Trapping Light Into Nano-Discs Much Smaller Than Its Wavelength

  • Scientists used 2D boron nitride crystals to build nano-discs that trap light and create polaritons. 
  • These nano-discs concentrate light in thousands of times smaller volume than is possible with any other material. 
  • The technique could be used to detect tiny matter nearby polaritons, including hazardous compounds.

Over the last couple of years, there has been keen interest in nanophotonics in 2D materials due to its new properties and applications. Polaritons in 2D materials (especially in graphene) have attracted much attention as a system for high interaction of light and matter, with a wide range of applications, from sensing and light modulation to configurable antennas.

In case if you don’t know, polaritons are quasiparticles generated by high electromagnetic wave coupling with a magnetic or electric dipole carrying excitation.

Recently, a team of scientists at the Harvard John A. Paulson School of Engineering and Applied Sciences came up with a new method for squeezing infrared light into narrowest spaces. This produces an extreme atomic-scale antenna capable of detecting individual biomolecules. They utilized the power of polaritons – particles responsible for blurring the contrast between matter and light.

How Did They Do It?

The scientists used 2D boron nitride crystals to build nano-discs, which were nearly 200 nm wide and 50 nm high. These 2D crystals behave as micro-resonators: they trap photons (infrared light) and produce polaritons.

When exposed to infrared light, the nano-discs concentrated light in thousands of times smaller volume than is possible with any other material, like glass. At these levels of concentrations, scientists noticed something interesting about the polaritons’ behavior: they were oscillating exactly like water sloshing in a cup. The change in oscillation is directly related to the incident light’s wavelength.

Polaritons oscillating inside nano-discs | Image credit: Harvard SEAS 

If a cup is tipped back-and-forth, the water inside the cup oscillates in a single direction, and if the cup is swirled, the water oscillates in a different direction. This is what happens in polaritons: the discs are to incident photons what a glass is to water.

The 2D crystals of boron nitride can be further squeezed to make nano-discs even smaller. In fact, there isn’t any limit to how small they could be. Moreover, these crystals exhibit small optical losses, which means the light trapped inside the disc continues to oscillate for a quite long duration before settling down. This makes the incident light even more intense.

Reference: ScienceAdvances| doi:10.1126/sciadv.aat7189 | Harvard University

More specifically, researchers examined the interaction of light and matter in hexagonal boron nitride planar nanostructures through scattering-type scanning near-field optical microscopy, photo-induced force microscopy and Fourier transform infrared spectroscopy. And, the results matched with numerical simulations.

Trapping Light Into Nano-Discs of 2D materialNano-discs behave as micro-resonators | Credit: Harvard SEAS

To further concentrate light, scientists placed 2 nano-discs with similar oscillations close to each other (50 nm apart), producing an infrared antenna. The intensity of light raises, as it concentrates in tinnier volume, generating optical fields strong enough to exert significant force on neighbor particles.

Applications

The infrared antenna could be used to identify small matter nearby polaritons. This includes several hazardous compounds like formaldehyde. Also, it’s possible to configure the size and structure of the polaritons, paving the way to advanced infrared biosensors and detectors.

Read: Transparent Materials Can Absorb Light | An Unusual Optical Effect

The future work includes optimizing these incident photons to obtain high intensities so that they can effectively interact with individual molecules to measurable values.

Written by
Varun Kumar

Varun Kumar is a professional science and technology journalist and a big fan of AI, machines, and space exploration. He received a Master's degree in computer science from Indraprastha University. To find out about his latest projects, feel free to directly email him at [email protected] 

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