A Newtonian approach to extraordinarily strong negative refraction

An extremely large, negative refractive index is produced in a two-dimensional electron gas by exploiting its kinetic inductance, which is a manifestation of acceleration of the electrons by electromagnetic fields. Taking negative refraction to extremes Metamaterials with a negative refractive index have been at the forefront of photonics research in recent years because they can produce startling effects such as superlensing and cloaking. Here, Yoon et al . demonstrate a 'Newtonian' route to negative refraction that exploits the property of kinetic inertia of electrons in a two-dimensional semiconductor. In this method, electrons are accelerated across an array of metallic strips using microwave radiation. The corresponding refractive index is – 700. Such a large negative index and corresponding wavelength reduction could bring the technology of negative refraction to a drastically miniaturized scale. The authors believe the approach could also be scaled up to design devices displaying negative refraction effects at higher, terahertz frequencies. Metamaterials with negative refractive indices can manipulate electromagnetic waves in unusual ways, and can be used to achieve, for example, sub-diffraction-limit focusing 1 , the bending of light in the ‘wrong’ direction 2 , and reversed Doppler and Cerenkov effects 2 . These counterintuitive and technologically useful behaviours have spurred considerable efforts to synthesize a broad array of negative-index metamaterials with engineered electric, magnetic or optical properties 1 , 2 , 3 , 4 , 5 , 6 , 7 , 8 , 9 , 10 . Here we demonstrate another route to negative refraction by exploiting the inertia of electrons in semiconductor two-dimensional electron gases, collectively accelerated by electromagnetic waves according to Newton’s second law of motion, where this acceleration effect manifests as kinetic inductance 11 , 12 . Using kinetic inductance to attain negative refraction was theoretically proposed for three-dimensional metallic nanoparticles 13 and seen experimentally with surface plasmons on the surface of a three-dimensional metal 14 . The two-dimensional electron gas that we use at cryogenic temperatures has a larger kinetic inductance than three-dimensional metals, leading to extraordinarily strong negative refraction at gigahertz frequencies, with an index as large as −700. This pronounced negative refractive index and the corresponding reduction in the effective wavelength opens a path to miniaturi