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Photoacoustic spectroscopic analysis allows for evaluating biological microscopic features, such as morphology and viscoelasticity, which offers the opportunities of comprehensively understanding the biological specimens. However, an ongoing challenge arises from inadequate response to the photoacoustic impulses owing to limited bandwidth of the piezoelectric transducer. Here, we develop spectroscopically resolved optical-resolution photoacoustic microscopy (OR-PAM) by incorporating a broadband surface plasmon resonance sensor (∼169.5-MHz bandwidth) as the ultrasonic detector. The photoacoustic spectra from polystyrene and polymethyl methacrylate microspheres represent the close dependence upon the density and sound speed in addition to the diameter. The photoacoustic structural images of the two types of microspheres with the same morphology manifest the nearly identical appearances, while the photoacoustic spectroscopic analysis permits them to be clearly distinguished because of the different densities and sound speeds. The results suggest that, by simultaneously revealing the anatomic structures and acoustic spectra, our spectroscopically resolved OR-PAM system is potentially valuable in characterizing the microscopic features of biological samples at a cellular level.

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