Phononic crystals bring sound to a focus

Three years ago researchers at the University of California at San Diego created a storm in the world of optics when they announced that they had made a material that exhibited negative refraction. Light rays entering such a 'negative index' material are not bent towards the normal -- as happens in all known materials in nature -- but beyond the normal. Now physicists in China and Canada have demonstrated the equivalent phenomenon with sound waves in a material known as a 'phononic crystal'. (U.K.)