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Bulletin of the American Physical Society

Density-Matrix Theory with Renormalized Electron-Laser Coupling for Quantum-Dot Nonlinear Optical Response

Danhong Huang, Xuejun Lu, Shanhui Fan

Published Mar 16, 2021• 0 citations
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Quantum dots have been the focus of both fundamental and applied research due to their atomic-like properties. The delta-function-like density of states along with strong nonlinear optical properties have made quantum dots the focus of novel opto-electronic devices and of quantum computing proposals. The key issue for implementing quantum dots in devices such as quantum computers is the ability to coherently entangle separately localized quantum dot states. Using continuous wave coherent nonlinear optical spectroscopy techniques, this thesis focuses on creating coherent super-position states built from separately localized quantum dot states. The narrow bandwidths and mutual coherence of the applied laser fields allow for the creation of radiative and nonradiative coherences which can be understood from the coherent nonlinear optical response. The quantum dots under study are formed due to the width fluctuations of a thin quantum well. By applying nondegenerate mutually coherent fields on the sample in the traditional pump and probe geometry and homodyne detecting the nonlinear signal field, nonradiative coherences between two laterally coupled quantum dots are created and the coupling between their states examined. The coherent nonlinear optical response is compared with theoretical predictions based on the density matrix formalism for the interacting two exciton model. From this comparison, binding energies of the two-exciton bound state created with separately localized excitons is found to be on the order of 20 mueV for like-polarized excitons and on the order of 100 mueV for unlike-polarized excitons. A second order coherence between the ground and two-exciton state is also noted in the experimental data after comparison with theory. Further work attempting to study a vertically coupled quantum dot system was unsuccessful. The spectrum of a quantum dot is further understood from the comparison of the linear and nonlinear excited state spectra. Excited states which appear in both spectra imply a Coulomb interaction between them and their respective ground states. Coherent nonlinear optical signatures of excited states correlated with their linear excitation spectra are presented, and the importance of such states for a quantum computer read-out is described.
In this work, we develop a physically transparent Coulomb-interaction model from simplification of a general many-body theory and apply this model to study interacting electron dynamics for transient occupation and quantum coherence in both single and double quantum dots under laser irradiation. Our theory considers self-consistently the Coulomb-renormalized Rabi coupling to light by using an induced optical-depolarization field, corresponding to dynamical exchange interaction between two electrons within the same quantum dot. Meanwhile, we employ evanescent-field coupling for dynamical Coulomb interaction between two electrons in different quantum dots based on a surface-plasmon model. In particular, we explore the quantum interference between a pair of laser-induced quantum coherence in three-level quantum-dot systems, which gives rise to indirect transition of electrons for sum- and difference-frequency transient optical responses. By varying laser frequency detuning, the control of laser-dressed electronic states becomes possible and can be utilized for switching among off-state, partial, and complete on-states. This study will be useful for controlling the phase entanglement of two laser-dressed states of quantum dots, as well as for enhancing the electro-optical performance through sum- and difference-frequency transitions in many optoelectronic devices.