A base of miniaturized photonic circuits, undetectable to the human eye and assembled on top of a silicon chip, has enabled a German research team to develop an ultrasound detector they say is the smallest in existence. The detector is 100× smaller than a human hair (about 0.5 µm) and achieves superresolution imaging at a previously unattainable level.

Known as the silicon waveguide-etalon detector (SWED), the detector monitors changes in light intensity propagating through the minitiaruized photonic circuits instead of recording voltage from piezoelectric crystals. Since its development in the 1950s, core ultrasound detection technology has been heavily reliant on piezoelectric detectors, which convert ultrasound wave pressure into electric voltage. The imaging resolution achieved with the ultrasound depends on the size of the piezoelectric detector in use. Reducing its size increases resolution, offering smaller, densely packed 1D or 2D ultrasound arrays.

These arrays improve the probability of effectively discriminating features in imaged tissue and/or material. Decreasing piezoelectric detector size, however, significantly impairs sensitivity, rendering the detectors unusable for practical application.

If a piezoelectric detector were to be miniaturized to the scale of SWED, it would be 100 million times less sensitive, said Rami Shnaiderman, developer of SWED. Its size corresponds to an area that is at least 10,000× smaller than the smallest piezoelectric detectors used in clinical imaging applications. In addition, SWED is up to 200× smaller than the ultrasound wavelength employed in those applications — a factor that allows it to visualize features less than 1 µm, or superresolution image.

Shnaiderman is part of a team of researchers from Helmholtz Zentrum München and the Technical University of Munich (TUM) that introduced SWED.

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Silicon chip (approx. 3 × 6 mm) with multiple detectors. The fine black engravings on the chip's surface are photonic circuits interconnecting the detectors (and not visible to the naked eye). In the background a larger scale photonic circuit is positioned on a silicon wafer. Courtesy of Helmholtz Zentrum München.

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The silicon platform boasts simple manufacturability, simplifying production processes and lowering potential costs. Though its developers initially theorized and constructed SWED to propel optoacoustic imaging performance, the researchers said they foresee broader sensing and imaging applications.

One such identified application is the investigation of the fundamental properties of ultrasonic waves and their interactions with matter on a scale that was not previously possible.

The researchers’ principal targets, though, are currently applications in clinical diagnostics and basic biomedical research, as well as those in industry settings. A first line of investigation involves superresolution photoacoustic tissue cell imaging and the imaging of tissue microvasculature.

With a clear path to feasible development and manufacturing, the research team and SWED developer(s) are now focusing on device refinement. “We will continue to optimize every parameter of this technology — the sensitivity, the integration of SWED in large arrays, and its implementation in hand-held devices and endoscopes,” Shnaiderman said.

The research was published in Nature (www.doi.org/10.1038/s41586-020-2685-y).

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