New Frontiers of Infrared Optics: Tunable Metamaterials Studied at the Frascati National Laboratories of INFN
A recent research conducted by a group from the SBAI Department (Sapienza University of Rome) and published in the journal Scientific Reports investigates an innovative hybrid metamaterial based on a thin film of vanadium dioxide (VO2) and an array of gold nanodisks, capable of actively tuning its optical response in the infrared. The resulting metamaterial structure can modify its absorption or reflection of infrared light in a specific wavelength range (3-5 μm) by simply varying its temperature.
(a) 3D schematic of the unit cell of the metamaterial on CaF2 substrate. (b) SEM image of the sample.
VO2 is a material with a fascinating property: it undergoes a semiconductor-to-metal phase transition when heated to a relatively low temperature, around 68 °C. This structural change results in drastic modifications of its electrical, thermal, and optical properties. Exploiting this characteristic, the researchers designed a metal-insulator-metal (MIM) type metamaterial, interposing a thin layer of VO2 between a flat gold layer and an array of gold nanodisk resonators.
(a) Experimental spectra showing the resonance tuning of the metamaterial with the temperature. (b) Simulation using Bruggeman formalism and Looyenga mixing rule; f is the fitting parameter which represents the fraction of metallic VO2 during the heating.
A crucial aspect of this research was the experimental characterization of the metamaterial, conducted at the Frascati National Laboratories of the National Institute of Nuclear Physics, using a Bruker VERTEX 70v FTIR spectrometer, coupled with a Hyperion microscope at the DAFNE-Light facility. In order to check the dynamic spectral behavior, the infrared reflectivity of the metamaterial was characterized at different temperatures, from room temperature up to 100 °C.
The experimental results highlighted a continuous spectral tuning of the resonance with increasing temperature. When the VO2 was in its semiconductor state (at room temperature), the metamaterial exhibited a strong resonance in the mid-infrared (MWIR) range, centered at 4.72 μm. By heating the material, the resonance gradually shifted towards longer wavelengths, reaching 5.43 μm at 66 °C. This shift of almost 1 μm demonstrates the effective control over the optical response of the metamaterial through temperature. The researchers also verified the reversibility of the process, observing the return to the initial resonance by cooling the sample.
(a) Simulated and experimental absorption intensity at the resonant wavelength for the purely semiconductor VO2.
To fully understand the behavior of the metamaterial, numerical simulations were also conducted. These allowed analyzing how the phase transition of VO2 influences the resonances and how these depend on the fraction of material that has completed the transition to the metallic phase.
This research opens up interesting prospects for various applications. The ability to actively control absorption and reflection in the infrared range could be exploited in thermal camouflage systems, tunable infrared optical devices, and other technologies that require precise control of infrared light. The obtained results represent an important step forward in the development of advanced metamaterials with dynamic functionalities for infrared photonics.
Petronijevic, E., Larciprete, M.C., Centini, M. et al. Active infrared tuning of metal–insulator-metal resonances by VO2 thin film. Sci Rep 14, 25324 (2024). https://doi.org/10.1038/s41598-024-75430-0