Metasurfaces are artificially constructed sub-wavelength patterned structures containing arrays of tiny elements such as rods and rings that interact with light and other electromagnetic waves in unusual ways. For example, in a metasurface, light waves can be steered in the opposite direction to normal materials. Such a unique property means that these structures have already been used to make super-lenses, holograms and gratings.

All-dielectric metasurfaces could be ideal for a new generation of flat optical devices, particularly for use in telecommunications applications, thanks to the fact that they are highly transparent in the infrared part of the electromagnetic spectrum. The problem, however, is that these surfaces are difficult to tune.

Resonating nanoparticles

A team of researchers led by Andrey Miroshnichenko, Lei Xu and Mohsin Rahmani have now designed a metasurface based on a 2D array of silicon disks whose refractive index can be changed by applying heat to it. The disks are made from nanoparticles resonating at a particular frequency.

“By changing the refractive index of these particles, we can tune their resonance,” explains Rahmani. “Therefore, by choosing a certain frequency we can design a metasurface with the required resonant behaviour before and after heating. This can produce a material that goes from reflecting light to one that transmits it, and vice-versa.”

Protective applications

This surface is only a few tens of nanometres thick and could be applied to a variety of surfaces, including textiles, which would then, for example, reflect ultraviolet or infrared light. It might, for instance, be applied to a spacesuit that would protect astronauts in the environment of space or to clothing for the beach that would protect us from UV rays here on Earth. It might also be used to make metalenses and metaholograms.

Such a technology significantly increases the resistance threshold against harmful radiation compared to today’s technologies that rely on absorbing radiation with thick filters, he explains.

Tailored for other wavelengths

The invention might even be tailored for other wavelengths, including visible light, adds Miroshnichenko. This means that it might be used in architectural and energy-saving applications.

“We could, for instance, have a window that can turn into a mirror on demand, or one through which we would control the amount of light that passes, depending on whether it was a sunny or cloudy day,” he suggests.

“The most important aspect is that we avoid the need for having a number of different protection layers against different frequencies,” Xu tells nanotechweb.org. “In current technology, each bulk protection layer is designed to block or transmit a certain frequency but our metasurface can be used to protect devices and surfaces against different frequencies on demand.”

The researchers say that they will now be working on expanding their technology to a wider frequency range. “At the moment, it works from the ultraviolet to the infrared with silicon nanoparticles,” says Rahmani. “We plan to employ another bulk material, which, via nano-patterning, will allow us to go beyond UV and perhaps even beyond X-rays, by exploiting the same physics.”

The research is described in Advanced Functional Materials DOI: 10.1002/adfm.201700580.