Update: Valentine鈥檚 research has now been published in () and another Berkeley group has published on an alternative 3D visible light metamaterial in ()
A California nanotechnology research lab says it has created the first 3D material able to bend light in the opposite direction to natural materials. But some other specialists in the field remain sceptical about the claim.
Physicists have in recent years made it possible to bend, or refract, light in the opposite direction to any natural materials. These metamaterials make it possible to create invisibility cloaks that hide an object by steering light around it.
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The refractive index of a material is a measure of how it bends light and for natural materials it is always positive. Metamaterials, though, can have negative refractive indexes.
This is achieved with tiny periodic structures that interact with the electric and magnetic fields that comprise light. The repeating structures need to be smaller than the light waves themselves, something that has limited them to long-wavelength light, or microwaves.
Added depth
So far, researchers have created negative refractive-index materials and even an 鈥渋nvisibility cloak鈥� for visible light and microwaves. But they have all been flat, working only in two dimensions.
Now Jason Valentine, a graduate student in the at the University of California at Berkeley, US, claims to have made a 3D metamaterial with a negative refractive index.
Valentine鈥檚 鈥減rism鈥� is made from 21 alternating layers of silver and magnesium fluoride, arranged in a 鈥渇ishnet鈥� structure. He claims that the refractive index is negative in a small region of the near-infrared spectrum.
Gunnar Dolling of the University of Karlsruhe, Germany, has made a flat negative refractive-index material for light on the boundary between red and infrared light. He is unconvinced by Valentine鈥檚 claims, he says.
Light reading
Valentine based his figures on measuring that light was bent backwards by the prism. But Dolling says that he showed last year that, for metamaterials similar to Valentine鈥檚, that method .
Dolling and colleagues found that the complex interactions between light waves and such metal-based metamaterials can deflect a light beam the 鈥渨rong way鈥� without a negative refractive index.
鈥淵ou can only measure a negative refractive index by measuring the phase velocity鈥�, meaning the actual speed of light in the medium, he told New Scientist.
Valentine did not respond to New Scientist鈥榮 enquiries, saying that a future paper would expand on the new prism鈥檚 performance.
A was presented on 8 May at the in San Jose, California, US.
