Electron Spin Manipulation Via Ultrafast Optical Field In Ⅱ-Ⅵ Semiconductor Quantum Dots

The electron spin state of semiconductor quantum dots is one of the ideal candidates for solid-state quantum information processing. Ⅱ-Ⅵ semiconductor quantum dots normally have a weak electronic-nuclear spin hyperfine coupling effect, and nuclear spin influence on electron spin relaxation is weaker, so it is expected to get a longer spin lifetime. In this thesis, we investigate the ultrafast spin generation process, spin signal control, and hole trapping dynamics detection based on spin manipulation in CdS and CdSSe Ⅱ-Ⅵ colloidal quantum dots. The main works and research results are as follows:(1) At room temperature, time-resolved Faraday rotation spectroscopy is used to investigate the ultrafast spin generation process under the irradiation of femtosecond laser light. Through the comparative experiment between carrier dynamics and spin dephasing dynamics and g factor analysis, the spin carrier is shown to be resident electrons in the quantum dot. Via an excitation process from electron to trion states determined by polarization selection rules, the electron spin generation is completed in a short time of femtosecond pulse durations.(2) The electrons originally resident in quantum dots are usually sparse, leading to a weak spin signal. Based on the traditional pump-probe setup, a linear polarized prepump pulse is introduced before the pump laser pulse. Due to the hole surface trapping process, the number of the net electrons in the dot core will effectively increase, which significantly enhance the electron spin signal. By changing the delay time between the prepump and pump pulses and monitoring the spin enhancement effect, we further developed a new technology used for hole surface trapping dynamics based on the prepump-pump-probe ultrafast spectroscopy.