NTT and Okayama University have developed the world’s first gigahertz ultrasonic circuit using the principle of topology. This innovation allows the control of ultrasonic wave flow within microscopic spaces on semiconductor chips, unaffected by reflections.
In recent years, the deployment of wireless communication technology has accelerated, driven largely by the advent of 5G, which enhances connectivity among IoT devices. However, this advanced connectivity brings challenges, such as interference from multiple radio signals. IoT devices, including smartphones, must precisely identify and receive the correct signal amidst these interferences.
This achievement was made possible by creating miniature, energy-efficient, high-performance ultrasonic filters composed of electronic components, using topology, a mathematical theory, to realise a topological ultrasonic circuit that propagates gigahertz ultrasonic waves with reduced backward reflection. This research will be presented at META 2024, the 14th International Conference on Metamaterials, Photonic Crystals and Plasmonics, in Toyama City, from 16-24 July 2024.
Experimental outline
The waveguide structure features two types of topological configurations with periodic holes tilted five degrees counter clockwise or clockwise. When an external ultrasonic wave is applied to the edge of this structure, valley pseudospin is generated, causing ultrasonic wave propagation to proceed unidirectionally along the edge. This phenomenon, known as valley pseudospin-dependent conduction, results in a robust, stable travelling wave protected by topological order. Consequently, even sharp bends do not cause reflections as normal ultrasound waves would, allowing smooth travel along the edge.
Using this characteristic, the team resolved reflection issues in the folded small waveguide structure, previously problematic with conventional technology, enabling the miniaturisation and integration of ultrasonic devices. They constructed a ring/waveguide coupling structure, reducing the space required by conventional technology by more than 100-fold, and demonstrated the basic operation of a gigahertz ultrasonic filter.
This research was supported by the Japan Society for the Promotion of Science (JSPS) Grants-in-Aid for Scientific Research (S) projects: “Development of Ultrasonic Topological Phononics for Multifunctional Elastic Wave Devices” (Project/Area Number: JP21H05020) and “Ultrahigh-speed magnophononic resonator devices” (Project/Area Number: JP23H05463).
Outlook
Future work will introduce magnetic materials to dynamically control ultrasonic waves with an external magnetic field. This capability could enable the integration of high-frequency analogue operations such as frequency converters and amplifiers with ultrasonic filters on a single chip, facilitating further miniaturisation and energy efficiency in antenna systems.
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