A Versatile Negative Index Metamaterial Design for Visible Light
Stanley P. Burgos (left) and Harry A. Atwater (right)
[This is an invited article based on a recently published work by the authors and their collaborators from the Netherlands. -- 2Physics.com]
Authors: Stanley P. Burgos and Harry A. Atwater
Affiliation: Kavli Nanoscience Institute, California Institute of Technology, USA
Link to ATWATER Research Group >>
Negative index metamaterials (NIMs) are artificial optical materials that cause light to bend in the “wrong" direction and phase fronts to move backwards in time – exactly the opposite of what is observed in naturally-occurring positive index materials . What we have accomplished at the Caltech Light-Matter Interactions -- Energy
Frontier Research Center (LMI-EFRC) is to have developed the first wide-angle negative index material (NIM) operational at visible frequencies.
Our work, reported in Nature Materials on April 18th , presents an innovative design for an artificial material, or metamaterial, with an effective refractive index that is negative and insensitive to the direction and polarization of light over a broad range of angles – a level of isotropy which has not been possible with previous negative index metamaterial designs. By designing a nearly isotropic negative index metamaterial that operates at visible frequencies 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 .
The innovation of our metamaterial design is that the source of the negative index response is fundamentally different from that of previous NIM designs. Whereas other NIM designs use multiple layers of “resonant elements” as the source of the negative index, our design is composed of a single layer of coupled “plasmonic waveguide” elements . The fact that these waveguides are plasmonic allows for easy tuning of the waveguide’s negative index response into the visible simply by tuning the waveguide materials and geometry, and since the characteristic material symmetry is cylindrical, the negative index response is independent of polarization and angle of incidence over a broad range of angles . By carefully engineering the coupling between such waveguide elements, it was possible to develop a material with nearly isotropic refractive index tuned to operate in the visible.
Arrays of coupled plasmonic coaxial waveguides offer a new approach by which to realize negative-index metamaterials that are remarkably insensitive to angle of incidence and polarization in the visible range.
For practical applications, it is very important for a material’s response to be insensitive to both incident angle and polarization. Take eyeglasses for example – in order for them to properly focus light reflected off an object to the back of your eye, they must be able to accept and focus light coming from a broad range of angles, independent of polarization. Said another way, their response must be nearly isotropic. Our metamaterial has the same capabilities in terms of its response to incident light.
This means that our metamaterial design is particularly well suited for use in solar cells. The fact that the design is tunable means that the material’s index response could be tuned to better match the solar spectrum, allowing for the development of broadband wide-angle metamaterials that could enhance light collection in solar cells. 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. For the case of solar cells, this means more light collection and less reflected or “wasted” light.
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