Metasurface Encodes IR Images

Irregular metasurfaces can be made to encode multiple images at IR frequencies in much the same way as the pixels in a television screen can form a visible-light image.

This was demonstrated with an array of subwavelength metal-insulator-metal (MIM) resonators that revealed different letters and even a famous painting when heated and imaged at different wavelengths and polarizations.

Applications of the technology could include biochemical sensing, optical storage and anticounterfeiting devices, according to its developers at the French Aerospace Lab (ONERA) and the Laboratory of Photonics and Nanostructures.

"MIM metasurfaces are great candidates for infrared emitters thanks to their ability to completely control thermal emission, which is groundbreaking compared to the usual thermal sources, such as a blackbody," said Patrick Bouchon, a researcher at ONERA. "Moreover, this study shows the possibility to create infrared images with the equivalent of visible colors."

Typical MIM resonators consist of a rectangular metallic patch on top of an insulating material, atop another metallic layer. In this case, the researchers deposited 50-nm-thick rectangular patches of gold on top of a 220-nm silicon oxide layer, which sat atop an opaque 200-nm gold layer.

A visible image of the metasurface (top left) and its emission response at different polarizations and wavelengths. The top right image reproduces a painting of French playwright Molière by Nicolas Mignard. Courtesy of M. Makhsiyan/ONERA.

Most metasurfaces feature a regular pattern of nanoantennas and exhibit homogenous optical properties. Here the researchers used electron-beam lithography to create an irregular arrangement.

Each nanoantenna in the array acted as an independent emitter that controls emission properties such as wavelength, polarization and intensity based on its geometry and orientation. Arranging the antennas across a large surface allowed the researchers to encode several images. Heating the metasurface to 373 K allowed for the creation of static IR images, similar to an LCD screen.

"We had to theoretically predict the response of 100 million antennas, and to subsequently fabricate it," said Mathilde Makhsiyan, a doctoral student at the French Aerospace Lab.

In the future, the researchers hope to develop methods for dynamic light emission — a first step toward an IR TV — and to create a startup business focused on anticounterfeiting devices.

The research was published in Applied Physics Letters (doi: 10.1063/1.4937453).