Acoustic imaging modalities such as multispectral optoacoustic tomography (MSOT) have recently matured to a point which allows retrieval of anatomical, molecular and dynamic information with resolutions of several tens of microns
These advancements are going hand in hand with the continuous development of ultrasound detectors. Yet, the traditional piezoelectric transducers have a distinct disadvantage: the detector size is proportional to its sensitivity. This limits miniaturization and prevents the development of point-like detectors as well as the subsequent construction of ultra-dense detector arrays.
Consequently, research has shifted towards all-optical ultrasound detectors, such as Fabry-Pérot resonators, where miniaturization does not affect detector sensitivity. In this context, we present a novel optical resonator on a silicon chip with a sensing area of 220 x 500 nm that - to the best of our knowledge – is the smallest ultrasound detector ever created. These dimensions are 77 and 34 times smaller than the acoustic wavelength at the central detection frequency, hence our detector is truly a point detector.
Using the scalable silicon photonics platform we constructed an array of eight detectors. The archived density of 125 detectors/mm2 is larger by orders of magnitude compared to arrays of piezoelectric and capacitive micromachined ultrasound transducers.
We describe the working principle of the detector and characterize its sensitivity, spatial response, and bandwidth. We demonstrate its applicability for optoacoustic tomography and perform the first SOI based tomography ever reported, by imaging micron-sized phantoms at light fluences well below the ANSI limit for human skin.