A group of researchers from the California Institute of Technology (Caltech) has engineered a type of artificial optical material – a metamaterial – with a particular three-dimensional structure, such that light exhibits a negative index of refraction upon entering the material.
In other words, this material bends light in the ’wrong’ direction from what would normally be expected, irrespective of the angle of the approaching light.
This new type of negative-index metamaterial (NIM) is simpler than previous NIMs, which used multiple layers of ’resonant elements’ to refract the light.
Instead, it requires only a single layer of silver permeated with waveguide elements that route coupled waves of surface plasmons – or light waves coupled to waves of electrons at the interface between the material’s surface and air – through the material.
Yet it is claimed to be more versatile in that it can handle light with any polarisation over a broad range of incident angles.
’It can do all of this in the blue part of the visible spectrum, making it the first NIM to operate at visible frequencies,’ said graduate student Stanley Burgos, a researcher at the Light-Material Interactions in Energy Conversion Energy Frontier Research Center at Caltech.
’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 the potential enhancement of light collection in solar cells,’ said Prof Harry Atwater, the director of Caltech’s Resnick Institute and leader of the research team.
According to Burgos, not only is the material more feasible to fabricate than those previously used, it also allows for the simple ’tuning’ of the negative-index response; by changing the materials used, or the geometry of the waveguide, the NIM can be tuned to respond to a different wavelength of light coming from nearly any angle with any polarisation.
’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,’ he said.
’The fact that our NIM design is tuneable 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,’ added Atwater. 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’, he said.