紧聚焦条件下相干反斯托克斯拉曼散射信号场的矢量分析∗

摘要

In a coherent anti-Stokes Raman scattering (CARS) microscope, when samples with different shapes and dimensions are excitated by collinearly introduced and tightly focused Gaussian beams, the microscopic structure will be determined by the spatial distributions of generated CARS signals. Therefore, we build a theoretical model for CARS signals from spherical sample under the tightly focused condition. The intensity and phase distributions of tightly focused linear polarization Gaussian beams are analyzed with vector wave equations. The vector wave equation of CARS signals is derived from Green’s function. The far-field CARS radiation patterns of spherical scatters with different diameters are simulatively calculated. Theoretical analysis and simulative calculation results show that the intensities of forward and backward CARS signals from the small spherical sampler are similar. The images with high contrast can be obtained by backward detection method from an objective with a high numerical aperture. For big spherical samplers, intensities of CARS signals are greatly increased. The emission direction is mainly concentrated in a spatial angle. The forward CARS signals can be effectively collected by an objective with low numerical aperture.%在相干反斯托克斯拉曼散射(coherent anti-Stokes Raman scattering, CARS)显微镜中,共线传输的紧聚焦高斯光束激发具有不同形状和尺寸的待测样品所产生的CARS信号场的空间分布决定了整体系统的结构特点。建立了紧聚焦条件下球形样品产生CARS信号场的理论模型。利用矢量波动方程分析了紧聚焦条件下线偏振的高斯光束的光场强度和相位分布。利用格林函数求解该模型中CARS信号场的矢量波动方程,模拟计算得到了不同直径球形样品的远场CARS信号场的空间分布。理论分析和模拟计算结果表明,对于小体积的球形样品,前向和背向传输的CARS信号场强度接近,因此采用大数值孔径物镜背向探测方式即可获得高对比度图像。对于大体积球形样品, CARS 信号场的强度大幅增强,且发射方向主要集中在前向的一定立体角内。因此,采用小数值孔径物镜即可有效收集前向传输的CARS信号。