Three‐color coherent anti‐Stokes Raman scattering with optical nonresonant background removal

Broadband coherent anti‐Stokes Raman scattering (B‐CARS), excited through intrapulse three‐color processes in which both the pump and Stokes photons are derived from a broadband beam, enable diffraction‐limited hyperspectral Raman imaging over the entire fingerprint spectral region with a speed and sensitivity far superior to that possible with conventional spontaneous Raman confocal microspectroscopy. We extend the theory of spectral interferometric polarization coherent anti‐Stokes Raman scattering (SIP‐CARS) to include three‐color excitation processes. This allows for the B‐CARS measurement of polarized Raman‐equivalent spectra free of the nonresonant background, without the requirement for post‐acquisition computational processing. We demonstrate the simultaneous measurement of both three‐color and two‐color SIP‐CARS, at low and high Raman shifts, respectively, in good agreement with theory. These proof of concept results pave the way for photon‐efficient SIP‐CARS signal generation with ultrafast lasers, for all‐optical, fast, quantitative Raman hyperspectral imaging.