Originally developed for diagnostic ultrasound imaging, piezoelectric transducers are the most widespread technology employed in optoacoustic (photoacoustic) signal detection. However, the detection requirements of optoacoustic sensing and imaging differ from those of conventional ultrasonography and lead to specifications not sufficiently addressed by piezoelectric detectors. Consequently, interest has shifted to utilizing entirely optical methods for measuring optoacoustic waves. All-optical sound detectors yield a higher signal-to-noise ratio per unit area than piezoelectric detectors and feature wide detection bandwidths that may be more appropriate for optoacoustic applications, enabling several biomedical or industrial applications. Additionally, optical sensing of sound is less sensitive to electromagnetic noise, making it appropriate for a greater spectrum of environments. In this review, we categorize different methods of optical ultrasound detection and discuss key technology trends geared towards the development of all-optical optoacoustic systems. We also review application areas that are enabled by all-optical sound detectors, including interventional imaging, non-contact measurements, magnetoacoustics, and non-destructive testing.

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

A critical link exists between pathological changes of cerebral vasculature and diseases affecting brain function. Microscopic techniques have played an indispensable role in the study of neurovascular anatomy and functions. Yet, investigations are often hindered by suboptimal trade-offs between the spatiotemporal resolution, field-of-view (FOV) and type of contrast offered by the existing optical microscopy techniques. We present a hybrid dual-wavelength optoacoustic (OA) biomicroscope capable of rapid transcranial visualization of large-scale cerebral vascular networks. The system offers 3-dimensional views of the morphology and oxygenation status of the cerebral vasculature with single capillary resolution and a FOV exceeding 6 × 8 mm , thus covering the entire cortical vasculature in mice. The large-scale OA imaging capacity is complemented by simultaneously acquired pulse-echo ultrasound (US) biomicroscopy scans of the mouse skull. The new approach holds great potential to provide better insights into cerebrovascular function and facilitate efficient studies into neurological and vascular abnormalities of the brain.