Ultrasound imaging is commonly used in medicine, but also has applications in the non-destructive testing and quality control of industrial parts. The most common way of detecting ultrasound waves uses piezoelectric detectors, which convert the pressure from ultrasound waves into electric voltage. The imaging resolution depends on the size of the detector: The smaller the detector, the higher the resolution. However, the sensitivity of piezoelectric detectors decreases with their size, which limits the miniaturization of such devices. Ultrasound detectors based on optical methods, in contrast, can be miniaturized without sacrificing sensitivity.

Rami Shnaiderman, Vasilis Ntziachristos, Technical University of Munich, Germany, and Helmholtz Zentrum München, Neuherberg, Germany, and colleagues have used such miniaturized photonic circuits to build the smallest ultrasound detector so far, a point-like silicon waveguide–etalon detector (SWED). One such detector has dimensions of 220 nm by 500 nm. Instead of detecting pressure changes like piezoelectric crystals, a SWED measures changes in light intensity that are caused by the ultrasound waves. The SWED consists of a single continuous silicon waveguide. A laser pumps light into the SWED and the ultrasound-induced changes in the reflected optical intensity are measured using a photodiode.

The SWED detector has a per-area sensitivity that is 100 million times higher than that of piezoelectric detectors. It can be used to visualize features that are smaller than one micrometer, which could allow researchers to study ultra-fine details in tissues and materials. Because manufacturing silicon devices is a common and well-developed technology, detectors can be mass-produced at a lower cost than piezoelectric detectors.

• A submicrometre silicon-on-insulator resonator for ultrasound detection,
  Rami Shnaiderman, Georg Wissmeyer, Okan Ülgen, Qutaiba Mustafa, Andriy Chmyrov, Vasilis Ntziachristos,
  Nature 2020.
  https://doi.org/10.1038/s41586-020-2685-y