SpectroscopyNOW- Geometric resonator: Tuning the infrared

• Author: David Bradley
• http://www.spectroscopynow.com/ir/details/ezine/15216bf9014/Geometric-resonator-Tuning-the-infrared.html

Meta resonator

Meta material resonators that allow emission in the infrared to be tuned through changes in the geometry of the resonator have been demonstrated by scientists in France.

Researchers at MINAO, a joint lab between The French Aerospace Lab in Palaiseau and the Laboratoire de Photonique et de Nanostructures in Marcoussis, have used a sub-wavelength scale metal-insulator-metal, or MIM, resonator to spatially and spectrally control emitted light up to the diffraction limit. This, the team says, allows them to create arrays of resonators that can be used to form an image in the infrared region of the spectrum analogously to the way in which the pixels of a television screen form a visible light image. The new technology has potential in biochemical sensing, optical storage, anti-counterfeit devices and in infrared imaging.

“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,” explains Patrick Bouchon, of The French Aerospace Lab, also known as ONERA. “Moreover, this study shows the possibility to create infrared images with the equivalent of visible colours.”

Funnel

Writing in the journal Applied Physics Letters, Bouchon and his colleagues explain how they had previously demonstrated the ability to manipulate light through tailoring its absorption or converting its polarization, and investigated the “funnelling effect,” in which incoming light energy is coupled to a nanoantenna. Now, they have made a MIM nanoantenna consisting of a rectangular metallic patch on top of an insulating material, on top of an additional metal layer. The majority of metasurfaces, the aggregate of many nanoantennae on a substrate, contain a periodic repetition of a given pattern, and exhibit no spatial modulation. For their MIMs, Bouchon and his colleagues deposited 50 nanometre-thick rectangular patches of gold on top of a 220 nanometre silicon oxide layer, on top of an opaque 200 nanometre gold layer. The idea of modifying the emissivity with nanostructures is relatively recent, with this same team showing that it is possible to combine several antennae in the same subwavelength period as recently as 2012.

“We had to theoretically predict the response of 100 million antennae, and to subsequently fabricate it,” explains team member Mathilde Makhsiyan from The French Aerospace Lab. To do this, the researchers developed their own software, as well as specific software to generate the electron-beam files for the fabrication of spatially modulated emissivity metasurfaces.

IR pixels

They explain that once constructed, each nanoantenna acts as an independent deep subwavelength emitter for a given polarization and wavelength of infrared. This allows them to control emission properties such as wavelength, polarization, and intensity through the device's specific geometry and orientation. When juxtaposed on a large scale, these MIMs cause the emissivity to be defined at the sub-wavelength scale, allowing the researchers to encode several images on the same metasurface.

Importantly, the emission information is encoded within a unit cell that is smaller than the infrared wavelength. As such, two neighbouring cells can have different encoded information and encode the information spatially. This is what allows a static infrared image to be generated on a display. The next step will be to develop a way to contr