Breakthrough could create True Wide-Spectrum Optical Invisibility Cloak

07/03/2013 - 00:00

Brian Dodson

To make a Harry Potter-style invisibility cloak requires the use of materials that have what's known as a negative refractive index over all optical wavelengths, from red to violet. However, the artificially-structured optical materials from which cloaks are made thus far have been restricted to a very narrow range of optical wavelengths, limiting their ability to cloak over a range of colors. That obstacle to progress looks to be at an end, as a group of optical engineers at Stanford has succeeded in designing a broadband metamaterial that exhibits a negative refractive index over nearly the entire rainbow.


Ref: A Broadband Negative Index Metamaterial at Optical Frequencies. Advanced Optical Materials (1 March 2013) | DOI: 10.1002/adom.201200022

ABSTRACT A broadband metamaterial presenting negative indices across hundreds of nanometers in the visible and near-infrared spectral regimes is demonstrated theoretically, using transformation optics to design the metamaterial constituents. The approach begins with an infinite plasmonic waveguide that supports a broadband but dark (i.e, not easily optically accessed) negative index mode. Conformal mapping of this waveguide to a finite split-ring-resonator-type structure transforms this mode into a bright (i.e, efficiently excited) resonance composed of degenerate electric and magnetic dipoles. A periodic array of such resonators exhibits negative refractive indices at optical frequencies in multiple regions exceeding 200 nm in bandwidth. The metamaterial response is confirmed through simulations of plane-wave refraction through a metamaterial prism. These results illustrate the power of transformation optics for new metamaterial designs and provide a foundation for future broadband metamaterial devices.