Free-Electron Laser-Based Extended Wide-Field Mid-Infrared Photothermal Imaging for Biomedical and Microplastic Analysis.

Wide-field mid-infrared photothermal (MIP) imaging offers rapid label-free chemical contrast for biomedical and polymer analysis. Its field of view (FOV) depends on the mid-infrared pump power of infrared lasers. Here, a wide-field MIP microscope is presented using up to 150 nJ pulse energies of a free-electron laser (FEL) as the pump source to achieve a larger FOV compared to a quantum cascade laser (QCL) excitation with typically 1 nJ pulses. Both implementations use counter-propagating beam paths with a microsecond pulsed 450 nm LED as the probe source and a CMOS camera that records images using a virtual lock-in detection scheme. FEL's higher pulse power expands the FOV by approximately a factor of 20, enabling submicron-resolution wide-field MIP imaging of polystyrene beads, single cells, and a murine brain tissue section. QCL systems with less intense pump pulses achieve only 45 μm FOV for samples including polystyrene beads, Mycobacterium tuberculosis-infected fixed tissue sections, and laryngeal cancer cryosections. IR spectra are reconstructed by tuning FEL and QCL wavelengths and collecting a series of wide-field images. We discuss current challenges and further improvements to implement high-power mid-IR pump lasers and shorter pulse probe sources for wide-field MIP imaging with even larger FOVs in the context of biomedical diagnostics and microplastic screening.