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-resolution and nanoparticle detection sensitivity. By using short-wave infrared light for overtone excitation of CH bonds and photothermal probing, our modality can reach millimeter deep into scattering phantom and tissue. With a fast digitization method, we showed that in our setting the photothermal signal amplitude is 100 times larger than photoacoustic thus enabling higher sensitivity. SWIP can resolve intracellular lipids in tumor spheroid and layered structure in the liver, skin, and brain tissue.
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