Nondegenerate Two-photon Absorption Properties Of Fluorescent Dyes

Two-photon absorption(TPA),a third-order nonlinear optical effect,is an interaction between light and matter in strong light field.It has been widely used in physics,biological and chemistry fields because it can absorb long wave and emit short wave,and break through the optical diffraction limit.In recent years,with the development of ultra-short pulsed laser,two-photon fluorescence microscopy plays an increasingly important role in the field of biomedicine.However,investigations on two-photon fluorescence microscopy,especially for two-color two-photon fluorescence microscopy,are focused on fluorescent proteins,regardless of fluorescent dyes which are also important parts in fluorescence microscopy.In this thesis,we first elaborated on the process and characters of two-photon absorption,and introduced the applications in physics,biology and chemistry,especially for two-color two-photon fluorescence microscopy.Second,we measured nondegenerate two-photon absorption cross sections of three commercial fluorescent dyes,Coumarin 120,Coumarin 4 and Rhodamine 6G through pump-probe technique,and investigated the fluorescence signals of Coumarin 120 and Coumarin 4 in experiment.Contrasting nondegenerate two-photon absorption cross sections and homologous degenerate two-photon absorption(D-TPA)cross section,the effect of intermediate state resonance enhancement was proved in experiment.In the last,the effect of intermediate state resonance enhancement during two-photon absorption process is qualitatively analyzed based on one-dimensional harmonic oscillator model.And then we improved the essential-state model and quantitatively analyzed the two-photon absorption of the three fluorescent dyes.The results of the experiment and the theory are in good agreement.It proved the correctness of the improved essential-state model.The theoretical results of nondegenerate and degenerate two-photon absorption proved the effect of intermediate state resonance enhancement in theory.The main points are as follows:1.Descriptions and applications of two-photon absorption.We first described the process of two-photon absorption.Second,we introduced several applications of two-photon absorption,for example,optical data storage,optical limiting,microfabrication,and fluorescence microscopy.Then,we introduced applications of nondegenerate two-photon absorption,for example,infrared detection,quantum count,optical sampling and two-color two-photon fluorescence microscopy.In the last,we established the purpose and significance of the subject.2.Experiments of two-photon absorption of fluorescent dyes.In experiment,we measured nondegenerate two-photon absorption cross sections through pump-probe technique of the three commercial fluorescent dyes,Coumarin 120,Coumarin 4 and Rhodamine 6G,and investigated the fluorescence signals of Coumarin 120 and Coumarin 4.The degenerate two-photon absorption cross sections of Rhodamine 6G were obtained from literature and those of Coumarin 120 and Coumarin 4 were obtained from Z-scan experiments.The two-photon cross sections were obtained from fitting experimental data.It provides fundamental researches for the applications of fluorescent dyes in two-color two-photon fluorescence microscopy.Contrasting nondegenerate two-photon absorption cross sections and homologous degenerate two-photon absorption(D-TPA)cross section,the effect of intermediate state resonance enhancement was proved in experiment.3.Theory of two-photon absorption of organic molecule.The effect of intermediate state resonance enhancement during two-photon absorption process is qualitatively analyzed based on one-dimensional harmonic oscillator model.The expression of two-photon absorption cross section of the essential-state model was derived from the Sum-Over-States based on quantum-mechanical perturbation theory.The essential-state model was improved,only dipole term(D-term)and several two-photon terms(T-term)that contribute to the two-photon absorption cross section are retained,and other terms are ignored.Theoretical two-photon absorption cross sections were obtained by fitting the shared parameters with quantum chemical calculation.The results of the experiment and the theory are in good agreement.D-term and every T-term in the improved essential-state model was analyzed severally.It proved the correctness of the improved essential-state model.Contrasting theoretical degenerate two-photon absorption cross sections with theoretical nondegenerate two-photon absorption cross sections,it proves the effect of intermediate state resonance enhancement in theory.