Normal refraction in a glass of soda. In the case of negative refraction, the underwater part of the drinking straw would look flipped, as if the straw were bent at an angle. Credit: imagenavi/ Getty Images.
Artificial structures known as metamaterials can bend light, sound and other waves to generate effects not seen in nature. Now physicists in Italy have shown1 how a metamaterial made up of tiny L-shaped holes gives rise to a previously unobserved phenomenon known as asymmetric negative refraction, which they reckon could potentially be exploited in everything from seismic isolators to telecommunication technology.
Metamaterials consist of large numbers of tiny identical resonators laid out in a grid-like pattern. It is the design and arrangement of these resonators that determines the material's response to incoming waves, rather than the objects' chemical composition — leading to novel behaviour when exposed to electromagnetic, acoustic and other types of radiation.
One of the most striking properties of metamaterials is negative refraction. In normal refraction, a wave entering a dense medium at an angle, bends towards what would be the forward direction were it to strike the material head on. The negative version of the phenomenon instead sees the wave bent past the forward direction, or ‘normal’. The result is a partial reversal of the wave’s trajectory.
Simone Zanotto, at the CNR Nanoscience Institute in Pisa, and colleagues at Istituto Officina dei Materiali - CNR in Trieste, the Paul Drude Institut in Berlin and Istituto di Nanoscienze CNR in Pisa, have shown an asymmetric variation on this second effect. They revealed that if a wave enters the metamaterial from one side of the normal it experiences positive refraction, while from the other side it undergoes negative refraction, meaning that it leaves the material along a similar trajectory in both cases.
To look for this effect, Zanotto and colleagues etched an array of L-shaped holes, each just one thousandth of a millimetre long, in a thin piece of the semiconductor gallium arsenide. By applying a fast-varying voltage at one edge of the gallium arsenide, they were able to send a train of mechanical waves across the surface of the metameterial at radiofrequencies. They then used laser pulses to record the very tiny up and down motion of the material caused by the passing waves.
Rather than observing the asymmetric refraction directly, the researchers inferred its existence by measuring an asymmetry in what are known as Bloch modes – constraints imposed on waves when they travel through a periodic array of bumps or holes. Zanotto says that the result provides physicists and engineers with "a new tool” when it comes to manipulating waves.
One possible use of the new knowledge, he suggests, could be in protecting buildings from earthquakes. Concrete pillars placed at specific points in the ground can divert seismic waves around an object, and he reckons that the new asymmetry might help to optimize the position of those pillars.
Smartphone technology might also benefit, according to Chen Shen, of Rowan University, in the United States, who was not involved with the research. Phones rely on tiny piezoelectric devices to convert radio waves to mechanical waves and back again, exploiting the resonant behaviour of those devices to filter out unwanted background noise. By also making use of the asymmetric refraction, explains Shen, these filters could target multiple signals simultaneously – so expanding capacity. "That would be really exciting to further reduce the size of current components in phones or tablets," he says.
