Pushing the resolution of infrared imaging by mid infrared photothermal microscopy

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.