Circularly polarized coherent anti-Stokes Raman scattering microscopy.

We demonstrate circularly polarized coherent anti-Stokes Raman scattering (CP-CARS) microscopy that significantly suppresses the nonresonant background for high-contrast vibrational imaging. Circularly polarized pump and Stokes fields with opposite handedness are used to excite CARS signal. In this case, theoretically the nonresonant CARS signal and resonant CARS signal from isotropic media will completely vanish, while the resonant CARS signal from anisotropic structures can still exist. This allows CARS imaging of anisotropic samples with enhanced resonant contrast. Furthermore, we performed CP-CARS imaging on fibroin fibers from silkworm silk, and the results confirmed its effectiveness in background suppression.