Hyperbolic Metamaterials Enable Nanoscale Fingerprinting

A beam inside the hyperbolic metamaterial changes its direction when the wavelength of the light is swept from 800 nanometers to 1,600 nanometers. (Source: Z. Huang)

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 now, 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 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 scatter out light; this scat­tering is strongest when the narrow beam is directed toward them. “We can measure the scattered light power using a photo­detector and plot the scattered light power versus the wavelength of the incident light,” said Zhengyu Huang, a graduate student at the Univer­sity of Michigan. “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.”

The plot serves as a finger­print, which allows the researchers to determine the distance of a bottom nano-object to be sensed relative to the top nano­particle, as well as the separation between two nano-objects, and their material compo­sition. Gaining access to the nanoscale world via optics has been one of the most vigo­rously pursued frontiers in optics during the past decade. “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.”

Moving beyond this limit and resolving smaller features will require some advanced tech­nologies. “Most are imaging methods, with images containing the objects of interest as the measure­ment,” explained Huang. “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.” Signi­ficantly, it can resolve two objects that are just 20 nanometers apart from each other – well beyond the Abbe limit.

“Our work could potentially find appli­cations in biomo­lecular measure­ment,” Huang said. “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.” (Source: AIP)

Reference: Z. Huang et al.: Nanoscale fingerprinting with hyperbolic metamaterials, APL Photonics 4, 026103 (2019); DOI: 10.1063/1.5079736

Link: Center for Photonic and Multiscale Nanomaterials, University of Michigan, Ann Arbor, USA

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