Higher-order coherent anti-Stokes Raman scattering microscopy realizes label-free super-resolution vibrational imaging

Coherent Raman scattering (for example, coherent anti-Stokes Raman scattering (CARS) and stimulated Raman scattering) microscopy has emerged as a powerful tool for label-free biomolecular imaging in biological and biomedical systems, but its spatial resolution is diffraction limited. Here, we report a higher-order coherent anti-Stokes Raman scattering (HO-CARS) microscopy to break the diffraction limit for label-free, super-resolution vibrational imaging. The resolution enhancement of HO-CARS microscopy has been analysed and demonstrated in biological samples (for example, live HeLa and buccal cells). The HO-CARS technique provides an inherent high resonant to non-resonant background ratio compared with conventional CARS microscopy. We affirm that under a tight focusing, the HO-CARS signal originating from the higher-order nonlinear process (χ(5), χ(7)) dominates over the cascaded lower-order nonlinear process (χ(3)), yielding much richer spectroscopic information. This study illustrates that HO-CARS microscopy can be an appealing tool for label-free, super-resolution imaging in biological and biomedical systems with high image contrast.