Optical waves propagating away from a point source typically exhibit circular (convex) wavefronts. "Like waves on a water surface when a stone is dropped", explains Peining Li, EU Marie Sklodowska-Curie fellow at nanoGUNE and first author of the paper. 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 surfaces, called 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 and co-author of the paper. These unusual waves are called hyperbolic surface polaritons. Because they 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, PhD student at nanoGUNE, and Saül Vélez, former postdoctoral researcher at nanoGUNE (now at ETH Zürich) 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 nanoptics 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 so-called 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.

The research has been mainly funded by individual fellowship grants of the European Union Marie Sklodowsca-Curie Actions and the pre-doctoral research grants program of the Basque and Spanish Governments, as well as by the National Science Foundation (USA), and has been carried out in line with nanoGUNEs projects within the EU's Graphene Flagship.