NTT and Okayama University develop world's first gigahertz ultrasonic circuit based on the topological principle

NTT and Okayama University have realized the world's first gigahertz ultrasonic circuit based on the principle of topology. This technology makes it possible to freely control the flow of ultrasonic waves without being affected by reflections even in the microscopic space on a semiconductor chip. The problem of reflections in a small folded waveguide structure was thereby eliminated, which has been difficult to achieve using conventional technologies. This will make it possible to further miniaturize and improve the performance of ultrasonic filters used in wireless terminals such as smartphones and IoT devices. This result was presented as an invited lecture at META2024 (14th International Conference on Metamaterials, Photonic Crystals and Plasmonics) held in Toyama City, Japan, from July 16 to 19.

Due to the presence of many radio waves, including 5G, and wireless communication and broadcasting waves, wireless terminals such as smartphones need to avoid interference and precisely extract and receive only the desired signal. The device used for this purpose is the ultrasonic filter. Ultrasonic waves are elastic waves that vibrate at frequencies ranging from kilohertz (kHz) to gigahertz (GHz). They are composed of much finer waves than ordinary radio waves and have the outstanding property of extremely low energy leakage outside the element. Therefore, this filter is smaller and more power efficient than those composed of electronic components. Existing ultrasonic filters have only a single filter function for one device.

Meanwhile, topological ultrasonic filter circuits can integrate many filters on a microsubstrate using a fine circuit structure, enabling multiple filter functions per device. Wireless terminals using various communication methods and frequency bands are equipped with multiple ultrasonic filters. For example, top-of-the-line smartphones are equipped with nearly 100 ultrasonic filters, which efficiently transmit and receive signals in different bands. As the IoT society moves toward greater sophistication, more filters will need to be installed in wireless terminals, which will require their miniaturization. Therefore, an ultrasonic circuit that can confine vibrations in a narrow path (waveguide), such as electrical wiring, and direct them in the required direction is needed. However, ultrasonic waves are difficult to bend, and abrupt changes in their direction quickly cause backward reflections, making it difficult to realize fine ultrasonic circuits until now.

From this background, NTT and Okayama University have developed a "topological ultrasonic circuit" that can propagate gigahertz ultrasonic waves with less backward reflections based on the mathematical theory of topology. Ultrasonic waves propagating through this circuit are protected by the topological order created by the shape of the surrounding periodic pores and propagate stably without undergoing reflections. Therefore, regardless of the shape of the waveguide, ultrasonic waves are transmitted smoothly without undergoing reflections.

Using this "topological ultrasonic circuit," they succeeded in miniaturizing an ultrasonic filter to a few hundred square micrometers, less than 1/100th of the size of an ultrasonic filter that would have been several tens of thousands of square micrometers in size if conventional technologies were used. They also demonstrated the basic operation of the gigahertz ultrasonic filter. The technology developed in this research deserves attention as it is a promising technology that will enable the miniaturization, integration, and multifunctionality of ultrasonic filters, which are widely used in wireless terminals. NTT and Okayama University plan to introduce magnetic materials and develop a technology that can dynamically control ultrasonic waves using an external magnetic field.

■ Topology: A branch of mathematics that studies the shape of objects and properties of space. The focus of topology is on the property of an object that the object is not changed even if it is "continuously deformed" by bending and stretching. Topology places importance, not on the "shape of the object," but on how it is connected. For example, a doughnut and a coffee cup, although different in shape, are considered the same because of the way they are connected.