Nanocomposites for a New Kind of Metalenses

Schematic of the working principle of a disordered metalens made of titanoxide-nanocomposites. (Source: Kazan Fed. U.)

A new metalens made of nano­composite ceramics is a thin composite metal-dielectric film placed on a dielectric substrate; the width is several dozen nanometers. “The light has a wave nature, so there is a diffrac­tion limit which confines the resolution of tradi­tional optical micro­scopy,” explains Kharintsev. “Our metalens is an optical device capable of sur­passing that diffraction limit. Such a solution paves way for using optical technologies in nanoscale integral circuits and sensors.”

The ultra-high resolution is based on an unusual behavior of the metalens in optical and infrared ranges. “The material part of the dielec­tric constant oscillates near zero. This property can be used to enhance sti­mulated Raman scattering of light in a spatially limited medium illu­minated by low-intensity conti­nuous laser light. For most materials found in nature, nonlinear effects are weak, and to observe them it is neces­sary to increase the length of the medium (for example, using optical fibers) and to increase the laser pump power using high-power pulsed lasers”, the researcher said.

“We used a 50 nm thick titanium oxy nitride film as a disordered nonlinear medium. The film was synthesized by magnetron sputtering and subsequent oxidation in air. As a result of a two-stage procedure, metal (TiN) and dielectric nano­particles were formed in the film. An increase in the amplitude of the Stokes wave in a TiN / Titan dioxide film occurs due to the enhancement of the cubic suscepti­bility because of localized plasmon resonance and a small refractive index of the effective medium. Such metal-insulator nano­composite films having several epsilon-near-zero frequencies in the visible and infrared ramges have found application in creating broad­band metal technologies providing resolution beyond the limits of light diffrac­tion,” adds the author.

The scientists have succeeded in visualizing 40 nm multiwall carbon nanotubes scattered along the surface of the metalens created by them, and the resolution was below 100 nm. “Nano­composite epsilon-near-zero film works as a surface-enhanced Raman scat­tering substrate, and it helps not only enhance the scattered signal, but also achieve beyond-diffrac­tion reso­lutions. Metalenses and ENZ films can be used to create broadband absorbers for solar panels,” concludes Kharintsev. (Source: Kazan Fed. U.)

Reference: S. S. Kharintsev: Far-field Raman color superlensing based on disordered plasmonics, Opt. Lett. 44, 5909 (2019); DOI: 10.1364/OL.44.005909

Link: Dept. of Optics and Nanophotonics, Institute of Physics, Kazan Federal University, Kazan, Russia

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