As wireless technology continues to advance, with smartphones, IoT devices, and the spread of 5G networks, effective signal processing is more and more crucial to ensure device performance remains strong.

In response, researchers from NTT and Okayama University have created the first-ever gigahertz ultrasonic circuit based on topological principles, promising smaller and more efficient radio frequency (RF) filters.

Ultrasonic filters reduce interference...

Ultrasonic filters are especially important in wireless devices, because they allow signals at specific frequencies to pass through while blocking others, which reduces interference. Smartphones, for example, can contain around 100 filters to manage communication bands like Wi-Fi and Bluetooth across various regions. Unlike traditional filters, which rely on electric currents, ultrasonic filters use sound waves, specifically ultrasound. Ultrasound involves vibrations at frequencies from kilohertz (kHz) to gigahertz (GHz), with wavelengths short enough to allow compact designs that lose minimal energy, making for smaller and more efficient filters.

... but they're not easy to make

Building reliable ultrasonic filters on a microscopic scale has been tricky so far. It's been difficult to avoid problems such as wave reflections and backscattering, where waves bounce back and interfere with the intended signal path. NTT and Okayama University's new gigahertz ultrasonic circuit tackles these problems.

Topology to the rescue

Using a field of mathematics called topology, which studies the properties of shapes and how they connect rather than their exact form, researchers created a unique material called an ultrasonic topological phononic crystal, an artificial elastic structure that consists of a grid of microscopic holes, allowing ultrasonic waves to travel without interference, even on complex paths. The structure relies on "valley pseudospin-dependent transport," where ultrasonic waves flow smoothly in one direction, unaffected by curves or obstacles. These unidirectional flows are made possible by "valley pseudospins" in the topological structure. The pseudospins push waves forward as they reach the edges of the grid, keeping them stable even through sharp turns.

A key component of the technology is the topology-based waveguide, which directs waves along a desired path without backscattering. Traditional circuits often struggle with reflections, where waves get scattered at turns, which makes it hard to contain them within tiny waveguides. With the new topological structure, ultrasonic waves can travel in precise paths confined to areas as small as hundreds of square micrometers—1/100th the size of standard ultrasonic filters, which generally span tens of thousands of square micrometers.

To create the new type of filter, researchers used a topological design method, arranging microscopic holes in a two-dimensional grid with a specific rotational pattern. By adjusting the angle of the rotations, they were able to balance the grid's topological properties with its waveguiding capabilities. This configuration protects the waves' direction and stability, ensuring they travel as intended.

Tiny design = easy integration

Additionally, the team designed a composite structure called a topological ring, a closed loop that acts as a filter. Ultrasonic waves in the ring amplify specific frequencies while suppressing others, a feature that allows fine-tuning for specific applications. Until now, ultrasonic filters needed large rings to prevent backscattering; however, the new topological structure now allows for a much smaller design—just 10 micrometers in radius—enabling integration into compact wireless communication devices.

Future radio frequency filters

NTT and Okayama University's research sets a foundation for future RF filtering advancements. Besides enabling smaller filters, the technology may also support other high-frequency processing functions, such as converting and amplifying frequencies, all on a single chip. Additionally, the possibility of adding magnetic materials to these circuits could enable precise control over ultrasonic waves through external magnetic fields. This could lead to RF filters that not only perform more efficiently, but also help save power and reduce antenna sizes in devices.

For further information, please see this link:
https://group.ntt/en/newsrelease/2024/07/22/240722b.html

If you have any questions on the content of this article, please contact:
NTT Science and Core Technology Laboratory Group
Public Relations
nttrd-pr@ml.ntt.com

NTT—Innovating the Future

Daniel O'Connor

Daniel O'Connor joined the NTT Group in 1999 when he began work as the Public Relations Manager of NTT Europe. While in London, he liaised with the local press, created the company's intranet site, wrote technical copy for industry magazines and managed exhibition stands from initial design to finished displays.

Later seconded to the headquarters of NTT Communications in Tokyo, he contributed to the company's first-ever winning of global telecoms awards and the digitalisation of internal company information exchange.

Since 2015 Daniel has created content for the Group's Global Leadership Institute, the One NTT Network and is currently working with NTT R&D teams to grow public understanding of the cutting-edge research undertaken by the NTT Group.