Infrared (IR) spectroscopy depicts molecular structure and dynamics based on vibrational absorption of chemical bonds. Spatially resolved IR spectroscopy, i.e. IR imaging, further enabled label-free in situ chemical imaging for dynamics in complex systems. However, IR imaging suffers from low spatial resolution at a few micrometers due to diffraction limit, thus having difficulty in applications such as sub-cellular imaging. Recently, by visible light probing of the photothermal effect of vibrational absorption, mid-infrared photothermal imaging (MIP) overcomes the limitations of conventional IR microscopy and has achieved sub-micron resolution. In this work, we built an optimized MIP system to boost the spatial resolution and sensitivity, and demonstrated MIP imaging of nanometer-sized polymeric microspheres and living cells with a high spatial resolution of 200 nm.
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