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PF-SNOM Characterization Technique Reveals 3D Shape of Polariton Interaction Around Nanostructures

The holy grail of novel materials is believed to be nanostructures. For instance, the wonder material - graphene - is a one layer of carbon atoms that are organized in a hexagonal pattern, and thanks to its strength, transparency, conductivity, and flexibility, it can possibly lead to more efficient solar cells, faster and smaller microchips and electric circuits, high-density batteries and capacitors, and transparent displays.

This is an image of Xiaoji Xu, Assistant Professor, Department of Chemistry, Lehigh University. (Image credit: Douglas Benedict/Academic Image)

Another quality that makes graphene and other nanomaterials so unique is their ability to create a physics phenomenon known as a polariton, says Xiaoji Xu, assistant professor in the Department of Chemistry at Lehigh University.

An intense coupling of electromagnetic waves with a magnetic or electric dipole-carrying excitation results in quasiparticles called polaritons. Some refer to this as light-matter coupling. These polaritons allow nanostructures to confine and compress light around the material. For future computing and optical communications, the ability to compress light is very important to scale down devices. In fact, it could lead to sensing at a scale less than one nanometer, which is significant for realizing biomedical advancements in detection, prevention, and treatment of various diseases.

For those who are exploring these materials, the challenge would be to how to expose and define the polaritons at the nanoscale because this cannot be done by traditional microscope, says Xu.

Now, Xu and his co-workers have discovered a technique to expose the 3D shape of the polariton interaction that takes place around a nanostructure. Their method improves upon the standard spectroscopic imaging method called scattering-type scanning near-field optical microscopy (s-SNOM). The new technique, known as peak force scattering-type scanning near-field optical microscopy (PF-SNOM), functions through a combination of time-gated light detection and peak force tapping mode. The scientists have described their work in an article titled:

"Tomographic and multimodal scattering-type scanning near-field optical microscopy with peak force tapping mode" (DOI: 10.1038/s41467-018-04403-5) published online in Nature Communications on May 21, 2018. Besides Xu, Le Wang, Haomin Wang, and Devon S. Jakob, Ph.D. students in Xu's lab, are the paper's co-authors.

The authors state in the paper: "PF-SNOM enables direct sectioning of vertical near-field signals from a sample surface for both three-dimensional near-field imaging and spectroscopic analysis. Tip-induced relaxation of surface phonon polaritons are revealed and modeled by considering tip damping."

According to the team, PF-SNOM also provides an enhanced spatial resolution of 5 nanometers, instead of the usual 10 nanometers provided by the conventional s-SNOM technique.

"Our technique could be beneficial to scientists studying nanostructures enabling them to better understand how the electrical field is distributed around a given nanostructure," says Xu.

The researchers’ PF-SNOM characterization technique is more direct than current methods and at the same time obtains the polaritonic, electrical, and mechanical information. With a single measurement, multiple modes of information can be achieved, which is indeed a special advantage, explains Xu.

The advancement of the PF-SNOM characterization method emerged from the researchers’ analysis of gap mode—when a pair of plasmonic structures approaches within a few nanometers, a large improvement of the plasmon intensity is observed in the gap between both the structures as energy is moved from one structure to the other structure. Thanks to their ability to close this gap mode response in simulations, the team decided to extend it to the non-gap mode as well - when increasing the distance between the sample and the atomic force microscopy (AFM) probe tip.

An interesting fact is that when the researchers started their experiments they anticipated a different result, but during the simulations, they noticed a unique shape of light scattering and observed a clear enhancement of the gap mode.

"It turned out that we could section the light in different tip-samples distances and use those signals to view the near-field response at different layers and in vertical directions," says Wang.

He adds: "Though this work was done with infrared, in principle it could also be extended to other frequencies, such as visible and terahertz."

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