  Mid-infrared photothermal (MIP) microscopy has been a promising label-free
chemical imaging technique for functional characterization of specimens owing
to its enhanced spatial resolution and high specificity. Recently developed
wide-field MIP imaging modalities have drastically improved speed and enabled
high-throughput imaging of micron-scale subjects. However, the weakly scattered
signal from sub-wavelength particles becomes indistinguishable from the
shot-noise as a consequence of the strong background light, leading to limited
sensitivity. Here, we demonstrate background-suppressed chemical fingerprinting
at a single nanoparticle level by selectively attenuating the reflected light
through pupil engineering in the collection path. Our technique provides over
three orders of magnitude background suppression by quasi-darkfield
illumination in epi-configuration without sacrificing lateral resolution. We
demonstrate 6-fold signal-to-background noise ratio improvement, allowing for
simultaneous detection and discrimination of hundreds of nanoparticles across a
field of view of 70 um x 70 um. A comprehensive theoretical framework for
photothermal image formation is provided and experimentally validated with 300
and 500~nm PMMA beads. The versatility and utility of our technique are
demonstrated via hyperspectral dark-field MIP imaging of S. aureus and E. coli
bacteria.
