Nanophotonic Transparent Broadband Ultrasonic Detector Enabling longitudinal in vivo Functional Photoacoustic Imaging

Functional photoacoustic microscopy (PAM) has been studied extensively for its unique capability in noninvasive label-free imaging of biological samples in 3D. PAM does not rely on contrast agent to image the optical absorption contrast in biological tissue. It is uniquely suited for measuring several tissue physiological parameters, such as hemoglobin oxygen saturation, that would otherwise remain challenging. However, the commonly used sizeable and opaque piezoelectric ultrasonic detectors featuring limited ultrasound detection bandwidth often impose a serious constraint. To this end, optical-based ultrasonic detection techniques may offer a more desirable solution. Because light oscillates more than five orders of magnitude faster than ultrasonic waves, optical-based detection methods can potentially allow more sensitive ultrasonic detection over a much wider frequency band. Furthermore, the rapid progress in integrated photonic circuits offers an opportunity for creating miniaturized and transparent ultrasonic detectors, which are the necessary features for integrating PAM with optical microscopic systems. In this talk, I will review our effort in developing a coverslip-style optically transparent ultrasound detector based on a polymeric optical micro-ring resonator (MRR). In the early proof-of-concept study, we have demonstrated an optically transparent ultrasound detector with the total thickness of 250 um. It enables highly-sensitive ultrasound detection over a wide receiving angle with a bandwidth from DC to 140 MHz, which corresponds to a photoacoustic saturation limit of 287 cm−1, at an estimated noise-equivalent pressure (NEP) of 6.8 Pa. We also established a theoretical framework to provide general design guideline for optical-based ultrasound detectors. 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. We have further demonstrated the functional integration of PAM with the optical microscope and endoscope, by making use of the transparent MRR detectors. In a recent study, we have successfully integrated the MRR to the inner surface of cranial window, which enables the experimental demonstration of long-term in vivo intravital cortical photoacoustic microscopy of live rodents over a 28-day period.