artist rendering of metastructure

Researchers have demonstrated the first nanophotonic microwave imager chip, which could enable the realization of hand-held microwave imagers for various applications. [Image: Ella Maru Studio]

Since gigahertz microwaves have a wavelength range comparable to smartphones or desktop computers, it’s no surprise that most equipment used to create images in that spectral region is similarly bulky. Now, researchers at a U.S. university have harnessed silicon nanophotonics to create a miniaturized near-field imager on a chip (Optica, doi:10.1364/OPTICA.6.001255).

The team at the University of Pennsylvania, USA, fit more than 1,000 photonic components onto a wafer roughly the size of a fingernail clipping. The technique could lead to a new generation of handheld microwave cameras for detecting tumors in tissue or finding hidden objects.

Harnessing silicon photonics

Firooz Aflatouni, a University of Pennsylvania assistant engineering professor, and his colleagues designed a silicon photonic platform that collects microwave signals reflected from the target object, upconverts them to the optical regime and runs them through a network of miniaturized delay lines. These delay lines are silicon waveguides, ranging from 500 nm to 1.2 μm in diameter, that wind back and forth on a silicon dioxide substrate and create a signal delay of 9.8 ps.

According to the research team, the silicon-on-insulator delay lines avoid problems with electromagnetic interference that crop up in benchtop-sized microwave imaging systems. From there, an 11×11 array of silicon-germanium photodiodes picks up the optical signals—121 simultaneous beams—which are further amplified and assembled into a final image.

Pixelated but distinguishable

To test the system, the researchers placed different metallic shapes inside an anechoic chamber and illuminated them with an ultra-wideband microwave source. The resulting images look rather pixelated—after all, the “camera” has only 121 pixels—but the shapes can be distinguished from each other. The device has a spatial resolution of 4.8 degrees and a field of view of roughly 27 degrees.

The Pennsylvania group says that four imager chips could be placed together to form a 22×22 array and, from there, scaled up to larger sizes.