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Point-like broadband ultrasound detection can significantly increase the resolution of ultrasonography and optoacoustic (photoacoustic) imaging 1,2 , yet current ultrasound detectors cannot be miniaturised sufficiently. Piezoelectric transducers lose sensitivity quadratically with size reduction 3 , while optical micro-ring resonators 4 and Fabry-Pérot etalons 5 fail to adequately confine light at dimensions smaller than ~50 microns. Micromachining methods have been used to generate arrays of capacitive 6 and piezoelectric 7 transducers, but at bandwidths of only a few MHz and dimensions not smaller than 70 microns. Here we use the widely available silicon-on-insulator (SOI) platform to develop the world's smallest ultrasound detector with a sub-micron sensing area of 220 x 500 nanometers. The SOI-based optical resonator design can provide per-area sensitivity that is 10 4 -fold higher than for micro-ring resonators and 10 8 -fold higher than for piezoelectric detectors. We also demonstrate ultra-wide bandwidth reaching 230 MHz and conduct the first imaging based on an SOI ultrasound detector. The technology showcased is suitable for manufacturing ultra-dense detector arrays (>125 detectors/mm 2 ), which have the potential to revolutionise ultrasonography and optoacoustic imaging.Ultrasound detection based on optical methods has a fundamental advantage over piezoelectric detection because the detectors can be miniaturised without sacrificing sensitivity 3 . One highly miniaturisable approach for ultrasound detection is the use of optical interferometry with a shifted Bragg grating etalon embedded in a fibre waveguide 8 . In this configuration, ultrasound waves perturb an optical cavity established between two Bragg gratings, which act as optical mirrors. The ultrasound waves alter the optical path by changing the cavity's length and refractive index 9 , allowing the waves to be detected. However, such etalons are unattractive for biomedical imaging because their large sensing length (100-300 microns) 9,10 and narrow

Optical and optoacoustic (photoacoustic) microscopy have been recently joined in hybrid implementations that resolve extended tissue contrast compared to each modality alone. Nevertheless, the application of the hybrid technique is limited by the requirement to combine an optical objective with ultrasound detection collecting signal from the same micro-volume. We present an all-optical optoacoustic microscope based on a pi-phase-shifted fiber Bragg grating (π-FBG) with coherence-restored pulsed interferometry (CRPI) used as the interrogation method. The sensor offers an ultra-small footprint and achieved higher sensitivity over piezoelectric transducers of similar size. We characterize the spectral bandwidth of the ultrasound detector and interrogate the imaging performance on phantoms and tissues. We show the first optoacoustic images of biological specimen recorded with π-FBG sensors. We discuss the potential uses of π-FBG sensors based on CRPI.

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