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Device miniaturization with advanced fabrication techniques has revolutionized the semiconductor industry along with innovative concepts of carrier spin, potentially important at the fundamental physical limits of scalability. For spin-based information processing, semiconductor coupled quantum dots (CQDs) provide excellent control in spin dynamics due to 3-D confinement, discrete energy levels, and optical orientation and coupling. The research presented in this dissertation investigates spin interactions and exciton relaxation channels in semiconductor CQDs measured through optical control and time-resolved experimental techniques.Our experiments involving photoluminescence (PL) and photoluminescence excitation (PLE) methods revealed effects arising from the structural properties of semiconductor nanostructures, including quantum rings and CQDs. High resolution PL measurements on positively charged exciton states demonstrated experimental evidence of isotropic exchange interaction. Controlling exchange interaction in different spin configurations is fundamental to quantum logic operations. Hence, polarization dependent PL experiments were executed and electric field tunable exchange interaction effects were reported on the neutral exciton states. Next, time-resolved measurements were performed while pumping above the InAs wetting layer (WL) energy and probing below the WL to determine the dynamics of the optically generated electric field in CQDs. The observed, rapid onset of the optically generated electric field may provide the use of CQDs for optical switching applications.Finally, carrier relaxations in the CQDs were identified through the dynamics of the spatially indirect exciton state using a mode-locked laser excitation source and standard time-resolved single photon counting technique. Wave function distribution, carrier tunneling, and phonon scattering led to the observed lifetime and intensity modulations. With time-resolved PLE, bi-exponential lifetime decay revealed non-monotonic phonon relaxations as a result of the structure factor of the CQDs. Furthermore, with resonant excitation, carrier tunneling into charged exciton states was also eliminated. These results demonstrated tunable exciton relaxation rates in CQDs, which are useful for quantum information, optoelectronics, and photonics applications.

Coherent optically driven semiconductor quantum dot systems are proposed for the implementation of quantum information processing devices. Fundamental quantum computation units, the quantum bits (qubits), are realized and optically manipulated in the exciton and spin bases. In this thesis, consecutive phase-sensitive rotations of a single qubit and their importance in the measurement of the full physical density matrix have been demonstrated experimentally in the exciton system in neutral GaAs/Al0:3Ga0:7As interface fluctuation quantum dots. The experimental work done on the exciton system serves as a foundation for consequent work on the long coherence lifetime spin-based qubits in charged quantum dots. In the spin-based system in the charged GaAs/Al0:3Ga0:7As interface fluctuation quantum dots, we demonstrate the selective excitation from the two spin ground states to the charged exciton (trion) state through phase sensitive control of the spin coherence in an ensemble of three-level spin-trion Lambda systems. This controlled excitation utilized the optically coupled and decoupled states of the electron spin in the presence of coherent transient excitation of the spin ground states in singly charged quantum dots. This progress lays the ground work for achieving complete ultrafast spin rotations.

In this thesis, the coherent nonlinear optical spectroscopy of negatively charged interface fluctuation GaAs/Al0.3Ga0.7As quantum dots is studied by utilizing differential transmission techniques in both frequency and time domains. In the frequency domain, a spectral hole burning (SHB) method with two independently tunable frequency-stabilized continuous wave lasers is employed to eliminate the broad inhomogeneous broadening effect. In SHB experiments, not only the stimulated Raman spin coherence is excited and measured, but spin population pulsation is also demonstrated. Both electron decoherence and population relaxation times (∼ 3 nsec) are extracted from the SHB experiments, which are theoretically discovered to be limited by the spin spectral diffusion processes. A novel nonlinear coherent optical phase-modulation spectroscopy is developed to measure the real electron spin relaxation time. Since the phase-modulation measurement is only sensitive to processes with relaxation rates on the same order or slower than the modulation rate, the relatively slow electron spin-flip relaxation process is distinguished from the fast spectral diffusion processes. The measured result of the electron spin-flip relaxation rates as a function of magnetic field and temperature is best explained in term of spin relaxation processes by the phonon-assisted Dresselhaus spin-orbit scattering. The spin-flip relaxation time approaches 46 musec for zero magnetic field at 4.5 K. With the help of sub-micron aperture arrays on the sample surface, coherent nonlinear optical response in the frequency domain is clearly observed in a single charged QD with magnetic field in both Faraday and Voigt geometries. The optically forbidden transitions are optically excited and detected experimentally through the spin ground state mixing with an applied in-plane magnetic field. Pauli blockade of the electron spins is clearly observed when the system is excited with different polarization. In addition, extremely long quantum beat oscillations due to the long spin Raman coherence excited through a sub-micron aperture are demonstrated using ultrafast laser pulses. By analyzing the dependence of the amplitude and phase of the quantum beat oscillations at various magnetic fields, it is evident that the spin Raman coherence not only arises from optical stimulated Raman transitions, but also from spontaneously generated coherence due to the trion population relaxation.

The focus of the thesis is nonlinear transient optical properties and coherent control of single interface fluctuation quantum dots (QD's). When single exciton and biexciton resonances were driven by strong resonant laser pulses, Rabi oscillations of these transitions were observed in the differential transmission (DT) geometry. These results extend the concept of coherent control to the limit of single quantum confined systems in solids and have important implications for the feasibility of building quantum information processing devices using semiconductor QD's. Exciton Rabi oscillations are equivalent to arbitrary single-bit operations. The pulsed control of the exciton-biexciton transition is utilized as the optical operation for building a two-bit controlled rotational gates (CROT) in a single QD. A fidelity of 0.79 was obtained from numerical evaluation of the gate operation itself assuming typical decay parameters measured from these dots. Next, the nonradiative quantum coherence between two orthogonally polarized excitons confined in a single QD was studied. By using pulses with large enough bandwidth and the proper polarization, two exciton states are simultaneously excited. The quantum beats between the two exciton states were temporally resolved as the relative phase between the two dipole oscillations changed. It was found that the nonradiative coherence time is limited by the lifetimes of the excitons involved even in the strong field regime. In the basis of the pseudo-spin Bloch vectors of exciton transitions, the total wavefunction following the pulsed excitation is an entangled state, i.e. the wavefunction is non-factorizable. Finally, the quantum coherence between the biexciton, exciton, and the crystal ground states was studied in a novel quantum beat experiment, where the oscillations only existed when the pump and probe pulses overlapped in time. Laser pulses generated after a passive pulse shaper with just enough bandwidth to cover both the exciton and biexciton transitions were used in this experiment. The appearance of the beats is indicative of a reasonably long coherence time between the biexciton and other states even in the presence of strong pulsed excitations.