Turning Light Upside Down

An illustration of waves propagating away from a point-like source: Details of a wave propagation on a hyperbolic metasurface. (Source: P. Li, CIC nanoGUNE)

Optical waves propa­gating away from a point source typically exhibit circular convex wave­fronts. The reason of this circular propa­gation is that the medium through which light travels is typi­cally homo­genous and iso­tropic i.e. uniform in all directions. Scientists had already theore­tically predicted that speci­fically structured surfaces can turn the wave­fronts of light upside down when it propa­gates along them. “On such hyber­bolic meta­surfaces the waves emitted from a point source propa­gate only in certain direc­tions and with open concave wave­fronts”, explains Javier Alfaro, PhD student at nanoGUNE. These hyper­bolic surface polari­tons propa­gate only in certain direc­tions, and with wave­lengths that are much smaller than that of light in free space or standard wave­guides, they could help to minia­turize optical devices for sensing and signal proces­sing.

Now, the researchers developed such a meta­surface for infrared light. It is based on boron nitride, a graphene-like 2D material, and was selected because of its capa­bility to mani­pulate infrared light on extremely small length scales, which could be applied for the develop­ment of minia­turized chemical sensors or for heat manage­ment in nano­scale opto­electronic devices. On the other hand, the researchers succeeded to directly observe the concave wave­fronts with a special optical micro­scope, which have been elusive so far.

Hyper­bolic meta­surfaces are chal­lenging to fabri­cate because an extremely precise struc­turing on the nano­meter scale is required. Irene Dolado and Saül Vélez at nanoGUNE mastered this challenge by electron beam litho­graphy and etching of thin flakes of high-quality boron nitride provided by Kansas State Univer­sity. “After several optimi­zation steps, we achieved the required precision and obtained grating structures with gap sizes as small as 25 nm”, Dolado says. “The same fabri­cation methods can also be applied to other materials, which could pave the way to realize arti­ficial meta­surface structures with custom-made optical proper­ties”, adds Saül Vélez.

To see how the waves propa­gate along the meta­surface, the researchers used a state-of the-art infrared nano­imaging technique that was pioneered by the nano­optics group at nanoGUNE. They first placed an infrared gold nanorod onto the meta­surface. “It plays the role of a stone dropped into water”, says Peining Li. The nanorod concen­trates incident infrared light into a tiny spot, which launches waves that then propa­gate along the metasurface. With the help of a scattering-type scanning near-field micro­scope (s-SNOM) the researchers imaged the waves. “It was amazing to see the images. They indeed showed the concave curvature of the wave­fronts that were propa­gating away form the gold nanorod, exactly as predicted by theory”, says Rainer Hillen­brand, Iker­basque Professor at nanoGUNE, who led the work.

The results promise nano­structured 2D materials to become a novel platform for hyber­bolic meta­surface devices and circuits, and further demon­strate how near-field micro­scopy can be applied to unveil exotic optical pheno­mena in aniso­tropic materials and for verifying new meta­surface design prin­ciples. (Source: Elhuyar Found.)

Reference: P. Li et al.: Infrared hyperbolic metasurface based on nanostructured van der Waals materials, Science 359, 892 (2018); DOI: 10.1126/science.aaq1704

Link: CIC nanoGUNE, Donostia–San Sebastián, Spain

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