  Deep-tissue chemical imaging plays a vital role in biological and medical
applications. Here, we present a shortwave infrared photothermal (SWIP)
microscope for millimeter-deep vibrational imaging with sub-micron lateral
resolution and nanoparticle detection sensitivity. 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
microparticles located millimeter-deep in a highly scattering phantom. By fast
digitization on the optically probed signal, the amplitude of photothermal
signal is shown to be 63 times larger than that of photoacoustic signal, thus
enabling highly sensitive detection of nanoscale objects. SWIP can resolve the
intracellular lipids across an intact tumor spheroid and the layered structure
in millimeter-thick liver, skin, brain, and breast tissues. Together, 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.
