Metamaterials twist sound

In 2006 John Pendry of Imperial College London and his coworkers tackled a problem straight out of Star Trek—electromagnetic cloaking, the deflection of waves around an object to render it invisible. Within five months, they had a working device, at least for microwaves (see Physics Today, February 2007, page 19 ). The trick was to design artificial structures known as metamaterials and configure them in a given region of space such that their electric permittivity and magnetic permeability produced the optimal refractive-index profile to deflect the waves. Jian-chun Cheng (Nanjing University in China) and his colleagues have now demonstrated that the same principles can be used to manipulate sound waves. The Nanjing metamaterials, though, produce variations not in electric and magnetic properties of the propagation medium but in mass density. As proof of concept, they built a device that rotates an incident broadband acoustic field by an arbitrary angle, so that the waves inside it appear to propagate from a different direction. The device, shown here, consists of nested rings, each composed of numerous plastic rectangles a centimeter tall, that are sandwiched between two plexiglass plates; the rectangles are oriented at angles that produce the position-dependent mass density needed to rotate the incident field. The researchers’ measurements of the acoustic pressure throughout the device’s interior closely matched their simulations. They expect that their approach to controlling acoustic waves will find application in medical ultrasound imaging. (X. Jiang et al., Appl. Phys. Lett., in press.)