"Hyper-Focusing" a Sound Wave

"Hyper-focusing" a sound wave with time-reversed acoustics has been experimentally demonstrated by researchers in France (Julien de Rosny, CNRS/ESPCI/University of Paris, julien.derosny@espci.fr), providing a new way of breaking the so-called "diffraction limit" when imaging an object.

Even when a sound wave is launched by the tiniest nanomachine, it's often difficult or impossible to focus the sound wave down to the size of the machine itself. The same idea holds for any other type of wave, including light.

That's because conventional lenses don't capture a wave at its source, but many wavelengths away, in the "far-field." As a result, the lens cannot focus the wave to a spot smaller than half a wavelength. This roadblock, called the "diffraction limit," usually dictates the smallest details one can see with a common optical microscope and the tiniest circuits that one can carve in a computer chip using light and lenses.

But researchers can surmount the diffraction limit--and achieve higher-resolution microscopes, smaller circuits, and better focused sound--by capturing a wave's "near-field" components, the fields that exist within a wavelength of the source of the sound or light.

Researchers have now demonstrated a new way of breaking the diffraction limit by using "time-reversed" (TR) acoustics, a technique that takes an incident sound wave, produces a backwards-sounding version of it, and sends the reversed version right back to the source of the original sound. However, conventional TR acoustics itself is limited by diffraction, because previous TR devices only captured a sound wave in the far field rather than at the source.

In the new experiment, researchers connect a loudspeaker to a 1.9-mm-thick glass plate. From a 100-micron contact point on the plate, they launch a 5-microsecond-long, 500 KHz sound wave that travels inside the plate and bounces chaotically from many points on the plate's rounded outer boundary. A laser interferometer records the initial wave, including its near-field components, and its trajectory for 1.5 milliseconds.

Using this information, they launch, from the same contact point, a time-reversed version of the original sound wave. The glass plate's boundary, which bounced around the initial wave in a chaotic fashion, acts remarkably as many individual small lenses for the TR wave! It excellently focuses the wave (albeit only its far-field components), and sends it back to the tiny 100-micron spot where the sound originated. However, the focused wave develops an undesirable "diverging" component that spreads out (see figures and animations).

To eliminate this component, the researchers generate the missing TR near-field components at just the right time and this cancels out this diverging component. What's left is the original wave that focuses on the 100-micron contact point with a spot size that's 1/14 of the initial sound's wavelength, 7 times smaller than that allowed by the diffraction limit. (de Rosny and Fink, Physical Review Letters, 16 September.)