Optical Nonlinearities and Femtosecond Dynamics of Quantum Confined CdSe Microcrystallites

Abstract

Pump-probe spectroscopic techniques with nanosecond pulses are used to investigate the size quantization effects in CdSe microcrystallites in glass matrices (quantum dots). Nonlinear properties of the transitions between quantum confined electron and hole states are reported for low temperatures and at room temperature. Femtosecond four-wave mixing and differential transmission spectroscopic techniques were also employed to study the excited state dynamics and relaxation times of the quantum dots. The homogeneous and inhomogeneous contributions to the lowest electronic transitions are measured by femtosecond spectral hole burning at various temperatures. The inhomogeneous linewidth is due to size and shape distribution of the crystallites. Our experiments indicate that the hole-width increases with increasing light intensity. The optical nonlinearities as a function of microcrystallite size were investigated using single beam saturation experiment for the three quantum confined samples. A simple absorption saturation model was used to analyze the data. The results indicate that the saturation intensity is larger for smaller semiconductor sizes. Therefore, the index change per unit of intensity, Δn/I which is proportional to (α-α_B)/I_s is larger for larger sizes. Here, Δn is the index change, α is the absorption at the peak of the transition, α_B is the background absorption, and I_s is the saturation intensity.