Metamaterials are man-engineered, artificial structures, exhibiting properties usually not found in nature. Caltech researchers have just discovered a metamaterial with a particular three-dimensional structure that exhibits a negative index of refraction for the light entering it. It simply bends the light in another angle than it would normally be expected, no matter what angle the incident light might have had.
This is the simplest approach so far to this kind of materials, involving only a single functional layer. The metamaterial they invented can direct light with any polarization over a broad range of incident angles. Graduate student Stanley Burgos says that this is “the first negative index metamaterial to operate at visible frequencies”. For the moment, though, the material can do this in the blue part of the visible spectrum.
“By engineering a metamaterial with such properties, we are opening the door to such unusual — but potentially useful — phenomena as superlensing (high-resolution imaging past the diffraction limit), invisibility cloaking, and the synthesis of materials index-matched to air, for potential enhancement of light collection in solar cells,” says Harry Atwater, Howard Hughes Professor and professor of applied physics and materials science, director of Caltech’s Resnick Institute, founding member of the Kavli Nanoscience Institute, and leader of the research team.
There had been other attempts to make negative-index metamaterials with more complicated structures, by using multiple resonant elements to refract light in an unusual way, but this version is only composed of a single layer of silver permeated with “coupled plasmonic waveguide elements” (surface plasmons are light waves coupled to waves of electrons at the interface between a metal and a dielectric, while plasmonic waveguide elements route these coupled waves through the material).
All these facts suggest that Caltech’s new metamaterial is perfectly suited for use in solar cells, where anti-reflective layers are being experimented to keep the light onto the cells’ surface, including a surface plasmons approach, involving a silver coating, like this one we presented just yesterday.
“The fact that our NIM design is tunable means we could potentially tune its index response to better match the solar spectrum, allowing for the development of broadband wide-angle metamaterials that could enhance light collection in solar cells,” explains Atwater. “And the fact that the metamaterial has a wide-angle response is important because it means that it can ‘accept’ light from a broad range of angles. In the case of solar cells, this means more light collection and less reflected or ‘wasted’ light.”
Kylie Catchpole’s thin film solar cells permeated with silver nanoparticles turned out to improve their output by 30% – we’re expecting to see Caltech’s results applied in this field.