The term infrared signature generically describes how objects appear to infrared sensors. In most cases, infrared (IR) emissions from vehicles are used to detect, track, and lock-on to the target. The infrared signature of a given object depends on several factors, including the shape and size of the object, its temperature and its emissivity, as well as external conditions (illumination, background, to name some). One of the most challenging tasks regarding the IR vision is to reduce the infrared signature of objects. Although the IR spectrum extends from the red light to microwave radiation, i.e. 0.77 to 1000 m, there are only two wavelength ranges showing high IR transmittance in the atmosphere, i.e. 3-5 and 8-12 m, known as mid (MWIR) and long (LWIR) IR windows, respectively. Outside these windows, CO2 and H2O vapour give rise to both absorption and scattering phenomena, determining strong attenuation of IR radiation. Thermochromic materials, changing their spectral properties as a function of the temperature, are extensively studied in the seek of active control of thermal emission. Among the different thermochromic materials, the most known and widely diffused is vanadium dioxide, VO2 that is also the object of the present study . Its crystalline lattice exhibits an abrupt semiconductor-to-metal phase transition at a temperature TC= 341 K (68°C) characterized by an increase of reflectivity as well as a decrease of emissivity in the IR range. At a microscopic scale, through the phase transition VO2 undergoes a physical change of its crystalline cell from monoclinic to tetragonal. As well as other oxides showing this peculiar characteristic, as Nb dioxide (NbO2) and V2O3, VO2 has an insulating behavior under TC, while above this temperature it exploits a metallic nature, dramatically changing its optical, electrical and magnetic properties. In particular, optical properties are sharply changed during the phase semiconductor-to-metal transition, thus the dispersion law of the complex refractive index n+ik is strongly modified. [2,3]. As a consequence, the phase transition, occurring in a very short temporal range of the order of few picoseconds, can be exploited for the realization of an optical component switching from transparent (in the semiconductor state) to reflective (in the metallic state), as well as an efficient thermal switch. [4,5] In general, the performance effectiveness of either optical or thermal switches can be quantified and estimated through the so-called dynamic range, which is the difference between the largest and smallest possible values of a changeable quantity. Within the present work we define this figure of merit as the difference between the emissivity values, averaged in the IR range 3-5 m and calculated for the two different regimes, i.e. below and above TC, respectively. Given this assumption, it’s worth to note that the sign of the dynamic range, i.e. if positive or negative, completely changes the filter behavior and thus determines the type of application. A thermochromic filter displaying positive dynamic range, i.e. its IR emissivity decreases with increasing temperature, is suitable for IR signature reduction as well as for smart windows for thermal control . On the other side, a negative dynamic range is required for space applications and emissivity control of spacecraft . Recent works have shown, both theoretically and experimentally, that the thermal emissivity behaviour with temperature (i.e. dynamic range) of VO2 thin films is strongly influenced by the substrate used for the deposition. In what follows we consider simulations of the optical response of VO2 thin films first deposited on different substrates (section 2), and then in multilayer structures (section 3), below and above the TC. We discuss the effect that different substrates as well as VO2 layer thicknesses have on the sign of the dynamic range. Finally, we introduce some metallo-dielectric multilayer structures, composed by copper or silver and VO2 alternating layers, where the layer thickness is systematically varied in order to further increase and optimize the dynamic range value.
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|Titolo:||Nanostructures for Infrared Management|
|Data di pubblicazione:||2012|
|Appartiene alla tipologia:||04d Abstract in atti di convegno|