Nonlinear interference of two successive coherent anti-Stokes Raman scattering signals for the biological imaging applications

Nonlinear interference of two successively generated coherent anti-Stokes Raman scattering (CARS) signals from thin glass slabs is demonstrated, in which a collinear phase matching geometry is tried. We used 76 MHz mode-locked Nd:YAG pulsed laser at 1064 nm and its frequency-doubled optical parametric down-converted signal at 817.2 nm as Stokes and pump beams, respectively. The pulse duration time is around 7 ps for both incident laser beams. The relative phase of two CARS signals is controlled by a phase shifting element made of dispersive glass material of which thickness can be varied. The clear interference fringes are observed as the thickness of the phase shifting element changes. The interference effect is utilized to achieve better CARS image quality. We first try imaging the polystyrene beads immersed in water to estimate how the nonlinear interference could improve the contrast of the beads image. Performing the raster scan of the laser beams on the sample, we can get the CARS image and investigate the image quality as a function of the relative phase and amplitude.