| In recent years,it has been an active area of research to manipulate the spectral absorption properties of optical materials from ultraviolet to microwave region.Near-infrared broadband perfect absorbers can find applications in thermal photovoltaic cells?TPV?,infrared thermal detection,sensing,thermal emitters and so on.Present absorbing structures,such as Fabry-Perot cavities,metamaterials and metasurfaces have a tradeoff among the absorber thickness,sample size,preparation cost and absorber performance.In order to achieve an ultrathin,broadband,tunable and layer-structure absorption,more investigations on near-infrared perfect absorbers are needed.In order to realize ultrathin planar broadband near-infrared absorber with thermal tunability,this paper studied the conditions for achieving targeted absorbing performances based on the thermochromic VO2 thin films and ultrathin multilayer absorbing structures.The phase transition and optical properties of VO2-based single and multi-layer films were studied.The relationships between preparation process and optical constants of multi-layer films were also analyzed.Based on these results,we proposed and fabricated a tunable broadband near-infrared absorber with a sandwich structure.It consists of tunable VO2 film on the top,Al2O3 dielectric layer in the middle and backed up ITO reflection layer.Absorption performance and mechanism of the structure were also studied.The main work and results are listed as follows:?1?Based on a single-layer film structure,we find that ultrathin feature and high absorption requires that the real part of the refractive index ns of the substrate is smaller than 1,and the imaginary part of the refractive index ks should not be too large,and it should be in the range of 02.It was proposed that ITO could be used as the reflection layer material.And the lossless Al2O3 dielectric layer was introduced for broadband absorption.?2?Single ITO,Al2O3 and VO2 film was prepared and its optical constant was obtained by spectroscopic ellipsometry,respectively.The influences of Al2O3 and SiO2substrates on VO2 properties were analyzed.It was found that the optical properties and surface morphology of SiO2/VO2 thin films were obvious different.The electronic transition of E1 in SiO2/VO2 thin films in metal phase was not zero due to the impacts of compressive stress or donor and acceptor levels on the electronic band structures.?3?In the preparation of multilayer structure,the introduction of non-destructive dielectric layer Al2O3 leads to the increase of imaginary part of dielectric and infrared absorption of ITO thin films.And it also made the phase transition temperature of VO2films approach to bulk phase transition temperature.The dielectric constant of VO2films is also enlarged.?4?Based on the optical parameters obtained,the tunable near-infrared ultrathin planar absorber,which consists of 260 nm ITO,75 nm Al2O3 and 35 nm VO2,was optimized by transfer matrix method and FDTD simulation.In the low temperature semiconductor phase of VO2,the strong absorption peak was mainly located in the range of 1.6-2.4?m.In the phase transition intermediate state of VO2,the resonance absorption peak is the strongest and the absorber shows near perfect absorption at 1.2-2.5?m.In the metal phase of VO2,the absorption intensity and bandwidth decreases,with only about 90%absorption in the range of 1.2-1.5?m.At the same time,the structure shows very weak angular sensitivity,and the absorption performance doesn't decay at the incident up to 70°for TE and TM polarizations. |
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