Dynamics Study On Nonlinear Optical Process Of Two-photon Absorption Materials

After emergence of lasers,nonlinear optics become an important sub-discipline of modern optics.Nonlinear optics display good prospects in the fields of optoelectronics,medicine,and biology,etc.On the one hand,with the development of design and synthesis technologies,organic molecular materials have attracted much attention because of their excellent nonlinearity characteristic,good flexibility,and ease to be synthetic.The nonlinear properties of organic molecular materials are greatly affected by the geometric structure,such as donor/acceptor strength,conjugated bridge properties,and conjugation length,etc.However,the structures of organic molecules are wide and varied,how to effective design and synthesis organic molecular materials which have excellent nonlinear characteristics become one of the key research.On the other hand,two-dimensional semiconductor materials have attracted wide attentions in the field of photoelectric devices due to their unique optoelectronic properties.The analysis of the energy transfer and the carrier dynamics within the materials are of great significance for developing and improving performance of the optoelectronic devices.The thesis mainly divides two parts,the first part is about the performance of two-photon absorption and optical limiting.Two types of organic molecular materials with the different of terminal groups and the different positions of the substituents are studied theoretically.The second part is about the photoluminescence and photoluminescence life of ultra-thin CdS films,single to multi-layered MoS₂,and CdS-MoS₂ heterojunctions using photoluminescence spectroscopy and time-resolved fluorescence spectroscopy techniques.The charge transfer and carrier dynamics are measured and studied.The following conclusions are obtained:1.The difference of terminal groups result in the change of electronic structure and transition dipole moment of organic molecules.The-NO₂ as terminal groups have a larger dipole moment which enhanced two-photon absorption cross section,and superior optical-limiting ability compared with-N?CH₃?₂ as terminal groups.2.Two-photon absorption and optical limiting properties of the organic molecules depend crucially on the substituent positions.When the substituent in the meta position,the optical limiting ability of the organic molecule is stronger.It is indicated that the nonlinear optical properties of the medium can be controlled by subtle manipulation on the molecules.3.The photoluminescence properties and photoluminescence lifetime of MoS₂ are related to the layer number.The photoluminescence intensity decreases with the increase of the layer number.The energy gap changes from the direct band gap of the single layer MoS₂ to the indirect band gap of multilayer MoS₂.The photoluminescence lifetime of bilayer MoS₂ have longer lifetime compared with single layer MoS₂.The photoluminescence intensity of the ultra-thin CdS films increases with the sample thickness increasing,and the peak position red shift due to the weakening of the quantum confinement effect.The photoluminescence lifetime of the CdS bound excitons is longer than that of free exciton due to the surface states capture electrons to increase the lifetime of the electrons.4.CdS-MoS₂ heterojunction is a type II van der Waals heterojunction.Photogenerated electrons transfer to the conduction band of MoS₂ from the conduction band of CdS,and photogenerated holes transfer to the valence band of CdS from the valence band of MoS₂,long-lived carriers are generated in the heterojunctions.Secondly,the surface states of ultra-thin CdS films block the occurrence of interlayer excitons in CdS-monolayer MoS₂ heterojunction and promote the generation of interlayer excitons in CdS-bilayer/trilayer MoS₂ heterojunctions.