  Infrared (IR) imaging has become a viable tool for visualizing various
chemical bonds in a specimen. The performance, however, is limited in terms of
spatial resolution and imaging speed. Here, instead of measuring the loss of
the IR beam, we utilize a pulsed visible light for a high-throughput, widefield
sensing of the transient photothermal effect induced by absorption of single
mid-IR pulses. To extract such transient signals, we built a virtual lock-in
camera synchronized to the visible probe and IR light pulses with
precisely-controlled delays, allowing sub-microsecond temporal resolution
determined by the probe pulse width. Our widefield photothermal sensing (WPS)
microscope enabled chemical imaging at a speed up to 1250 frames per second,
with high spectral fidelity, and offering sub-micron spatial resolution. With
the capability of imaging living cells and nanometer-scale polymer films, WPS
microscopy opens a new way for high-throughput characterization of biological
and material specimens.
