Lens-Antenna Lends Portability to THz On-Chip Technology

A miniature lens-antenna designed for sending and receiving terahertz (THz) signals is compact enough to be used as a portable device in a diverse range of industries, from communications and manufacturing to security, biomedicine, and food safety. While traditional THz systems demonstrate high tunability and strong signal quality, they are commonly too bulky for extensive use or to be considered portable.

“Today, you will find bulky THz devices in laboratory or industrial settings, in some airports, hospitals, and telescopes,” said Shihab Al-Daffaie, a professor at Skolkovo Institute of Science and Technology (Skoltech). “To enable new exciting applications, we need to get THz technologies out of the lab and into the hands and homes of ordinary people. And that means making them radically smaller.”

The Skoltech professor worked with colleagues at the Technical University of Darmstadt and the University of Kassel to develop a THz system platform based on photonic integrated circuits (PICs) and nanotechnology. One of the primary objectives of the Russian-German team was to eliminate the bulky silicon lens (Si-lens), found in conventional THz systems, to achieve better performance and portability. Si-lenses cannot be used as compact THz sources for portable applications.

In addition, they are highly sensitive to misalignments that can occur when mounting the photomixer on the lens.

“Almost 90% of THz devices use the bulky silicon lenses that are about 10 mm in diameter and 6 mm thick,” Al-Daffaie said. “There’s no way you can have a fingertip-size device with that thing, so we found a way to get rid of it.”

The researchers used a Fresnel zone plate to design the lens-antenna and integrate it with nanocontact-based photomixers. They chose a Fresnel zone plate because of its planar structure and directivity characteristics.

The team then replaced the bulky Si-lens with an integrated lens-antenna designed for a nanocontact-based THz photomixer. The system uses vertical nanophotomixers with integrated electronic and air interfaces to integrate the THz source and detector.

Compared with conventional photomixers, the nanocontact-based photomixers showed a reduction in capacitance and an increase in photocurrent.

The performance of the integrated lens-antenna is comparable to a bulky Si-lens, only with a considerable reduction in size in all three dimensions, especially the lens thickness. The flat antenna is just 0.3 mm thick, or about 20× thinner than a conventional Si-lens.

Using two different software programs to simulate the new lens-antenna — Meep, an MIT open-source tool for 2D simulations, and CST Microwave Studio for 3D simulations — the final design showed that with only two fabrication technology processing steps using standard optical lithography, the lens-antenna could be monolithically integrated with the THz nanophotomixer.

“This is more than a reduction in size,” Al-Daffaie said. “We can plant our THz device directly on the flat antenna, seamlessly integrating it into the system. Previously, you would put the device over the lens, sort of like a device within a device — but now we have it on the same platform.”

A European team has reported its design of a very small and flat antenna for receiving and transmitting THz signals. The team believes that the versatility of the device's design will be suitable to enable nanophotomixers to improve the performance of chip-based THz systems. Courtesy of Skoltech.

The new THz “system-on-a-chip” significantly reduces the amount of optical power required, without compromising performance. It eliminates the need for big lasers and enables the transmitting antenna to be placed close to the receiving antenna. “You can put them both on the tip of a pencil,” Al-Daffaie said. With traditional THz technology, two independent devices are needed to send and receive signals, and each device is about 0.5 m wide.

“To get an idea of how such a pencil-size device could be used, consider biocell, liquid, or gas analysis,” Al-Daffaie said. “You could use it to test milk quality, for example. You put a drop between the two antennas. The transmitter sends THz waves to the receiver, which picks up their modulation by the substance undergoing analysis — in this case milk — to infer its composition.

“But remember: THz radiation is also good for wireless communication, so the beauty of this setup is that this same antenna can also relay the analysis results to a smartphone or wherever it is you want them.”

The new lens-antenna has high directivity and less sensitivity to THz source or detector misalignments than the bulky Si-lens. The simple, two-step lithography fabrication process makes it compact and monolithically integrable and reduces the impact of misalignments. Finally, the THz system-on-a-chip could enable portable devices for biomedical, food analysis, and other industries to be driven by integrated nano and terahertz technologies.

The research was published in Scientific Reports (www.doi.org/10.1038/s41598-022-05338-0).