Caltech researchers, led by Harry Atwater, have engineered a type of metamaterial that allows it to bend light in part of the visible spectrum from nearly any angle toward an optimal direction. They report that this artificial optical material can handle light with any polarization over a broad range of incident angles, making it the first negative-index metamaterial to operate at visible frequencies.
So what? Atwater explains, in a Caltech release, that, “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,” he says.
I know from his previous work that Atwater is interested in maximizing the efficiency of solar cells. Recently I did a post on his team’s creation of a 3D photovoltaic array composed of a bed silicon wires given an antireflective coating intermixed with light-scattering particles to surpass convention light absorption techniques. Thus, it appears that Atwater’s group has worked out two methods to improve PV efficiency: increase the amount of light that gets to a PV system, and increase the amount of that light that then gets converted.
A number of other groups are working with metamaterials to achieve negative refractive indexes. The Caltech researchers, however, are using a different and somewhat simpler approach. They only need a single layer of silver permeated with coupled surface plasmonic waveguide elements.
The plasmonic waveguide elements route these coupled waves through the material.
One researcher, grad student Stanley Burgos, says the material can be tuned to respond to a different wavelength of light coming from nearly any angle with any polarization by changing the material or adjusting the geometry of the waveguides. “By carefully engineering the coupling between such waveguide elements, it was possible to develop a material with a nearly isotopic refractive index tuned to operate at visible frequencies,” explains Burgos.
“The fact that our [negative index metamaterial] 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,” says Atwater. “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.”
A paper on the team’s work has been published in Nature Materials.