The Research Of The Application Of Femtosecond CARS Spectrum In Molecular Coherent Vibrational Process And Microscopy

Coherent Anti-Stokes Raman Scattering (CARS) is a kind of third order nonlinear effect related to the vibrational energy level of molecule. CARS is a kind of coherent process a nd t hus ha s the a dvantage of hi gh s ignal i ntensity and good di rectivity. Therefore, CARS technology is often used to detect the trace molecule and measure the combustion temperature. By introducing femtosecond time-resolved technology to the CARS spectrum, we could obtain the ultrafast vibrational process of molecule. And By combining t he m icroscopy w ith C ARS s pectrum, w e c ould get the hi gh r esolution micrograph noninvasively. This means that we can get more information of the sample from both time resolved dynamics aspect and spatial structure aspect. This thesis will mainly focus on the two applications of CARS spectrum that have been just mentioned.This dissertation first does some theoretical research on femtosecond time-resolved CARS. By combing coupled wave theory and perturbation method, we obtain the basic expression of t ime r esolved C ARS signal. T hen w e mainly f ocus on t he t hird order susceptibility which is the main factor that determines the CARS signal. We discuss the concrete expression of the third order susceptibility of different resonant condition.After discussing the basic theory of CARS, we construct the time resolved four wave m ixing e xperimental system. T he sys tem expl oits space f iltering, f requency filtering and Lock-in Amplifier technology, which make the system have the advantages of high S/N and stability. The system could also effectively reduce the influence of the drifting of the laser source. We detect the CARS spectrum of alcohol and co-doped solid dye. We preliminarily research the raman energy level of the solid dye. We also prove that by altering the polarization of the incident pulses, we could selectively excite the raman mode of the molecule, which extends the application region of the system, this method may also be very useful in the application of CARS microscopy.Then we used two pulses and three pulses time resolved CARS spectrum to study the ul trafast dyna mic of B BO c rystal, carbon disulfide solution a nd R hodamine 590 solution. BBO crystal has two vibrational energy levels near 3000cm-1 and the energy cannot be transmitted between two levels. We also find that the dephasing time constant T2 due to homogeneous broadening is much longer than the dephasing time constant T2* due to inhomogeneous broadening. Disulfide solution has three vibrational levels near 3000cm-1, and two of them have a very fast energy transfer path. By adding CCl4 to the disulfide solution we find that the weakening of the interaction between CS2 molecules will change the energy structure of the solution and the energy level will shift to the red side. For Rhodamine 590 solution, the dephasing constants T2 and T2* are almost equal. Finally w e do s ome t heoretical research on C ARS microscopy, by us ing a ngle spectrum method w e obt ain t he l ight i ntensity di stribution ne ar t he f ocus of high numerical aperture objectives. We also obtain relationship between the transverse and longitudinal distribution of light intensity near the focus and numerical aperture and the waist radius of the incident pulse. We further research the CARS intensity of every point near the focus, we also obtain the change of the CARS intensity distribution in space with the scatter size.