picture of exciton relaxation in these materials. Here the total transition rate associated with the charge carriers is composed of a manifold of different contributions, each with a distinct size dependence.;These state-resolve methods were further applied to investigate the development of optical gain in CdSe quantum dots. It was observed that the capacity of these systems to achieve the regime of optical gain has a pronounced dependence on the initially prescribed excitonic state. In general, as the energy of the initial state was increased it was progressively more difficult to achieve optical gain due to the impeding influence of state dependent multiexcitonic interactions. This explains the inability of prior research which used fixed wavelength excitation sources to demonstrate optical gain in small particles. By maintaining the identity for the pumped state in different particle sizes, the long standing prediction that quantum dots would be a universal gain medium was recovered. In addition, evidence for the capacity of specific higher order multiexcitonic interactions to manipulate the resulting optical gain spectrum was provided.