Millimeter-deep micron-resolution vibrational imaging by shortwave infrared photothermal microscopy

Deep-tissue chemical imaging is essential for many biomedical applications. Here, we present a shortwave infrared photothermal (SWIP) microscope for millimeter-deep vibrational imaging with micron lateral resolution. By pumping the overtone transition of carbon-hydrogen bonds and probing the subsequent photothermal lens with shortwave infrared light, SWIP can obtain chemical contrast from one-micron polymer particles located at 800-μm depth in a highly scattering phantom. We demonstrated that SWIP can resolve intracellular lipids across an intact tumor spheroid and the layered structure in thick liver, skin, brain, and breast tissues. SWIP microscopy fills a gap in vibrational imaging with sub-cellular resolution and millimeter-level penetration, which heralds broad potential for life science and clinical applications.