NTT develops first ultrasonic filters to enhance wireless comms

IoT devices, such as smartphones, have to accurately identify and receive the correct signal. NTT Corporation (NTT) and Okayama University have developed the world's first gigahertz ultrasonic circuit utilising the principle of topology. This technology makes it possible to freely control the flow of ultrasonic waves in microscopic spaces on semiconductor chips without being affected by reflections. This breakthrough was achievable by developing miniature, energy-efficient, high performance ultrasonic filters made of electronic components by using topology, a mathematical theory, to realise a topological ultrasonic circuit that can propagate gigahertz ultrasonic waves with reduced backward reflection. This research result will be presented at META 2024, the 14th International Conference on Metamaterials, Photonic Crystals and Plasmonics, held in Toyama City, Toyama Prefecture from July 16-24, 2024.

Background

In recent years, wireless communication technology has been developing rapidly as it’s led by 5G (5th generation mobile communication system), and we have become an IoT society in which everything, including household appliances and cars, are connected to the Internet and communicate with each other. To avoid interference from countless radio waves, a wireless communication terminal 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). It is composed of much finer waves than ordinary radio waves and has the excellent property that the leakage of energy outside the element is extremely small. This makes it possible to realise filters that are much smaller and more energy-efficient than filters made from electronic components.

Figure 1 Schematic diagram of an existing ultrasonic filter (top) and a topological ultrasonic circuit (bottom) Note. Existing ultrasonic filters have a single filter function per device. On the other hand, topological ultrasonic filter circuits can integrate multiple filters on a substrate, which makes it possible to realise multiple filter functions per device.

Wireless communication terminals use various communication bands depending on the communication method such as Wi-Fi and Bluetooth, and the country or region in which they are used. For example, high-end smartphones are said to be equipped with 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 miniaturisation will be important. This requires an ultrasonic circuit that can trap vibrations in a narrow path (waveguide) and guide them in a desired direction, similar to electrical wiring. However, ultrasonic waves are difficult to bend, and sudden changes in direction can quickly cause backward reflection, making it difficult to realise fine ultrasonic circuits.

NTT and Okayama University have newly applied topology¹, a mathematical theory, to realise a topological ultrasonic circuit that can propagate gigahertz ultrasonic waves with reduced backward reflection. 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, they succeeded in miniaturising the size of an ultrasonic filter to hundreds of square micrometres, less than 1/100 of the size of a conventional ultrasonic filter, which is tens of thousands of square micrometres. These results are expected to enable the miniaturisation, integration, and multifunctionality of ultrasonic filters widely used in wireless communication terminals.

Outline of the experiment

The waveguide structure has two types of topological structures consisting of periodic holes tilted five degrees counterclockwise or clockwise. When an external ultrasonic wave is applied to the edge (junction surface) of this structure, valley pseudospins³ is generated, which rotates in opposite directions, and ultrasonic wave propagation proceeds along the edge in one direction.

Figure 2 Schematic diagrams of electron micrograph of fabricated device (a) and ultrasonic waveguide (b), Note. At the boundary (edge) between two regions with different pseudospin rotation directions (i.e., different topologies), there is no backscattering and stable propagation of ultrasonic waves. The internal holes are tilted 5° clockwise (yellow) and counterclockwise (pink).

This phenomenon is called valley pseudospin-dependent conduction, and results in a robust and stable traveling wave protected by topological order. As a result, even if there is a sharp bend, it does not reflect backward like normal ultrasound waves, but instead travels smoothly along the edge.

By utilising this characteristic, they solved the problem of reflection in the folded small waveguide structure, which was difficult with the conventional technology, and miniaturisation and compounding of ultrasonic devices became possible. They fabricated a ring/waveguide coupling structure that reduced the space required by conventional technology to less than 1/100 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) ‘Development of Ultrasonic Topological Phononics for Multifunctional Elastic Wave Devices’ (Project/Area Number: JP21H05020) and the Grant-in-Aid for Scientific Research (S) ‘Ultrahigh-speed magnophononic resonator devices’ (Project/Area Number: JP23H05463).

Figure 3 Numerically simulated spatial evolution of gigahertz ultrasonic waves propagating through a conventional ultrasonic circuit (a) and a topological ultrasonic circuit (b). In conventional circuits, the area where periodic holes are removed is a waveguide, and ultrasonic waves are strongly reflected at the corner bent 120° in the middle. In topological circuits, on the other hand, waves travel smoothly to the exit without backscattering. The periodic hole spacing, and the input ultrasonic frequency of both circuits are similar: 4 micrometers and 0.5 GHz, respectively. (c) Measurements of ultrasonic wave propagation through topological circuits shaped like the letter "Z." The dashed yellow lines indicate the edges.

Figure 4 (a) Frequency response of the waveguide output in the topological ring-waveguide coupled system. There is a filtering effect around 0.495 GHz where the output drops significantly (red arrow). (b) Schematic diagram showing the principle of an ultrasonic filter. The ultrasonic waves circulate in the ring without backscattering, causing interference between the waves in the ring and the waveguide and suppressing the ultrasonic waves traveling through the waveguide. (c) Measured results of spatial propagation of ultrasonic waves at 0.495 GHz. Ultrasonic filtering significantly reduces the ultrasonic output in the waveguide.

