  Optical photothermal microscopy is a powerful, emerging method that overcomes
the diffraction limit in infrared hyperspectral imaging by utilizing a visible
probe laser beam to detect local temperature-induced modulation at the visible
diffraction limit. However, the spectral range of this technique has been
limited by the tuning range of infrared sources, which is generally restricted
to the fingerprint window with commercially available quantum cascade lasers.
In this work, ultra-broadband synchrotron infrared radiation was used for
infrared photothermal imaging and spectroscopy, spanning the entire
mid-infrared range. Both optical- and fluorescence-detected photothermal
modalities were performed, demonstrating improved spectral range when compared
to optical photothermal microscopy using commercial sources and improved
spatial resolution when compared to synchrotron micro-spectroscopy
measurements. Following these initial validation studies, synchrotron
Fourier-transform optical photothermal infrared spectroscopy (FT-OPTIR) in
combination with synchrotron micro-spectroscopy measurements were used to
differentiate cells in mouse brain tissue sections.
