August 17, 2008 Development in metamaterials - the so called “left handed” composite materials that negatively refract light waves and promise the sci-fi scenario of rendering objects invisible - is accelerating with news this week of two breakthroughs from scientists at the University of California, Berkeley.
We’ve written about metamaterials before, but the focus has primarily been on its cloaking capability. Metamaterials possess a negative refraction index, meaning they can bend electromagnetic waves, including the visible light spectrum, completely around an object. The press have dubbed metamaterials as the enabling technology for “invisibility cloaks”, though scientists have been less enthusiastic to throw around the term, stressing that such an application is, for the moment, well out of reach. Less publicized are the potential applications metamaterials could have in high-resolution optical microscopes, antenna performance, and nanocircuits for high-powered computers – not as exciting as something from Harry Potter, but probably more important.
The scientists led by Xiang Zhang, professor at UC Berkeley's Nanoscale Science and Engineering Center, have engineered 3-D metamaterials that negatively refract visible and near-infrared light for the first time. Unlike 2-D materials, 3-D materials are not restricted to a single monolayer of artificial atoms.
"What we have done is take two very different approaches to the challenge of creating bulk metamaterials that can exhibit negative refraction in optical frequencies," said Xiang Zhang. "Both bring us a major step closer to the development of practical applications for metamaterials."
The two breakthroughs have just reported separately in the journals Nature and Science. The metamaterial described in the Nature paper was created by stacking alternating layers of silver and non-conducting magnesium fluoride. A nanoscale fishnet pattern was then cut into the layers. Each pair of conducting and non-conducting layers creates a circuit – the cumulative effect of the circuits is to bend the magnetic field from incoming light with wavelengths as short as 1500 nanometers.
"Natural materials do not respond to the magnetic field of light, but the metamaterial we created here does," said Jason Valentine, UC Berkeley graduate student and co-lead author of the Nature paper. "It is the first bulk material that can be described as having optical magnetism, so both the electrical and magnetic fields in a light wave move backward in the material."
The metamaterial described in the Science journal is composed of silver nanowires grown inside porous aluminum oxide. The structure is more than 10 times the size of a wavelength of light, making it a bulk metamaterial. This form of metamaterial bent red light wavelengths as short as 660 nanometers, making it the first demonstration of bulk media bending visible light backwards. Unlike other metamaterials, the nanowire material does not technically create a negative index of refraction. For there to be a negative index of refraction in a metamaterial, its values for permittivity - the ability to transmit an electric field - and permeability - how it responds to a magnetic field - must both be negative.
"The geometry of the vertical nanowires, which were equidistant and parallel to each other, were designed to only respond to the electrical field in light waves," said Jie Yao, a student in UC Berkeley's Graduate Program in Applied Science and Technology and co-lead author of the study in Science. "The magnetic field, which oscillates at a perpendicular angle to the electrical field in a light wave, is essentially blind to the upright nanowires, a feature which significantly reduces energy loss."
"What makes both these materials stand out is that they are able to function in a broad spectrum of optical wavelengths with lower energy loss," said Zhang. "We've also opened up a new approach to developing metamaterials by moving away from previous designs that were based upon the physics of resonance. Previous metamaterials in the optical range would need to vibrate at certain frequencies to achieve negative refraction, leading to strong energy absorption. Resonance is not a factor in both the nanowire and fishnet metamaterials."