The possibility to control the infrared (IR) absorption and thermal emission on subwavelength scales has attracted large interest in the recent years thanks to the opportunities granted by nanostructured metamaterials. For example highly frequency selective and directional thermal emitters/absorbers have been proposed for a large variety of applications ranging from sensing and security to cooling and energy harvesting . In order to introduce a control of the emissivity as a function of the temperature thermochromic and phase change materials have been considered. In particular Vanadium dioxide (VO2) has become a widely-studied material for applications such as metamaterials, smart windows and supercapacitors, thanks to its strong optical transmittance changes at the IR and THz regions and huge resistance jump . The control mechanism is achieved by taking advantage of the metal-insulator phase transition of VO2 at its critical temperature (~68 °C).
We numerically show the control of spatial and spectral features of the far field thermal emission pattern of nanoantenna arrays, composed of alternating Gold and VO2 rods, as a function of the temperature. In this work we performed a numerical study by modifying a previously developed model  based on the fluctuational electrodynamics approach and on the discretization of the resulting volume integral equation to calculate relative emissivity and spatial emission pattern of nanoparticle ensembles smaller than the thermal wavelength lam=hc/kBT .
The drastic changes of the VO2 refractive index across its metal-insulator phase transition produce strong differences in the behavior of the overall system by creating or destroying evanescent wave coupling between different elements of the nanoantenna. The study of IR thermal nano-emitters is crucial for the realization of coherent thermal nano-sources in the mid and far IR for sensing applications and thermal management as well as thermal logic gates on the nanoscale.
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Marco Centini, Gianmario Cesarini, Maria Cristina Larciprete, Alessandro Belardini, Roberto Li Voti, and Concita Sibilia, "Spatial and spectral control of infrared thermal emission in VO2-based composite nanoantennas
(Conference Presentation)," Proc. SPIE 10671, Metamaterials XI, 1067116 (Presented at SPIE Photonics Europe: April 25, 2018; Published: 23 May 2018); https://doi.org/10.1117/12.2307185.5788808060001.
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