Whispering-gallery optical microprobe for photoacoustic imaging

Optical ultrasonic probes, exemplified by Fabry–Perot cavities on optical fibers, have small sizes, high sensitivity, and pure optical characteristics, making them highly attractive in high-resolution ultrasonic/photoacoustic imaging, especially in near-field or endoscopic scenarios. Taking a different approach, we demonstrate an ultrasensitive and broadband ultrasound microprobe formed by an optical whispering-gallery-mode polymer microcavity coupled to a U-shaped microfiber. With the high-quality ( Q ) factors (>10 6 ), the noise equivalent pressure of the ultrasound microprobe reaches 1.07 mPa/√Hz with a record broadband response of 150 MHz and a large detection angle of 180°. Our results show that this optical microprobe can overcome the strong decay resulting from ultrasound diverging and medium absorption through short working distances. We further demonstrate high-quality in vivo whole-body photoacoustic imaging of a zebrafish larva. Our implementation provides a new strategy for developing miniature ultrasound detectors and holds great potential for broad applications.