Investigation On Optical Nonlinearity And Dynamics Of Several Typical Optical Functional Materials

Two-photon absorption is an optical nonlinear phenomenon,which means that a molecule absorbs two photons at the same time to reach an excited state with two photon energies when the laser interacts with the medium.With the rapid development and progress in the field of optoelectronic information science,nonlinear optics,as a very important branch,relies more and more on the optical properties of the nonlinear optical functional materials,so the research on the optical nonlinearity of organic two-photon absorption materials has gradually become a focus.The structure-property relationship of optical functional materials has always been a research hot spot in physics,materials,chemistry and other fields.Typical optical functional molecular materials have special symmetrical or asymmetrical D-π-A,D-π-D,A-π-A structures together with long chains or multi-branched structures based on these basic molecular structures,and also have modifiable molecular structures.They exhibit good nonlinear optical physics properties,as well as excellent ultrafast response speed,and excellent two-photon absorption characteristics,so they have potential applications in the fields of optical limiting and two-photon fluorescence.In this thesis,the nonlinear optical properties and excited state dynamics of these typical optical functional materials are studied.First,these materials are characterized by linear spectroscopy,and these molecules are initially obtained through the obtained ultraviolet-visible absorption spectra and single-photon fluorescence spectra.The optical properties of the system.Then use the single-photon time-resolved fluorescence technology to test their fluorescence lifetime,and to understand the relationship between the excited state and the structure.The degree of charge transfer in the excited state can be expressed by the degree of charge transfer in the fluorescent ICT state.The result of the two-photon absorption cross-section reflects the optical nonlinear performance of the molecular system.It is calculated by the open-aperture Z-scan technology test.From the anti-saturation absorption properties shown by these several oligomer materials.Furthermore,the use of variable power to excite the femtosecond beam can obtain strong excitation fluorescence of the conjugate system,the linear relationship between the fluorescence intensity and the square of the laser beam intensity can prove that this excitation process is caused by two-photon absorption.Through the relationship between laser beam transmittance and laser power density,analysis shows that the molecular structure has better optical limiting properties.Through the use of femtosecond pump-probe technology to study the excited state dynamics of these conjugated systems,they all show a fast process and a long decay process.After data fitting and the analysis,the fast process and long decay process are ascribed to the formation and attenuation of the ICT state,respectively.The study of ultrafast dynamics provides a scientific basis for understanding the conjugation relationship of conjugated systems.Finally,the density functional theory is used to quantify their frontier molecular orbitals and energy level forbidden band widths,so as to theoretically understand the electron cloud density distribution and the charge transfer mechanism.Through the quantitative calculation results,we analyzed that a molecular system with a smaller energy level forbidden band width can show better nonlinear optical performance and more excellent ultrafast response speed.