Differential imaging in coherent anti-Stokes Raman scattering microscopy II: a filter-assisted Laguerre-Gaussian beam detection scheme

A new differential imaging technique to obtain contrast improvement in coherent anti-Stokes Raman scattering (CARS) microscopy is proposed through a spatial spiral phase modulation in the collected signal of CARS microscopy. A spiral phase mask makes the CARS signal from the bulk material to be distributed in a ring centered at the detecting pinhole of a confocal microscope resulting in a weak detected CARS signal from a bulk material. When tiny scatters are included in the focal volume of a CARS setup, the ring-shaped distribution of CARS field is distorted, leading to an increase in the detected signal through the pinhole. The sensitivity and the size selectivity of this proposed technique is studied with varying the particle size, and it is found that this method is to be efficient in edge detection. Simulation results obtained by finite difference time domain (FDTD) methods show that the image contrast is enhanced by many times as it is able to highlight the details of the specimen by suppressing the CARS signal from a bulk or a uniform material.