Nanooptics: Coherent optical spectroscopy of single semiconductor quantum dots.

Abstract

The objective of this Thesis is to present a series of novel experiments which directly probe single semiconductor quantum dots (QD's). These optical studies, performed at T = 6K on naturally formed QD's in a narrow single GaAs quantum well, remove the spectral blurring caused by inhomogeneous broadening in ensemble measurements and reveal extremely narrow resonances resulting from the complete energy quantization of the zero-dimensional exciton. Excitons in isolated QD's were probed by laser excitation through a submicron sized aperture in a 100-nm-thick Al mask deposited directly on the sample surface. Previous experiments on single QD's have been based solely on photoluminescence detection. In the present work, continuous wave coherent nonlinear optical spectroscopy is used to perform direct measurements of exciton dynamics and optical nonlinearities. The fully resonant nonlinear measurements allow for the extraction of both the excitation decoherence time and the energy relaxation time, showing that the dephasing process is dominated by contributions that lead to energy relaxation. These measurements also identify an incoherent and a coherent contribution to the resonant electronic response and demonstrate a behavior similar to two beam coupling. This atomic-like behavior extends the similarities between the two systems to the nonlinear optical regime. The nonlinear response between different states measures the interstate energy relaxation and reveals a feature of the state coupling that is not explained by simple atomic models. Using the knowledge acquired in the cw linear and nonlinear optical experiments, transient optical coherent control of the excitation of a single QD in times shorter than the excitonic lifetime was demonstrated. A two-pulse sequence of picosecond pulses excited a superposition of eigenstates belonging to the same fine structure doublet creating a nonstationary quantum mechanical state. The temporal evolution and the quantum phase of this state was manipulated and monitored by controlling the optical phase of the two-pulse sequence through timing and polarization. The results combine concepts of wavefunction engineering, developed in atomic and molecular systems, with coherent control in semiconductors to demonstrate wavefunction manipulation in the limit of a single quantum system in a zero-dimensional QD.