Superlens Squeezes Light into Nanospace

An artist’s conception of the superlens compressing a laser beam into more manageable, lower-wavelength electromagnetic oscillations. (Source: D. Sokol, MIPT)

Russian and Danish researchers have made a first-ever experimental obser­vation of a plasmon nanojet. This physical pheno­menon enables nanoscale focusing of light and, theoretically, allows engineers to bypass one of the funda­mental limi­tations of the ordinary con­verging lens. Tight compression of light waves is necessary to use them as signal carriers in compact devices that would work much faster than today’s electronics.

To make photonic devices smaller, engineers require a way to control photons on such a small scale that the light wave itself has to be localized, squeezed into a minimum space. Ideally, the light needs to be focused into a spot smaller than 50% of the original wave­length. While this feat is impossible in the classical optics due to the diffraction limit, modern research has already found several ways around it. And the newly observed plasmon nanojet is likely to become one of them. Now, the team has created a focusing component, or nanolens, capable of converting light into electro­magnetic waves of a special kind, compressing it to 60% of the initial radiation wavelength. This new contraption is made up of a square piece of dielectric material 5 by 5 micro­meters in size and 0.25 micrometers thick. The square particle lies on a thin 0.1-micrometer gold film, next to an etched grating that diffracts light.

Illuminating the grating in the gold film with a laser generates surface plasmon polari­tons, which travel along the metal’s surface. These SPPs are essentially two kinds of waves coupled to each other and propa­gating together. First, there’s the collective oscil­lation of electrons in gold and then there’s also a surface light wave, a polariton. The point of converting light to SPPs is that there are ways to focus them to a greater extent than the initial laser pulse. “One of the mechanisms that enable subwave­length focusing relies on the plasmon nanojet, a phenomenon we have observed in an experiment for the first time,” said Igor Minin of Tomsk Poly­technic Univer­sity.

The scientific expla­nation of why waves undergo compression in the superlens is as follows. “Using computer simu­lations, we figured out the appro­priate dimensions of the dielec­tric particle and the diffraction grid in the gold film. When these para­meters are right, SPPs have different phase velocities at different points in the particle. This causes the wavefront to bend, creating a vortex in the particle and there­fore a region dense with SPPs behind it, which we call a plasmon nanojet,” said Dmitry Ponomarev, a leading researcher at the MIPT Labora­tory of 2D Materials and Nanodevices. The study has demons­trated a new and efficient mechanism for strongly localizing radiation and mani­pulating it on the nanoscale, which is a prere­quisite for densely packing optical components in photonic and plasmonic devices that would operate much faster than conven­tional electronics. (Source: MIPT)

Reference: I. V. Minin et al.: Plasmonic nanojet: an experimental demonstration, Opt. Lett. 45, 3244 (2020); DOI: https://doi.org/10.1364/OL.391861

Link: Center for Photonics and 2D Materials, Moscow Institute of Physics and Technology MIPT, Dolgoprudny, Russia

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