Schematic showing the experimental setup. The sample with PI/NI interface is composed of an array of different designed Helmholtz resonators machined from an
aluminum plate. Unit cells of each half part and the corresponding inductor–capacitor circuit analogy are shown in the insets.
Metamaterials are progressing to enable invisibility to sonar and creating superlenses for ultrasound. Superlenses can enable resolution that is ten to twenties time shorter than the wavelength being used instead of the classic limit of half of a wavelength. Typical ultrasound physics is compared to X-rays here and it lists resolutions at around one millimeter. Currently the sonic superlenses are just at the best classical limit of half of a wavelength but these proof of concepts are expected to be improved. Improved ultrasound resolution would enable simple and cheap higher resolution scans for studies of the brain and other organs for science and diagnostics.
Note: The whole field of metamaterials is relatively new and a several years ago many believed metamaterials to be impossible. They believed that negative indexes of refraction were impossible. They were wrong because we did not really understand how light interacts with matter at small scales and in detail. Why is it then unreasonable to believe that we also have a very incomplete understanding of the details of what is going on with cold fusion and physical reactions on small scales ? How can scientists who now admit that there is real science and possibly new physics being uncovered with cold fusion (low energy nuclear reactions LENR) be so sure that it will not lead to anything important ? By definition of "new physics" they do not know what it is doing or what could be done.
This is the first experimental demonstration of focusing ultrasound waves through a flat acoustic metamaterial lens composed of a planar network of subwavelength Helmholtz resonators. We observed a tight focus of halfwavelength in width at 60.5 KHz by imaging a point source. This result is in excellent agreement with the numerical simulation by transmission line model in which we derived the effective mass density and compressibility. This metamaterial lens also displays variable focal length at different frequencies. Our experiment shows the promise of designing compact and light-weight ultrasound imaging elements.
The resolution of 0.5wavelength was recorded by focusing the acoustic field of a point source. This is not sub diffraction imaging, but among the best achievable passive acoustic imaging elements. The unit cell of the acoustic network is only one eighth of the operating wavelength, making the lens in a compact size. Compared with conventional lenses, the flat thin slab lens takes advantages in that there is no need to manufacture the shapes of spherical curvatures and the focus position is insensitive to the offset of source along the axis. Also this negative index lens offers tunable focal length at different frequencies. More generally, this design approach may lead to novel strategies of acoustic cloak for camouflage under sonar.
Metamaterials are also now able to work in wide band of optical frequencies that are currently the visible range. If the same width of frequencies was shifted into the visible range they would cover the entire visible range.
For the ultrasound metamaterial lenses, it was noted that single PI/NI interface does not allow the enough growth of evanescent fields to achieve sub diffraction focusing while sandwich structure (two PI/NI interfaces) offers better chance to overcome the diffraction limit.