Study On The Physical Mechanism Of Nonlinear And Chiral Optical Processes

The main work of this paper focuses on the establishment of theoretical calculation methods for nonlinear and chiral optical behavior and the development of corresponding calculation tools.These works mainly focus on three aspects.First,a calculation method of the two-photon absorption cross section based on the total state summation method under the framework of first principles and quantum chemical methods is established.Theoretical calculation of the twophoton absorption cross section is crucial for the precise guiding design of twophoton materials in nonlinear optics.Moreover,this method is superior to the traditional quadratic response method for visual analysis of multi-step transitions to study the charge transfer characteristics of different two-photon transition processes,which will be beneficial to the study of host-guest interactions in materials.It can also explain photoinduced charge transfer and electron-hole coherence,including optical properties of "hidden" intermediate states during two-photon absorption.These theoretical visualizations include transition density matrices,charge density differences,and transition dipole moments involving ground and excited state wave functions and transition dipole moments between electrical transitions from ground states to intermediate and final excited states.The main work in chapters 3 and 4 of this paper introduces the theoretical research method of this two-photon absorption process by taking molecular-based materials as an example,and discovers two new mechanisms of charge transfer in the two-photon absorption process:sequential and super-exchange charge transfer.This approach can be applied not only in two-photon processes,but can also be extended to other nonlinear optical processes.Secondly,a new theoretical visualization analysis method for generalized chiral optical characteristics of material systems is also developed in this paper.In the process of theoretical calculation,the transition electric dipole moment and transition magnetic dipole moment are innovatively decomposed to explore the asymmetric response of materials to electromagnetic fields during optical transitions.Chapters 5 and 6 of this paper introduce this method,explain the macroscopic sources of chirality in large systems,and establish a generalized and near-physical description of chiral optics.The above two calculation methods are based on the same theoretical framework,but in practice,due to the ever-changing materials themselves,a combination of methods is required.Therefore,in the last chapter of this paper,a theoretical calculation research paradigm based on first principles and finite element methods is introduced,and the dielectric function is used as a bridge to study the plasmon-enhanced interfacial charge transfer(ICT)of MoS2/WS2 heterostructures.excitons.