Transparent Broadband Ultrasonic Detector for Functional Photoacoustic Imaging

Functional photoacoustic microscopy (PAM) has been extensively studied for its capability in noninvasive label-free imaging of biological samples in three-dimension (3D) due to its physiologically relevant optical-absorption contrast and high spatial resolutions. The rapid advance motivates the integration of PAM with existing optical imaging modalities for more comprehensive clinical diagnosis and biomedical research, which further requires high performance ultrasound detectors without obstructing the optical path. However, traditional piezoelectric detectors, which are opaque and, sometimes, bulky, are incompatible with constrained physical dimensions in optical microscopic systems. Although it is possible to create opening at the center of a detector to reduce obstruction to the imaging path, their bandwidths and angular sensitivities are still limited. In contrast, using light to demodulate photoacoustic signal provides a better solution, where optical detector can offer broader detection bandwidth in a miniaturized and optically-transparent form. In this paper, we present a systematic study of an optically-transparent ultrasound detector using a polymeric micro-ring resonator on a 250-m thick microscope coverslip. Our analysis formulates the general guideline for the design of optical transducer. The optimal design was further validated experimentally for its key sensing characteristics including sensitivity, bandwidth, angular dependence, and functional imaging capabilities including lateral/axial resolution and saturation limit. Such a transparent ultrasonic detector opens a new window for developing multi-modal optic/photoacoustic imaging systems for both clinical applications and biological researches.