Encoding images in infrared signals using metasurfaces. Image courtesy of Mathilde Makhsiyan/ONERA
Encoding images in infrared signals using metasurfaces. Image courtesy of Mathilde Makhsiyan/ONERA
"This metasurface is a great candidate for infrared emitters thanks to its ability to control the thermal emission at given wavelengths."Patrick Bouchon

A new study by French researchers has shown control of thermal emissivity at the sub-wavelength scale, allowing them to encode images in infrared signals using metasurfaces and the possibility of creating infrared images with the equivalent of visible colors. These metamaterial resonators permit emission in the infrared to be tuned based on the geometry of the resonator.

The sub-wavelength scale metal-insulator-metal (MIM) resonators are able to control, both spatially and spectrally, emitted light up to its diffraction limit, so that an array of resonators can combine to provide an image in the infrared, a breakthrough that could lead to applications in areas such as optical storage, infrared televisions, biochemical sensing, and anti-counterfeit devices.

MIM metasurfaces are useful candidates as infrared emitters due to their ability to control thermal emission at given wavelengths, and also to artificially tailor an electromagnetic response on various spectral ranges. The team had previously shown how to manipulate light through altering its absorption or converting its polarization, and also explored the “funneling effect”, where incoming light energy is coupled to a nanoantenna. For this study, reported in the journal Applied Physics Letters [Makhsiyan et al. Appl. Phys. Lett. (2015) DOI: 10.1063/1.4937453], a MIM nanoantenna was comprised of 50 nanometer-thick rectangular patches of gold deposited on top of a 220 nanometer silicon oxide insulating layer, which was positioned on top of an opaque 200 nanometer metal gold layer.

The team had to theoretically predict the response of 100 million antennas, and then fabricate it, which was achieved by producing electromagnetic software, and software to generate the e-beam files for the fabrication of spatially modulated emissivity metasurfaces. Each nanoantenna can then operate as an independent deep sub-wavelength emitter for a given polarization and wavelength, and can control emission properties such as wavelength, polarization and intensity with its specific geometry and orientation. On being juxtaposed on a large scale, the MIMS cause the emissivity to be defined at the sub-wavelength scale so they could encode several images on the same metasurface.

The emission information is encoded in a unit cell that is smaller than the wavelength due to the effect of the antennas' varied geometries and orientations on the way the information is encoded. This means that two neighboring cells can possess different encoded information and encode it spatially, which ultimately allows for the development of a static infrared image. Further research could involve independently controlling each pixel through tunable thermal emission to create a dynamic emission of light.