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Mid-infrared photothermal (MIP) microscopy has been a promising label-freechemical imaging technique for functional characterization of specimens owingto its enhanced spatial resolution and high specificity. Recently developedwide-field MIP imaging modalities have drastically improved speed and enabledhigh-throughput imaging of micron-scale subjects. However, the weakly scatteredsignal from sub-wavelength particles becomes indistinguishable from theshot-noise as a consequence of the strong background light, leading to limitedsensitivity. Here, we demonstrate background-suppressed chemical fingerprintingat a single nanoparticle level by selectively attenuating the reflected lightthrough pupil engineering in the collection path. Our technique provides overthree orders of magnitude background suppression by quasi-darkfieldillumination in epi-configuration without sacrificing lateral resolution. Wedemonstrate 6-fold signal-to-background noise ratio improvement, allowing forsimultaneous detection and discrimination of hundreds of nanoparticles across afield of view of 70 um x 70 um. A comprehensive theoretical framework forphotothermal image formation is provided and experimentally validated with 300and 500~nm PMMA beads. The versatility and utility of our technique aredemonstrated via hyperspectral dark-field MIP imaging of S. aureus and E. colibacteria.

Background-Suppressed High-Throughput Mid-Infrared Photothermal Microscopy via Pupil Engineering