Graphene Can Be Used for Terahertz Hyperlens
Author: Andrei Andryieuski
Affiliation: DTU Fotonik, Technical University of Denmark, Denmark
Andrei Andryieuski and Andrei Lavrinenko, the researchers from the Metamaterials group at Technical University of Denmark (DTU) -- in collaboration with Dmitry Chigrin from the University of Wuppertal (BUW) -- made the first theoretical description of graphene hyperlens, able to work in the terahertz range.
Terahertz radiation, which occupies the spectral range between infrared and microwaves, is harmless to humans and animals and passes easily through many dielectric materials. Terahertz waves, which are able to detect drugs even in sealed vessels, to reveal hidden weapon and to detect cancer tumors, may revolutionize spectroscopy, defense and medical analysis.
Fig. 1. Artistic view of the graphene hyperlens in action. Multiple structured graphene layers resolve and magnify two subwavelength sources.
The wavelength of terahertz radiation is, however, relatively large (300 µm at 1 THz) so the image-quality is seriously limited by the natural diffraction limit. To overcome this limit, artificially structured metamaterial lenses or metallic funnels can be employed. The very fine details hindered in the evanescent waves, which rapidly decay from the radiating object, can be captured by negative index superlens or indefinite medium (hyperbolic dispersion) hyperlens. While negative index metamaterials are still very lossy and far from being employed for practical purposes, the hyperlens present a realistic approach to imaging.
To make the hyperlens a very special material is required. It should behave as a metal in one direction and as an insulator in another. To obtain such properties, thin multiple metal-dielectric layers are normally used in optics. The theoretical concept -- proposed by Evgenii Narimanov’s group from Princeton University (USA) in 2006  -- was checked experimentally by Xiang Zhang’s group from UC Berkeley (USA) in 2007 . To realize the hyperlens the researchers deposited many ultrathin (35 nm) layers of silver and alumina. Even though the metal based hyperlens shows a subwavelength resolution, it is prohibited in tunability; basically, once being fabricated, its properties cannot be changed.
Contrary to metals, graphene, an atomically thin layer of carbon atoms, easily changes its properties under the influence of electrostatic field, magnetic field or chemical doping. This is why the researchers from DTU and BUW decided to employ it for the hyperlens. They propose to construct the hyperlens from narrow (starting from 40 nm) tapered graphene wires embedded into polymer.
Fig. 2. The building block of the hyperlens is the narrow graphene wire embedded into polymer. Arranging such tapered wires radially gives the required material properties for the hyperlens, namely, negative dielectric permittivity in radial direction and positive in azimuthal direction.
Such structured graphene arranged into multiple layers has the very properties needed for hyperbolic dispersion: the wave feels it as metal along the wires and as dielectric perpendicular to the wires. The hyperlens showed subwavelength resolution at the wavelength 50 µm of two sources separated with 10 µm, thus giving the possibility to image the points as close as 1/5 of the wavelength. The resolution of the hyperlens depends on its radii, so with proper selection of geometrical parameter it is possible to separate the images far enough to be captured by the conventional terahertz camera. The hyperlens can be tuned by applying voltage to various graphene layers. It works reciprocally, thus being able not only to image, but also to concentrate terahertz radiation in small volumes.
Fig. 3. Electromagnetic waves emitted by two line sources are captured by the hyperlens and magnified to such extent that they can be resolved with a conventional imaging device.
DTU’s researchers are looking forward to realize and test the graphene hyperlens experimentally.
The paper presenting the above work was published last week in Physical Review B Rapid Communications .
 Zubin Jacob, Leonid V. Alekseyev, and Evgenii Narimanov, “Optical Hyperlens: Far-field imaging beyond the diffraction limit”, Optics Express, 14, 8247 (2006). Abstract.
 Zhaowei Liu, Hyesog Lee, Yi Xiong, Cheng Sun and Xiang Zhang, “Far-field optical hyperlens magnifying sub-diffraction-limited objects”, Science, 315, 1686 (2007). Abstract.
 Andrei Andryieuski, Andrei V. Lavrinenko, Dmitry N. Chigrin, "Graphene hyperlens for terahertz radiation", Physical Review B, 86, 121108(R) (2012). Abstract. Also available at: arXiv:1209.3951.