Regular readers will know that I love me some sub-diffraction-limited imaging. But even though I hate to admit it, light is not the only thing we use for imaging. You might imagine that I am thinking of electrons or neutrons or something more exotic. But, no, I'm talking about acoustic imaging. It's an oft-forgotten workhorse of the medical and engineering world, not to mention the Earth Sciences. So I would be remiss in not talking about a recent Nature Physics paper that describes sub-diffraction-limited acoustic imaging.

What the authors have published is an acoustic superlens made of metamaterials. In this case, the lens consists of 1600 square brass tubes in a 40x40 array, all clamped in an aluminum tube and held together by that standby of elephant removal men: superglue. The experimental geometry was pretty simple. Place the lens just in front of an object to be imaged and place a loudspeaker on the other side.

In this case, the object was a brass plate with some holes and other shapes milled into it and the sound frequency was around 2kHz. This low frequency, long wavelength sound made it easy to make both the superlens (which required subwavelength structuring) and objects that couldn't be resolved without it—that is objects with features that are closer spaced than the sound wavelength.

Using this setup, they showed that they could image features at something like 50 times smaller than the wavelength.

The basic principle is that each tube acts as a sound resonator, like an organ pipe. Each opening is smaller than the wavelength of sound, but the organ pipes get excited by both propagating sound waves and evanescent waves. Evanescent waves are waves that decay very fast as you move away from the object that created them.

It is these evanescent waves that carry the high spatial frequency information that allows resolutions better than the diffraction-limit. By placing the lens very close to the object, these evanescent waves scatter off the pipe openings and excite their resonant modes. Imaging the output from the lens provides a high resolution image.

It is unfortunate that these results are only a proof-of-principle. The sound wave is too low-frequency to be of much use; at high frequencies, where ultrasound imaging might be useful, the feature size of the superlens and the material losses become hard to manage. Additionally, being limited to near-field imaging means that only surface phenomena can be imaged. This is still useful, of course, but you should not anticipate getting a clearer picture of a body's internal structures.

Nature Physics, 2010, DOI: 10.1038/NPHYS1804