Wow! Cool Laboratory [Researcher Introduction]
Division of Applied Physics,
Research Group of Quantum Matter Physics,
Laboratory of Applied Solid State Physics
Field of research: applied engineering/quantum engineering, acoustic/optical physics of solids
Research themes: time-resolved 2D ultrasonic imaging, phononic crystals, acoustic metamaterials
Characterizing nanoscale structures with high-frequency ultrasonic waves
Research on acoustic metamaterials, poised for application in sound control
2D and 3D imaging with ultrasonic pulses
A frog jumps into a pond, making circular ripples on the water. But ripples are not only seen on the surface of liquids. They can also be generated on solids such as glass or metal if you strike the surface. Ripples on crystal surfaces have complex and strange shapes instead of a simple circular shape. So it is possible to evaluate the structure of crystals from the shape of the ripples. Professor Oliver Wright’s Laboratory of Applied Solid State Physics was the first to capture moving images of sound waves (phonons) traveling on the surface of crystals (note 1).
“Crystal ripples are definitely more beautiful than water ripples. The wave velocity depends on the direction on the crystal, so strange ripple patterns such as squares or stars are created. These patterns can help to test surface acoustic wave filter elements used for selecting frequencies in cell phones.”
After graduating from the University of Oxford, UK, Prof. Wright completed a doctoral degree program at the University of Cambridge. Among other appointments he worked as a researcher in a Japanese company before taking up the post of professor at Hokkaido University in 1996. He specializes in optical physics and acoustic engineering, and his research areas related to imaging are (1) the real-time imaging of surface acoustic waves, (2) ultrasonic technologies for observing nanostructures, and (3) acoustic metamaterials. In particular, the use of picosecond laser pulses allows the ultrasonic evaluation of nanoscale structures without destroying them. Use of this technology can create much higher frequency sound than other ultrasonic techniques, enabling the 2D imaging of surface acoustic waves and the 3D imaging of microstructures (note 2).
“Using our 2D imaging technique we can observe phononic crystals, that is, periodic acoustic structures. We are analysing movies of sound waves traveling on microscopic phononic crystals to clarify of their mechanisms for sound control. In collaboration with researchers in Austria, France and Thailand, we have also successfully visualized high-frequency sound waves guided in channels inside microscopic phononic crystals using an ultrafast imaging technique (note 3).”
“There is a circular whispering gallery in St. Paul’s Cathedral in the UK, in which whispers next to a curved wall can be heard on the other side of the gallery. This is a phenomenon in which sound waves travel along cylindrical walls. We have succeeded in the 2D imaging of sound in a copper whispering gallery that is 100,000 times smaller than that of St. Paul’s. The frequency of sound is approximately 1,000,000 times higher. In this way, you can estimate the size of a microstructure from its vibration frequency (note 4).”
Prof. Wright is also working on a range of themes including 3D animal-cell imaging with picosecond ultrasonics and ultrasonic excitation and detection in nanofibers and nanorings. In these studies, ultrasonic waves with extremely high frequencies can be used to clarify the microscopic world.
Acoustic metamaterials to harness sound
In recent years, Professor Wright has focused on the field of acoustic metamaterials (note 5). Metamaterials refer to artificial materials with a fine structure that show a peculiar response to waves, including electromagnetic, sound and water waves. Typical acoustic metamaterials show negative bulk modulus and negative mass density, and the direction of sound waves can be changed or the sound blocked by tuning the elastic modulus and density.
Acoustic 2D imaging experiments are conducted by irradiating an acoustic metamaterial made of an array of nanosize spheres with a laser pulse to generate vibrations, and then measuring the vibrations with a separate laser pulse to determine the vibrational resonances. In addition, an acoustic metamaterial whose motion can be seen with the naked eye has been created (photo 1). This metamaterial is made up of steel weights connected by springs, making it possible to watch acoustic metamaterials moving in response to specific frequencies.
“Since acoustic metamaterials can be created with a 3D printer, it is expected that they will be put to practical use in the near future. For example, window glass through which sound passes or new types of acoustic lenses and sensors can be made. Also, you should be able to create structures for sound-proof walls or walls in which sound only travels through them in one direction.”
|Note 1.||Jumping frogs (real-time imaging of surface acoustic waves)|
|Note 2||3D acoustic imaging|
|Note 3.||Watching phonons in k-space|
|Note 4.||Whispering gallery|
|Note 5.||Acoustic metamaterials
Structures designed to have a much smaller unit cell than the wavelength of sound. By careful design of the unit cell of an acoustic metamaterial, strange behavior such as acoustically invisible walls or efficient sound transmission through holes smaller than the wavelength of sound can be created.