A University of Nanyang Technological University team, has  show the broadband all-angle negative refraction of highly squeezed hyperbolic polaritons in 2D materials in the infrared regime, by utilizing the naturally hyperbolic 2D materials or the hyperbolic metasurfaces based on nanostructured 2D materials (e.g., graphene). The paper entitled “Broadband Negative Refraction of Highly Squeezed Hyperbolic Polaritons in 2D Materials” was recently published in Research (Research 2018 Article ID: 2532819 DOI: 10.1155/2018/2532819) https://spj.sciencemag.org/research/2018/2532819/.

Light refraction is a common optical phenomenon that occurs when an optical ray propagates from one medium to another through an interface. In general, refraction is positive—the incident ray and the refracted ray stay on opposite sides of the normal to the interface, as required by the so-called Snell’s law—but this is not always correct. Predicted in 1968 but only realized in early this century, negative refraction refers to the counterintuitive phenomenon in which the incident ray and the refracted ray stay on the same side of the normal to the interface. Strikingly, such negative refraction might lead to a superlens that can distinguish extremely small features, being possible to observe behaviors of living DNA and viruses in real time.

Apparently, the shorter wavelength for negative refraction, the finer features a superlens can see. It is thus desirable to achieve negative refraction for highly squeezed polaritons, such as graphene plasmons, with their wavelength squeezed by a factor over 100 compared to free-space photons. A research team led by Baile Zhang from Nanyang Technological University has proposed negative refraction for highly squeezed polaritons on 2D materials (Figure 1), in a wide bandwidth. Previously, the same team, collaborated with MIT, has proposed similar phenomena on the platform of graphene–boron nitride heterostructures, but only in a very narrow bandwith (<1 THz). Now they have switched to a new platform utilizing the naturally hyperbolic 2D materials or the hyperbolic metasurfaces based on nanostructured 2D materials such as graphene, which can in principle achieve negative refraction with the working bandwidth varying from several tens of THz to over a hundred of THz by simply tuning the chemical potential of graphene.

Figure 1  Negative refraction of highly squeezed polaritons supported by 2D hyperbolic materials

This research has revealed a viable way to realize all-angle negative refraction of highly squeezed polariton in a broadband infrared region. In view of the broad class of 2D materials, it is likely to envision more opportunities enabled by negative refraction for the manipulation of light-matter interaction at the extreme nanoscale and for the design of highly compact nano devices and circuits.

Tag: Applied physics