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The concept of pure optical photoacoustic microscopy(POPAM) was proposed based on optical rastering of a focused
excitation beam and optically sensing the photoacoustic signal using a microring resonator fabricated by a
nanoimprinting technique. After some refinedment of in the resonator structure and mold fabrication, an ultrahigh Q
factor of 3.0×105 was achieved which provided high sensitivity with a noise equivalent detectable pressure(NEDP) value
of 29Pa. This NEDP is much lower than the hundreds of Pascals achieved with existing optical resonant structures such
as etalons, fiber gratings and dielectric multilayer interference filters available for acoustic measurement. The featured
high sensitivity allowed the microring resonator to detect the weak photoacoustic signals from micro- or submicroscale
objects. The inherent superbroad bandwidth of the optical microring resonator combined with an optically focused
scanning beam provided POPAM of high resolution in the axial as well as both lateral directions while the axial
resolution of conventional photoacoustic microscopy (PAM) suffers from the limited bandwidth of PZT detectors.
Furthermore, the broadband microring resonator showed similar sensitivity to that of our most sensitive PZT detector.
The current POPAM system provides a lateral resolution of 5μm and an axial resolution of 8μm, comparable to that
achieved by optical microscopy while presenting the unique contrast of optical absorption and functional information
complementing other optical modalities. The 3D structure of microvasculature, including capillary networks, and even
individual red blood cells have been discerned successfully in the proof-of-concept experiments on mouse bladders ex
vivo and mouse ears in vivo. The potential of approximately GHz bandwidth of the microring resonator also might allow
much higher resolution than shown here in microscopy of optical absorption and acoustic propagation properties at
depths in unfrozen tissue specimens or thicker tissue sections not now imageable with current optical or acoustic
microscopes of comparable resolution.
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