Soda cans focus sound to subwavelength spots

Soda cans focus sound to subwavelength spots. Beating the diffraction limit requires accessing a field’s evanescent waves, which harbor its subwavelength spatial details but don’t propagate—they are stuck to the field’s source. The scanning probe of a near-field microscope can directly access those evanescent waves (see July 2011, page 47). But it’s also possible to directly convert the standing waves to propagating ones by scattering them with a metalens—a cluster of strongly coupled subwavelength resonators—an approach introduced last year by ESPCI ParisTech researchers led by Geoffroy Lerosey. The Paris group has now demonstrated that broadband, audible sounds can be manipulated and focused onto spots as small as 1/25th of their wavelength λ. In a refreshingly low-tech experiment, various tones near 400 Hz from eight loudspeakers surrounding an array of empty soda cans were used to excite resonant modes with spatial variations as small as the diameter of a...