Study On Coherent Anti-Stokes Raman Scattering (CARS) Microscopy

I introduce the history and the development of the coherent anti-stokes Raman scattering (CARS) in this paper. Also, I review recent advances in CARS microscopy. The intensity formula of CARS is obtained based on Maxwell equation when the plane wave injects. CARS signal generation in a sample under a tight-focusing condition is formulated by the Green's function method. The CARS radiation pattern and the forward- (propagate along the z axis) and backward-detected (propagate along the -z axis) CARS signals from a three-dimensional Raman scatterer are calculated. The coherent nature of CARS image formation and its consequences for image contrast and spatial resolution are investigated.The main contents as following:(1) Using the principle of the nonlinear optics, I have educed the distributions of CARS signals intensity based on the Maxwell equation. The properties of CARS microscopy are calculated by using the slowly varying envelope approximation (SVEA) and Hertzian dipole approximation respectively. I have simulated the field distribution under the loosely focused (low numerical aperture) and tightly focused (high numerical aperture) conditions. The dependence of the CARS Signal on Numerical Aperture is researched in the above two situations respectively. By the calculation, The radiation pattern of the CARS signal from spherical scatterers centered at the focus are obtained. The x-, y- and z-polarized components for a fundamental Gaussian beam under the tight-focusing are confirmed.(2) From the nonlinear dependence of the CARS intensities on the incident intensities, the available excitation volume decreased considerably and the resolution power increases accordingly. Generally, the forward CARS is more intensive than the backward CARS, so the detection of forward CARS is more sensitive. However, the specimen with less volume can emit more intensive CARS, so the epi-detection CARS microscopy will suit to the specimen with little size. Farther more, because of the less Rayleigh scattering and non-resonance background with epi-detection, a higher contrast can be expected. The intensity distributions of CARS signals depend on the shapes and the sizes of specimens, we can estimate the shapes and sizes of the specimens by the CARS distributions in principle.