Isometrically resolved photoacoustic microscopy based on broadband surface plasmon resonance ultrasound sensing.

Photoacoustic microscopy (PAM) can measure optical absorption-based molecular specificities within tissues. Despite the diffraction-limited lateral resolution in optical-resolution photoacoustic microscopy (OR-PAM), the ongoing challenge is poor axial resolution because of an insufficient ultrasound detection bandwidth, which hampers PAM volumetric imaging. We propose a polarization-differential surface plasmon resonance (SPR) sensing for broadband and high-sensitivity photoacoustic (PA) detection, allowing OR-PAM with comparable resolution along lateral and axial directions. This sensor possesses an estimated noise-equivalent-pressure sensitivity of ~477 Pa over an approximately linear pressure response up to 107 kPa. Moreover, an improved PA detection bandwidth of ~173 MHz permits an axial resolution (~7.6 µm) that approaches the lateral resolution (~4.5 µm) of our OR-PAM system. The capability in spatially-isometric micrometer-scale resolution enables in vivo volumetric label-free imaging of the microvasculature of a mouse ear. The SPR sensing technology promises broader applications of PAM in biomedical studies such as microcirculation.