Theoretical Analysis Of The Coherent Anti-Stokes Raman Scattering Signals Generated Under The Tightly Focused Condition

In a coherent anti-stokes Raman scattering (CARS) microscope, when collinearly introduced and tightly focused Gaussian beams excitated samples with different shapes and dimensions, the microscopic structure will be determined by the spatial distributions of generated CARS signals.In order to better understand the production of the CARS signal and its spatial distribution characteristics of strength under the condition of tightly focused, and theoretical basis and guidance for the structure design and the further system optimization of the CARS microscope, in this paper the exciting field expression of a tightly focused linear polarization fundamental Gaussian beam, the far field spatial intensity expression of the CARS signal and the point spread function of the CARS microscopy imaging are derived. Then we make a more intuitive analysis for the exciting field, the CARS field intensity spatial distribution and the spatial resolution of CARS microscopy imaging through the numerical simulation of the expressions.In this dissertation, the main research work has been shown below.(1)Based on the extensive research, the developments and the state-of-the-art of the CARS spectroscopic and microscopic imaging technique are introduced. Introducing the related basic theory of the CARS spectroscopic and microscopic imaging technique, including Raman scattering and nonlinear optical effect.(2)We develop a theoretical model of CARS signals from spherical sample under the tightly focused condition. The intensity and phase distributions of tightly focused linear polarization Gaussian beam are analyzed by use of vector wave equations. The vector wave equation of CARS signals is derived with Green’s function. The far-field CARS radiation patterns of spherical scatters with different diameters are simulated. The theoretical analysis and simulation results present that the intensities of forward and backward CARS signals from small spherical sampler are similar. The images with high contrast can be obtained by backward detection method by an objective with a high numerical aperture. For big spherical samplers, intensities of CARS signals are greatly increased. The emission direction mainly concentrates in a spatial angle. The forward CARS signals can be effectively collected by an objective with low numerical aperture. Furthermore, The radiation pattern depends not only on the sample size but also on the sample shape. The shape dependence of the signal intensity and the radiation pattern can be used to monitor the tumbling motions or conformation changes of macromolecules with CARS microscopy.(3)The theory of the spatial resolution for the CARS microscopic imaging is introduced, including the meaning and calculation of the point spread function, the definition of spatial resolution, the theory of improving resolution through selective excitation and how the con-focal microscopy can break through the diffraction limit. Finally, we obtain the point spread functions of the CARS spectroscopic and microscopic imaging technique under the condition of the con-focal and the non-con-focal microscopic imaging. The spatial resolution of the CARS microscopy under two conditions is obtained by numerical simulation of the point spread functions. It can be seen that the nonlinear optical can effectively improve the imaging resolution. And by putting a small hole in front of the detector to transform the point spread function can make a further improving of the spatial resolution.