Professor Hu-Jong Lee, a Step Closer to Realization of Electronic Transparent Cloak
A research team consisting of Dr. Gil-Ho Lee, Geon-Hyoung Park, and Prof. Hu-Jong Lee of the Department of Physics, reported the realization of the negative refraction of electrons across a p-n interface on a graphene sheet, a single atomic layer of graphite. The paper was published in the September 14 online issue of Nature Physics.
The backward bending of lights was predicted several decades ago in materials with a negative refractive index. This negative refraction of lights was realized more than a decade earlier in artificially designed metamaterials with periodic sub-wavelength structures. It is anticipated that this phenomena will lead to the development of a perfectly focusing lens and a fantasy cloak that makes an object invisible. Since electrons are often behave like waves in miniaturized devices, negative refraction of electronic waves has been conceived in electronic materials with ballistic and coherent transmission. An electronic counterpart to optical metamaterials has, however, not been demonstrated, mainly because of the difficulty of creating repeated structures at electron sub-wavelength scales.
Tremendous interest in negative electronic refraction has been rekindled more recently, since it was theoretically proposed [Science 315, 1252 (2007)] that ballistic p-n junctions of graphene can exhibit negative refractive behavior with electrostatic gates giving control of the required local carrier doping. Here, negative refraction results from the fact that the wave vector and wave velocity of carriers can be opposite to each other depending on the carrier type; electron-like or hole-like.
The team demonstrated the first clear realization of the electronic negative refraction in graphene. The existing main obstacles were the high scattering of carriers during propagation in a graphene layer and a p-n potential barrier smoother than the carrier wavelength. The team utilized very recent technological progress in vastly reducing carrier scattering by encapsulating a graphene layer between hexagonal boron nitride crystal flakes in combination with nanofabricated sharp gate edges. Accordingly, they clearly confirmed the predicted backward bending of electronic waves and succeeded in focusing electrons which were spread out from a single point back to a tiny point across a p-n interface, known as the Veselagon lensing effect.
Veselago lensing is one of the most strikingly unusual features sought in studies on the properties of graphene since its prediction. This study confirms great potential for engineering electronic wave propagation in any ballistic and coherent Dirac medium like graphene. This study also demonstrates that graphene promises highly novel components for electronic optics operating at high temperatures close to room temperature. Prof. Lee expresses high confidence that this study will be accepted as a milestone achievement in graphene research and will attract great interest from both the graphene community and those studying metamaterials and optics.
This work was supported by National Research Foundation of Korea (NRF) through the SRC Center for Topological Matter (Grant No. 2011-0030788) and the GRF Center for Advanced Soft Electronics (Grant No. 2011-0031640).