A new study has revealed that the optical waves or light waves can be turned upside down when they are allowed to propagate through specifically structured surfaces. Normally what happens is that the optical waves emerging out from a point source propagate circularly. That means the optical waves traveling away from a point source characteristically display circular, or convex, wavefronts.
The scientists compared these circular wavefronts to the waves seen on the water surface when a stone is dropped into the water. But the latest study revealed that these circularly propagating light waves’ wavefronts can be turned upside down with the help of a special surface. They developed a new material having a hyperbolic metasurface and successfully inverted the optical waves.
The study was led by Peining Li, an EU Marie Sklodowska-Curie fellow at nanoGUNE. According to him, the reason behind this circular propagation of optical waves is because of the fact that the medium through which light waves propagate is isotropic and homogenous. If the waves are isotropic in nature then their propagation is uniform in all direction and being homogenous means they carry the same characteristics throughout the propagation. But these optical waves can be inverted using specifically structured surfaces like the hyperbolic metasurfaces.
“On such surfaces, called hyperbolic metasurfaces, the waves emitted from a point source propagate only in certain directions and with open (concave) wavefronts,” said Javier Alfaro a Ph.D. student at nanoGUNE and also the co-author of the study. These inverted waves are called as hyperbolic surface polarizations. The hyperbolic metasurfaces do not allow light waves to propagate in a uniform direction and constrain their path. Hence, these inverted light waves travel only in certain directions and have wavelengths smaller than that of light in free space.
According to scientists, these unique set of inverted light waves can be used to design tiny optical devices for sensing and signal processing. For the research, the scientists at nanoGUNE developed the special hyperbolic metasurface using boron nitride, a graphene-like 2D material and allowed infrared light to fall on it. The scientists saw that the hyperbolic metasurface successfully converted the infrared light into hyperbolic surface polarizations. To make the unique hyperbolic metasurface, the scientists inscribed the nanoscale textures into ultra-thin flakes of boron nitride using electron beam lithography technology. Researcher Saul Velez said that 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. The latest study was published in the journal Science.