Acoustic superlens could cloak objects from sonar

Researchers have been messing about with optical metamaterials and invisibility cloaks for a few years now. And while progress has been rapid, nobody’s going to be fooling Voldemort any time soon.

But the same exotic tricks that apply to light can equally be applied to sound. And potentially more easily too because sound has a longer wavelength. The business parts of acoustic metamaterials should therefore be significantly easier to build than their optical counterparts.

And that’s just what Nicholas Fang and buddies from the University of Illinois at Urbana- Champaign have done: create a flat slab of acoustic metamaterial that focuses sound with a negative refractive index. They’ve even fashioned a design that works as a “superlens” that focuses the so-called evanescent sound waves that form within a single wavelength of the source– a world first apparently.

Fang and co have created an acoustic metamaterial by carving an array of holes into an aluminium sheet and filling the holes with water. The holes then resonate when water moves over them, like wind over the mouth of a bottle.

In theory, superlenses can far outperform the resolution of conventional lenses but Fang’s lens isn’t super just yet: its resolution is only about half the length of the incident waves. But that’s pretty good and among the best that has ever been possible with purely passive focusing elements. And while conventional optics can never beat this kind of resolution, Fang’s superlenses can almost certainly be improved.

Another big advantage is the shape of the lens which is entirely flat and just a few centimetres square. That makes it much easier to make than the spherical optics that have been necessary in the past.

Obviously, the new technique will be handy for medical imaging and nondestructive testing but the authors hint at a more exotic application. They say:

“This design approach may lead to novel strategies of acoustic cloak for camouflage under sonar.

Tantalising! What on Earth could they mean?

Ref: arxiv.org/abs/0903.5101: Focusing Ultrasound with Acoustic Metamaterial Network