Color Superlensing Could Break Through Diffraction Barrier

Researcher Sergey Kharinstev and his team at Kazan Federal University recently published a paper in Optics Letters where they detail the design of a new type of metalens capable of imaging beyond the optical diffraction limit.

Schematic of the working principle of a disordered TiN/TiO₂. Courtesy of Kazan University.

A metalens described in the article 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,” said Kharintsev, who believes his team’s discovery might lead to the use of optical technologies in nanoscale integral circuits and sensors.

Kharintsev pointed out that the material part of the dielectric constant, or the electric potential, 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.

“We used a 50-nm thick titanium oxynitride (TiON) film as a disordered nonlinear medium,” Kharintsev said. “The film was synthesized by magnetron sputtering and subsequent oxidation in air. As a result of a two-stage procedure, metal (TiN) and dielectric (TiO₂) nanoparticles were formed in the film.”

An increase in the amplitude of the Stokes wave in a TiN/TiO₂ film occurs due to the enhancement of the cubic susceptibility because of localized plasmon resonance and a small refractive index of the effective medium. Kharintsev said metal-insulator nanocomposite films such as these with epsilon-near-zero frequencies in the visible and infrared ranges have found application in broadband metal technologies providing resolution beyond the limits of light diffraction.

Kazan University researchers have created 40-nm multiwall carbon nanotubes scattered along the surface of the metalens with the resolution measuring below 100 nm.

“Nanocomposite epsilon-near-zero film works as a surface-enhanced Raman scattering substrate,” Kharintsev said. “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.”

The research was supported by a Russian Science Foundation grant under the title “Synthesis and research of a new class of nanocomposite ceramics with degenerate dielectric permeability for opto-plasmonic applications.”