A holey-structured metamaterial for acoustic deep-subwavelength imaging

For classical waves such as light or sound, diffraction sets a natural limit on how finely the details of an object can be recorded on its image. Recently, various optical superlenses based on the metamaterials concept have shown the possibility of overcoming the diffraction limit1–7. Similar two-dimensional (2D) acoustic hyperlens designs have also been explored8–10. Here we demonstrate a 3D holey-structured metamaterial that achieves acoustic imaging down to a feature size of λ/50. The evanescent field components of a subwavelength object are efficiently transmitted through the structure as a result of their strong coupling with Fabry–Pérot resonances inside the holey plate. This capability of acoustic imaging at a very deep-subwavelength scale may open the door for a broad range of applications, including medical ultrasonography, underwater sonar and ultrasonic non-destructive evaluation. The design and experimental realization of man-made electromagnetic11–13 and sonic14–16 metamaterials have created many fascinating avenues in optics and acoustics research in recent years. Among these new possibilities, the discovery that a thin slab of artificiallymicrostructuredmetamaterial can act as a perfect lens1 and restore all the evanescent components of a near-field image, has had a profound impact on optical subwavelength imaging, that is, in the search to overcome the so-called diffraction limit. To overcome the same limitation in acoustic imaging, previous studies have described strategies such as time-reversal techniques17,18 and the use of Bragg-scattering in phononic crystals19–26. In the time-reversal approach, the inherently complex set-up reduces its robustness, whereas for lenses based on phononic crystals, the lattice constant must be of the same order as the acoustic wavelength, which results in an imaging resolution that is limited by diffraction. Recently, 2D acoustic lenses based on metamaterials have been explored8–10. To the best of our knowledge, the best resolution achieved with this type of 2D lens is λ/7 (ref. 12). Here we report a new class of 3D acoustic metamaterials that operate as near-field imaging devices. The basic structure, which consists of a rigid block (impenetrable for soundwaves) of thickness h, perforated with deep-subwavelength square holes of side a forming a periodic array with lattice parameter Λ (Fig. 1a), is surrounded by air. Previous work suggests that it may be difficult to conduct 3D optical subwavelength imaging with holey metal structures because the holes must be filled with a material having a very high dielectric constant27. However, because of the absence of a cutoff frequency in the acoustic case, propagation of acoustic waves inside deep-subwavelength-sized apertures is possible and