A new device has properties that respond to light, sound, and other waves in ways that do not happen in naturally occurring substances. This ultrasonic metamaterial may hold great promise in ultrasound technology, enabling much higher resolution than what is currently available.
A basic element of metamaterial design is a lattice of identical building blocks, each smaller than the wavelength of the light or sound wave with which the material is designed to interact. As a result, when waves move through the material they do not see the individual blocks, but respond to the material as a whole, as if it were a homogenous substance.
The material, designed by Xiang Zhang, chancellor’s professor in mechanical engineering at the University of California, Berkeley, and his colleagues, consists of a series of water-filled chambers connected by a long channel built into a bar of aluminum. Known as Helmholtz resonators, the rigid-walled, narrow-mouthed chambers vibrate in response to sound waves at a certain frequency.
Designed to respond to 30 kHz sound waves moving through water, each chamber in the aluminum is a little smaller than a pencil’s eraser. The chambers are spaced at 9.2 mm, one-fifth the length of one 30 kHz sound wave. As sound waves pass through the water-filled channel, a significant amount of their energy gets stored in the connected chambers.
“There is a natural frequency that determines the tone of a resonator,” says Nicholas Fang, who designed the metamaterial when he was a postdoctoral researcher in Zhang’s lab. “In this material, we are trying to excite the resonators with a tone higher than the one that they are tuned to. And because there are so many resonators in the series all tuned to the same frequency, every one lags just a bit behind the other.”
This causes various phenomena to happen. As opposed to natural materials that compress when a force, such as a sound wave, is applied to them, the metamaterial expands. This response, called negative modulus, occurs when the fluid in the neck of the resonators oscillates in and out, causing the fluid in the chambers to spread apart and push into its walls. The response makes it seem as if the sound wave is propagating backward instead of moving forward. The materials support sound waves that are shorter and finer than sound waves that propagate through any other material.
The result is that “basically, the resonators work together, supporting a much higher modulation of the acoustic wave,” says Fang. “They are reacting as a very precise ruler, allowing us to measure the finer features of the wave.”
This ability provides the basis for the material’s usefulness in ultrasound imaging. One of the factors limiting resolution quality of sonograms is the ability of the ultrasound lens to capture sound waves. Currently these lenses are made with elastic materials such as polymers. The elasticity of the materials is what allows them to capture and focus the waves. There is a limit to resolution quality, however.
“With this new material with a negative modulus, all the limits can be overcome,” says Fang.
The material is 55 cm long and houses 60 resonators. Presently it can only be used for one frequency and can capture sound from only one direction. The developers plan to create three-dimensional materials that will not only be able to capture sound from every direction, but will also be tunable.
Because its resonators are many times smaller than wavelengths of the sound wave, Zhang says the material can be used to make compact sonar and ultrasonic devices. Conventional lenses in these devices must be at least as large as the waves they are meant to capture. Sonar devices, which use long-length waves, would particularly benefit from the miniaturization.