#hyperbolic metasurfaces

[materialsscienceandengineering]

A new way to measure the light-warping properties of hyperbolic metamaterials

Manipulating light in a variety of ways—shrinking its wavelength and allowing it to travel freely in one direction while stopping it cold in another—hyperbolic metamaterials have wide application in optical communications and as nanoparticle sensors. But some of the same optical properties that make these metamaterials so appealing make them frustratingly difficult to evaluate.

For example, the mismatch between the wavelength of incident light, traveling through air, and the much shorter wavelength inside these metamaterials typically prevents the incident light from penetrating very far. That property can be used to create a nanoparticle sensor but poses a problem for measuring just how well a hyperbolic metamaterial performs its light-warping feats, characterized by an electrical property known as permittivity. If light can't probe deeply into a hyperbolic metamaterial, it can't accurately assess the permittivity.

CNST researchers have now developed a new measurement method that circumvents this difficulty. Using an off-the-shelf glass prism to enhance the interaction of incident light with hyperbolic metamaterials, a team led by Cheng Zhang of the CNST and the University of Maryland's NanoCenter and Henri Lezec of NIST has devised a simple and much more accurate way to determine the permittivity.

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[materialsscienceandengineering]

Researchers Develop Metasurface to Manipulate Infrared Light

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.

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