Infrared Emitters coupled to NanoAntennas : the IRENA project

Mid-wave Infrared emitter with fast modulation rate for short range communications

Near infrared (NIR) sources are used for short range communication applications such as remote control or free-space optical links. Current infrared technology uses diodes operating in the near infrared because of the quality of the available detectors and sources. However, optical links operating at these wavelenghts are vulnerable to degradation because of Mie scattering from micron-sized particles such as dust or pollutants. Luckily the atmosphere has two transparency regions at longer wavelengths, in the mid Infrared (MIR) at 3-5 µm and 8-12 µm, that are much less affected by this problem.  Furthermore, MIR sources are needed for many applications such as gas detection, sensing, drying, heating.

Currently available sources in this part of the spectum are gas lasers and optical parametric oscillators, that are bulky, and quantum cascade lasers (QCL) that are quite expensive. The only available cheap and compact sources are incandescent ones such as globars or hot membranes. Yet, currently available incandescent sources cannot be modulated faster than 100 Hz because of their thermal inertia. In the IRENA project, we have explored two new paradigms for Mid-IR emission in order to achieve cheap and compact Mid-IR sources in the 3-12 µm wavelength range, i) a LED assisted by a cavity and ii) an incandescent metasurface modulated at high frequency by taking advantage of novel concepts of nanophotonics. In the rest of this summary, we describe the incandescent metasurface.

An incandescent metasurface for quasimonochromatic polarized mid-wave infrared emission modulated beyond 10 MHz.

To address the thermal inertia issue, we envision heating by Joule effect a grid of metallic wires (MWs) with thickness h_metal deposited on a cold substrate. The temperature dynamics is limited by the thermal diffusion time through the metal given by h_metal²/D where D is the metal diffusivity. A thickness of 50 nm yields a cooling time on the order of 0.25 ns. In other words, heat transport is fast at the nanoscale.

The price to pay is that a grid of thin MWs is partially transparent. In order to achieve perfect absorption by a thin film, we rely on the principle of the Salisbury screen: a thin metallic film placed above a perfect mirror can be perfectly absorbing provided that the film thickness and the film-mirror distance are properly chosen.  

To implement these ideas, we use a gold mirror deposited on a silicon substrate (see Fig. 1). A transparent dielectric spacer is then added to control the distance between the mirror and the periodic array of MWs. In the spectral range of interest (3-5 µm), silicon nitride (SiNx) has a low absorption and can be heated up to at least 650°C. Platinum is chosen for the MWs because it matches a large number of requirements: i) it can sustain high temperatures, ii) it is compatible with SiNx for fabrication purposes, iii) its refractive index in the mid-infrared is compatible with a nanoscale absorber as discussed above, iv) the electrical properties of platinum make it possible to design a device with an electrical impedance approaching 50 Ω, matching the electrical source impedance, and v) the conductivity of platinum depends on temperature and this feature can be conveniently used as a real-time thermometer.

Fig. 1 Schematic view of the emitter. The inset shows the platinum wires deposited on a SiNx substrate on top of a gold mirror. 

Key results

The key result is the demonstration of a modulation faster than 10MHz, 5 orders of magnitude faster than commercially available hot membranes. This devices made of parallel wires emits linearly polarized light with an electric field parallel to the wires.

By modifying the shape of the wires, and introducing zig-zags in order to have a chiral emitter, we have been able to design and fabricate the first incandescent metasurface emitting IR radiation with a large degree of circular polarization.

Scientific output

1. An incandescent metasurface for quasimonochromatic polarized Mid-Wave Infrared emission modulated beyond 10 MHz, L. Wojszvzyk, A. Nguyen, Anne-Lise Coutrot, C. Zhang, B. Vest, J.J. Greffet, Nat  Commun 12, 1492 (2021). https://doi.org/10.1038/s41467-021-21752-w
    
2. Efficiency optimization of mid-infrared incandescent sources with time-varying temperature, A.Nguyen and J.J. Greffet, Opt.Mat.Exp. 12, 225, (2022), https://doi.org/10.1364/OME.443129
    
3. Large circular dichroism in the emission of an incandescent metasurface, A. Nguyen, J.-P. Hugonin, A.-L. Coutrot, E. Garcia-Caurel, B. Vest, J.-J. Greffet (submitted for publication)

The project IRENA is a fundamental resaerch project coordinated by Jean-Jacques Greffet from Laboratoire Charles Fabry, Institut d'Optique Graduate School. The project gathers the groups led by Raffaele Colombelli at Centre de Nanosciences et Nanotechnologies (C2N) and Jean-François Lampin at Institut d'Electronique du Nord (IEMN). The project started on march 1, 2018 and ended on september 30, 2022. It received a grant of 494 k€ for a total cost of 1.1 M€.