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Bending sound the 'wrong' way sharpens scans

发布时间:2019-03-02 14:15:00来源:未知点击:

By Justin Mullins Ultrasound scans could soon be much more detailed, thanks to a novel material that can bend sound waves the “wrong” way. This property, known as negative refraction, means the material should bring sound waves to a focus far sharper than today’s medical scanners. Negative refraction of light was predicted in the 1970s by the Russian physicist Victor Veselago, but it was not actually achieved until 2000. Then in 2001, John Pendry, a theoretical physicist at Imperial College London, predicted that using negative refraction in lenses could generate images with a much higher resolution than is usually possible (New Scientist print edition, 14 April 2001). Instead of resolving objects down to only around half the size of the wavelength of light being used, he predicted that so-called superlenses could resolve objects just one-tenth of the wavelength. It was not clear that the same ideas would also apply to sound waves, but now John Page and his colleagues at the University of Manitoba in Winnipeg, Canada, have become the first group to demonstrate the negative refraction of sound. The material they used is called a phononic “crystal” – a synthetic structure made of tungsten carbide beads just 0.8 millimetres across, painstakingly packed to form a slab 12 layers thick. The crystal is then immersed in water, where its elastic and acoustic properties mean that any sound with a wavelength similar to the bead size is diffracted as it enters the material. It is this diffraction that leads to various curious properties. For example, physicists have known for some years that phononic crystals can block ultrasound at certain frequencies. In the Manitoba team’s crystal, this happens at frequencies of 0.98 to 1.2 megahertz. However, little work has been done on the properties of phononic crystals at other frequencies. “We knew that various novel effects should be possible, but nobody had looked for them,” says Page. He and his colleagues found that at a slightly higher frequency of 1.57 megahertz the crystal brings sound to a focus by bending it in a chevron pattern (see graphic) so that waves that are initially diverging are brought together. As the wavefront enters and then leaves the material, it bends in the direction opposite from the norm for most materials – hence the term negative refraction. The team also found that to change the focal length of the acoustic lens, all they had to do was change the frequency. “It gives us an entirely new way of focusing ultrasound,” says Page. The team is now working on improving the resolution of the lens in the hope of demonstrating the ultrasound equivalent of superlenses. Page says that the new material could dramatically improve ultrasound scans used in medicine or for finding cracks in buildings, bridges and other structures. The new acoustic lenses are also useful because being flat, they are easier to engineer into low-profile scanning systems. Pendry agrees that Page’s findings have potential,