Key points of the technology

(1) Application of optimisation design method for topological waveguide structures

Utilising the know-how of Okayama University's acoustic topological structure design, NTT fabricated an ultrasonic circuit consisting of a two-dimensional periodic elastic body in which holes with three-fold rotational symmetry are regularly arranged in a semiconductor thin film. The hole in the unit cell is a combination of four circular holes that can be easily reproduced by micromachining techniques. Furthermore, by simply rotating it counterclockwise or clockwise, an elastic structure with a different topology can be achieved. Okayama University and NTT used a numerical computation technique called the finite element method⁴ to investigate the dispersion relation⁵ of ultrasonic waves for various rotation angles and calculated the optimal rotation angle (5^o) at which topological order and waveguide formation are compatible. By using this optimisation method, excellent topological circuits are constructed without repeating many trials.

(2) Utilisation of valley pseudospin -dependent propagation conduction

The waveguide through which ultrasonic waves travel is made up of junctions (edges) sandwiched between clockwise and counterclockwise  regions with different topologies. The topological structure on both sides has ultrasonic vortices (valley pseudospins) rotating in opposite directions. As a result, ultrasonic waves that penetrate the edge are pushed out by vortices on both sides, and even if there is a bend or defect in the waveguide, they do not reflect backward, maintaining very robust and stable propagation. This characteristic that determines the direction of wave propagation depending on the direction of rotation of the ultrasonic vortex, which is the polarity of the valley pseudospin, is called valley pseudospin-dependent conduction. It has a unique property (topological order) that the propagation of the waveguide edge is protected as long as the shape does not change significantly.

(3) Integration of topological ring and edge waveguide

The ring consists of a closed waveguide in a loop shape, and a wave incident from another waveguide is output by strengthening only the components of a specific frequency and wavelength during the rotation in the ring. This composite structure can be used as a filter because the wave in the ring and the wave in the waveguide interfere to suppress the propagation of the wave at a specific frequency. In the case of gigahertz ultrasound, a large (low curvature) ring structure with a radius of 100 micrometres or more was conventionally required to avoid reflections from the curved waveguide sidewalls. On the other hand, by introducing a topological structure, the influence of reflection is suppressed even in a structure with a large curvature, and a minute ultrasonic ring with a radius of about 10 micrometres can be realised

Role of each institution

NTT: NTT Fabricated artificial acoustic structures called phononic crystals by microfabrication of compound semiconductors such as gallium arsenide. NTT then examined how ultrasonic waves flow inside the laser by measuring changes in the reflected light of the irradiated laser. In addition, the propagation characteristics were numerically evaluated by simulation using the finite element method.

Okayama University: In topology-based ultrasonic wave control, wave propagation characteristics are designed using artificial acoustic structures (phononic crystals). Okayama University has accumulated the know-how for phononic crystal design of various scales and has explored and designed band dispersion and topological order in a wide range from several hertz (Hz) to several terahertz (THz). (Reference DOI: 10.35848/1347-4065/acc6da)

Outlook

In this experiment, the pair established elemental technology for spatially controlling gigahertz ultrasonic waves. In the future, they will introduce magnetic materials and aim to establish technology that can dynamically control ultrasonic waves with an external magnetic field. If spatial and high-speed control of gigahertz ultrasonic waves becomes possible, it will be possible to use ultrasonic waves to process not only filters, but also high-frequency analog operations required for wireless communication terminals such as frequency converters and amplifiers on the same chip. This eliminates the need for interdigital conversion and substrate connection between the ultrasonic filter and the electronic components used in the existing system and is expected to lead to further miniaturisation of the antenna, and energy reduction.

• 1Topology: The branch of mathematics that studies the shape of objects and the properties of space. It focuses on properties that do not change when the object is subjected to "continuous deformation" such as bending and stretching. In other words, topology focuses on how objects are connected, not their shape. For example, a doughnut and a coffee cup are considered the same. They're different shapes, but they're connected in the same way, so you can make a continuous transformation from a doughnut to a coffee cup or vice versa.
• 2Topological order: A new order of matter that introduces the idea of topology into physics. It is a state in which matter is not determined by symmetry disturbances, local defects, or fluctuations in physical variables, but by how the matter is connected. For example, when two materials with different topologies are connected, such as the topological ultrasonic waveguide in this experiment, the conduction state of the ultrasonic wave appears at the junction to change the topology. The local order of the junction does not guarantee the existence of this state but by the nonlocal order of how the two topological structures that form the junctions are connected. This results in stable and robust ultrasonic propagation against structural fluctuations in the waveguide.
• 3Valley pseudospin: A quantum number that typically indicates that an electron or ultrasonic wave is in a particular valley (Valley is a specific region in phase space where the energy bands of electrons and ultrasonic waves dip like a valley). In this experiment, they define the state of ultrasonic waves in the valley. In the valley hole topological structure, there are energy minima called valleys at K and K′ points in momentum space. Ultrasonic waves traveling along the edge belong to either K or K’ valley.
• 4Finite element method: A computational method that divides objects into small parts and analyses their physical behaviour. A tool that accurately simulates phenomena caused by complex geometries and actions.
• 5Dispersion relation: An equation that describes the properties of waves such as ultrasound and light. It mainly shows the relationship between wave frequency and wavelength.