Multiphoton vibrational microscopy is opening a new window to elucidating the rules of life at molecular level. CARS and SRS microscopy has enabled high-speed vibrational imaging of living cells till video rate. However, the ultimate speed and imaging sensitivity are limited by the ultralow cross section of Raman scattering. We report a highly sensitive, high-speed chemical imaging platform based on visible beam sensing of the thermo-refractive effect of infrared absorption. A lab-built resonant circuit is used to extract the photothermal signal at the IR modulation frequency. A virtual lock-in camera is further developed to enable superfast IR-based wide-field chemical imaging at 1250 frames per second. Structural illumination is deployed to further break the diffraction limit of the visible probing beam. With molecular fingerprint information, micro-molar sensitivity and nanoscale spatial resolution, this platform is able to map drug distribution in a pharmaceutical formulation, metabolic activities inside a living cell and further brings IR spectroscopy to in vivo settings.
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