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Browsing by Author "Wang, Bo"

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    Development of novel schemes for treating subsystem boundaries and electrostatic potentials in simulations of complex systems
    (2014-01) Wang, Bo
    Fragmentation schemes provide a powerful strategy for calculating the potential energy surfaces of complex systems. The combined quantum mechanical and molecular mechanical (QM/MM) method, the electrostatically embedded many-body (EE-MB) method, and the molecular tailoring approach (MTA) are three examples. Two critical issues to be addressed in these methods are the treatment of the boundary between the subsystems when it passes between bonded atoms and the inclusion of the electrostatic potential of one subsystem in the Hamiltonian of another. This thesis involves the development and application of new schemes to treat both issues. The first part focuses on the development of a tuned pseudoatom scheme with a balanced redistributed charge algorithm to accurately model the QM-MM boundary that passes through a covalent bond, especially a polar covalent bond. Various redistribution schemes and ways of tuning the boundary treatments are tested and compared for the QM/MM method and the EE-MTA method. The second part of this thesis involves the development of screened charge models to include charge penetration and screening effects in generating electrostatic potentials for use in various methods, including QM/MM and EE-MB methods. The screened charge models are also used to derive partial atomic charges by fitting electrostatic potentials.
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    Study on dynamic behavior of ring-cavity quantum-cascade lasers with group velocity dispersion
    (2014-12) Wang, Bo
    Quantum cascade lasers (QCLs) have been the common mid-infrared sources because of their high output power and agile wavelength range owing to the intersubband transitions. Previous research on QCLs shows that the instability and dynamic behaviors are more complicated than conventional semiconductor lasers due to interaction among various intracavity effects, such as material nonlinearities, unique gain and dispersions. Group velocity dispersion (GVD) plays an important role in the study of QCLs because of its effects on pulse spectrum in both unsaturation and saturation regime. To investigate the effect of group velocity dispersion in QCLs, both instability analysis and time-domain numerical simulation are presented in this thesis. A model that accounts for GVD effect and a background saturable absorber is developed based on Maxwell-Bloch equations. Stability analysis of a ring cavity QCL is performed through the linearization of dynamic equations. Depending on the results of stability analysis, the numerical simulation for time-domain pulse progression based on the split-step method is demonstrated. It is found that the instabilities of QCL with GVD are highly related to the pumping strengths, and stronger saturable absorber effect tends to make the system more stable. Numerical simulations show that Rabi side modes are enhanced by saturable absorber effect and pumping strength; GVD effect lowers the intensity of stable pulses and shrinks the pulse width. The split-step method successfully combines nonlinear and linear effects in the dynamic equations and finally leads to stable results.