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Nanoscale Fingerprinting Using Hyperbolic Metamaterials

Hyperbolic metamaterials are structures created artificially by depositing alternating thin layers of a conductor such as graphene or silver onto a substrate. One of their distinct abilities is aiding the propagation of a very narrow light beam, which can be produced by positioning a nanoparticle on its top surface and using a laser beam to illuminate.

It is very hard to achieve in practice subwavelength images of unidentified and random objects, but as scientists from University of Michigan and Purdue University report in APL Photonics, from AIP Publishing, it is not always essential to obtain a complete image when something about that object is previously known.

One familiar example from everyday life is the fingerprint. A fingerprint recognition system doesn’t need to obtain a complete high-resolution image of the fingerprint—it only needs to recognize it.

Theodore B. Norris, Professor, University of Michigan.

So Evgenii E. Narimanov, one of the co-authors, started to ponder about whether nanometer-scale objects could be recognized without the need to acquire whole images.

The propagation direction of the beam within a hyperbolic metamaterial is governed by the wavelength of the light. Sweeping the wavelength of the incident light allows the narrow beam to scan across the bottom hyperbolic metamaterial and its air interface. If nano-objects are positioned near the bottom interface, they disperse out light; this dispersal is strongest when the narrow beam is aimed at them.

We can measure the scattered light power using a photodetector and plot the scattered light power versus the wavelength of the incident light. Such a plot encodes spatial information about the nano-objects through the wavelength of the scattering peak in the plot and encodes their material information through the height of the peak.

Zhengyu Huang, Graduate Student, University of Michigan.

The plot acts as a “fingerprint,” which enables the scientists to define the distance of a bottom nano-object to be sensed in relation to the top nanoparticle, as well as the separation between two nano-objects, and their material properties.

Gaining entry into the nanoscale world via optics has been one of the most strongly researched frontiers in optics over the last 10 years. “The traditional microscope is limited in resolution by the wavelength of light,” said Huang. “And, using a conventional microscope, the smallest feature one can resolve is about 250 nanometers for visible light—also known as the Abbe limit.”

Crossing over this limit and resolving smaller aspects will require some cutting-edge technologies.

Most are imaging methods, with images containing the objects of interest as the measurement. But instead of following the imaging approach, our work demonstrates a novel route to obtain spatial and material information about the microscopic world through the ‘fingerprinting’ process.

Zhengyu Huang, Graduate Student, University of Michigan.

Importantly, it can resolve two objects that are just 20 nm from each other—well over the Abbe limit.

Our work could potentially find applications in biomolecular measurement. People are interested in determining the distance between two biomolecules with nanoscale separation, for example, which can be used to study the interaction between proteins. And our method may also be used for industrial product monitoring to determine whether nanostructured parts were manufactured to specification.

Zhengyu Huang, Graduate Student, University of Michigan.

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