Hyperbolic metamaterials enable nanoscale 'fingerprinting'

Hyperbolic metamaterials are artificially made structures that can be formed by depositing alternating thin layers of a conductor such as silver or graphene onto a substrate. One of their special abilities is supporting the propagation of a very narrow light beam, which can be generated by placing a nanoparticle on its top surface and illuminating it with a laser beam.

It's extremely challenging to realize in practice subwavelength images of unknown and arbitrary objects, but as University of Michigan and Purdue University researchers report in APL Photonics, it isn't always necessary to obtain a full image when something about that object is already known.

"One familiar example from everyday life is the fingerprint," said Theodore B. Norris, at the University of Michigan. "A fingerprint recognition system doesn't need to obtain a complete high-resolution image of the fingerprint—it only needs to recognize it." So Evgenii E. Narimanov, one of the co-authors, began to think about whether nanometer-scale objects could be identified without the need to obtain complete images.

The propagation direction of the beam inside a hyperbolic metamaterial depends on the wavelength of the light. By sweeping the wavelength of the incident light, the narrow beam will scan across the bottom hyperbolic metamaterial and its air interface. If nano-objects are placed near the bottom interface, they scatteroutlight; this scattering is strongest when the narrow beam is directed toward them.