Physicists in the US have built an acoustic lens from artificial “metamaterials” and used it to focus ultrasound waves to a tight spot. They say that modifying their apparatus could lead to very-high resolution ultrasound imaging, for use in medicine and non–destructive material testing. It could even lead to the development of an acoustic cloak that can be used to hide objects from sonar, says the team.

Several research groups have already been successful at building “superlenses” for electromagnetic waves. Such a device forms an image of an object with perfect resolution, using materials with a negative refractive index to recover the sub–wavelength information about an object that is conveyed by the “evanescent” waves that decay within a very short distance of the object.

Sub-wavelength resolution

Encouraged by this success, researchers are also trying to build the acoustic equivalent. Such a device would allow ultrasound waves to be focused with sub-wavelength resolution, therefore providing much more detailed ultrasound images.

Researchers have already shown how to focus ultrasound waves using negative refractive devices built up from phononic crystals, which contain a series of air gaps that scatter sound in such a way that it refracts in the reverse direction. However, the spacing of the air gaps within such crystals has to be of the same order of magnitude as the wavelength of sound being focused, which would make these devices impracticably large.

Now, Shu Zhang, Leilei Yin and Nicholas Fang of the University of Illinois at Urbana-Champaign have focussed ultrasound using a network of Helmholtz resonators, cavities with short necks that house resonating waves.

Helmholtz resonators

The Illinois device consists of an aluminium plate — 1 cm thick, 15 cm wide and 30 cm long — machined into which are two adjacent 40x40 arrays of Helmholtz resonators, each of which is less than 3 mm in size. The fluid-filled resonators are connected by a network of channels, and in the left-hand array the volume of the resonators is around ten times that of one section of the connecting channels, while in the right–hand array the volume of the channels is some ten times greater than that of the resonators.

The difference in the way that the pressure gradient builds up in these two differently-constructed arrays means that when a sound wave travels through the fluid in the left-hand array it is positively refracted, whereas sound travelling through the right-hand one is instead negatively refracted. Zhang and colleagues were able to demonstrate this by emitting ultrasound waves at 60.5 KHz from a transducer with a 3 mm tip inserted into a hole in the left-hand array.

Converging waves

They then mapped the resulting pressure field in the right-hand array by mounting a hydrophone – which converts pressure differences into electrical signals — onto a mechanical stage and then moved the stage around the array. They found that the converging waves from the left–hand array reached the interface with the right–hand array then reconverged to form an image of the transducer point source with a resolution of half a wavelength of ultrasound in water (about 12 mm at 60 KHz), dimensions that agreed with a computer simulation of the experimental set up.

Fang says that if his team can achieve sub–wavelength imaging then they might be able to reduce the minimum spot size to about 0.1 mm, which he points out is about as small as early-stage tumours, potentially allowing ultrasound diagnosis and therapy of cancer (particularly breast and prostate cancer) much earlier than is currently possible. However, doing so means having a ratio of refractive indices of the two arrays of –1, so that the angles of incidence and refraction are equal for all rays and the rays can therefore be brought to a single focus. This is not possible in the current set up owing to machining errors but Fang believes that these errors can be largely smoothed out.

‘Acoustic cloak’

The researchers say that their device is compact, and therefore practical, because the unit cell of the device is just one eighth of the operating wavelength. They also point out that their flat lens does not require obtaining precise spherical shapes, as is the case with traditional lenses. In addition, the focal length of the lens — the distance from the interface to the focus — can be varied with frequency, allowing superior 3D imaging to conventional ultrasound. Finally, they say that their device could be used as the basis for an “acoustic cloak” that steers sound waves round an object and therefore renders it invisible to sonar.

The team has reported its findings on the arXiv preprint server.