1. A special material called ultrasonic topological phononic crystal was developed using an artificial elastic structure consisting of periodic arrays of microscopic holes
2. Using this structure, we succeeded in propagating ultrasonic waves stably without backscattering even on complicated and microscopic channels
3. This result is expected to lead to miniaturized and high performance ultrasonic filters that support high-quality wireless communication for smartphones and IoT devices

From the press release:

In recent years, wireless communication technology has been developing rapidly as it’s led by 5G (5th generation mobile communication system), and realized an IoT society in which everything, including humans, home appliances and cars, are connected to the Internet and communicate with each other. To avoid interference from countless radio waves in space, a wireless communication device such as a smartphone must precisely extract and receive only the desired signal. Ultrasonic filters play an important role in this process. Ultrasound refers to waves in which matter vibrates at frequencies between kilohertz (kHz) and gigahertz (GHz). They possess much shorter waves than radio waves and have the excellent property that energy leakage is extremely small. This will realize filters that are much smaller and more energy-efficient than filters made from electronic components.

Wireless communication devices use various communication frequency bands depending on telecommunication methods such as Wi-Fi and Bluetooth, and country or region in which they are used. For example, high-end smartphones are said to contain nearly 100 ultrasonic filters, which allow them to efficiently send and receive signals in different bands. In a more advanced IoT society in the future, more and more filters will be required, and further miniaturization will be important. This requires an ultrasonic circuit that can confine vibrations in a narrow path (waveguide) and guide them in a desired direction, similar to an electric circuit. However, ultrasonic waves are difficult to bend, and sudden changes in direction can quickly cause backscattering, making it difficult to realize fine ultrasonic circuits.

　NTT and Okayama University have newly applied topology¹, a mathematical theory, to realize a topological ultrasonic circuit that can propagate gigahertz ultrasonic waves with reduced backscattering. Ultrasonic waves traveling through this circuit are protected by topological order² created by the shape of the surrounding periodic holes and show stable propagation without reflection. Therefore, regardless of the shape of the waveguide, ultrasonic waves do not reflect and travel smoothly. Using this topological ultrasonic circuit, we succeeded in miniaturizing the size of an ultrasonic filter to hundreds of square micrometers, less than 1/100 of the size of a conventional ultrasonic filter, which is tens of thousands of square micrometers. These results are expected to enable the miniaturization, integration and multifunctionality of ultrasonic filters widely used in wireless communication terminals