Theoretical Investigation On Coherent Control Of Biexciton In A Semiconductor Quantum Dot

The characteristic of atomiclike discrete energy levels, make quantum dots (QDs) very promising for solid-state implementations of quantum information processing. The biexciton system is one of important quantum states in QDs, as it carries prospects in realization of two-bit quantum logic gates and in generation of entangled photon pairs. In this thesis, we theoretically investigate coherent control of the biexciton in a semiconductor QD, based on a resonant two-photon excitation. The main works and innovative results are as follow:1. Within the framework of picosecond laser pulses for excitation, supposing there exist both horizontally and vertically linearly polarized components with adjustable relative phase between them. After building up the system Hamiltonian, we numerically solve the optical Bloch equations. The results show that the relative phase (consequently the laser polarization) plays an important role on Rabi oscillations of the biexciton system, due to the interference between different excitation paths related to different polarization photons. Constructively interfered when the phase difference between two excitation paths is0, while destructively for π phase difference. Different from previous studies in the literature, in which Rabi oscillations are typically measured by controlling the pulse-area via the field intensity of the incident laser pulse, the phase is explored as an alternate control knob to coherently manipulate the biexciton states. The phase control can be facilely implemented by changing the light polarization with a quarter-wave plate.2. Under the excitation of broadband femtosecond laser pulses, we demonstrated that selectively removing certain spectral bands or changing certain spectral phase can remarkably enhance the two-photon absorption rate from the ground state to the biexciton state, with an increase up to13times. The fundamental is that destructive interference exists for different excitation paths with a transform limited pulse, while shaping the pulse could make all the transitions constructively interfered. It means that pulse shaping could be used as another control knob to manipulate the biexciton states in QDs.