Measurement Of Two-photon Absorption Cross Sections Of Organic Compounds

Strong two-photon absorption organic compounds attract much attention due totheir increasing application foreground in many high-tech areas such as optical powerlimiting and stabilization, up-converted lasing, three-dimensional data storage andmicro-fabrication, three-dimensional fluorescence microscopy, and photodynamictherapy. In the nineties, it has been the hotspot in material physics, material chemistry,and quantum chemistry.Two-photon absorption is the process that occurs through simultaneous absorptionof photons via virtual states in a medium. It is a kind of third-order nonlinear process,only occurs when the material is excited by very intensive laser. Two-photon absorptioncross section characterize the capability of two-photon absorption of materials,consequently further research on the measurement of two-photon absorptioncross-sections is quite significant. The development of more advanced equipment andcharacterization techniques for the study of optical non-linearity will not only helpsynthetic chemists gain insight into structure-property relationships and subsequentlycontribute to the strategy of molecular design but will also inspire materials scientists toexplore new applications based on the two-photon absorption phenomenon.Because two-photon absorption cross sections are very small, the requirements ofhigh intensities to induce the process have restricted the progress of experimentaltechniques and applications of materials. With the development of ultra-fast pulse laser,several techniques have been used to determine the two-photon absorption crosssections of materials for more than three decades. They include transmission methods,such as nonlinear transmission, Z-scan techniques, and excited-state methods, such astwo-photon induced fluorescence. In recent years, a new method was presented byOulianov etc. , which is based on two-photon transient absorption spectroscopy.The basic idea of two-photon induced fluorescence method is to investigate thedependence of the fluorescence intensity on the excitation intensity, which adapt tofluorescent molecules. The relationship between transmission and input laser intensitywas measured in the transmission method. According to the relationship, two-photonabsorption cross section can be evaluated. The method of Z-scan technique is anincreasing popular method for the measurement of optical nonlinearity, particularly fornonlinear refraction and nonlinear absorption. For measurements of two-photonabsorption, open-hole Z-scan technique was used. In this technique, the sample ismoved along the optic axis through the focus of a single laser beam, while the totalpower transmitted by the nonlinear medium is recorded as a function of sample position,thus the coefficient of two-photon absorption can be evaluated.Based on the theoretical and empirical study of the two-photon absorption process,we have accomplished the measurements of two-photon absorption coefficients andcross-sections of many organic nonlinear optical materials by the two-photon inducedfluorescence method, transmission method and Z-scan technique. In the experiments,we used a femtosecond pulse laser system as excited source, which is the product ofAmerican Spectra-Physics Company.The thesis includes:1. Review the history and development of research on two-photon absorption,introduce the fundamentals of two-photon absorption and several importantapplications. Also the main contents of this dissertation is outlined.2. In the second section , the theory of two-photon absorption process andtwo-photon absorption cross-section was mentioned. Then the femtosecondpulse laser system used in our experiments was introduced.3. Two-photon induced fluorescence method was used in the two-photonabsorption cross-section measurements. The experiments results anddiscussion were presented.4. Nonlinear transmission method was used in the two-photon absorptioncross-section measurements. The experiments results and discussion werepresented.5. Z-scan technique was investigated and experimental design was presented.