Design And Fabrication Of Smart Infrared Absorbers Based On Vanadium Dioxide

Vanadium dioxide?VO₂?undergoes a reversible phase transition from the monoclinic phase?semiconducting state?to the rutile phase?metallic state?at 68oC.During the transition,the mid-infrared?MIR?properties of VO₂ change abruptly:VO₂behaves highly reflective in the MIR region when the temperature is above 68oC,while it behaves highly transmissive in the MIR region when the temperature is below 68oC.In the dissertation,we designed and fabricated a smart MIR absorber based on VO₂metasurface,utilizing its switchable MIR properties during its phase transition.The smart absorber shows a tunable absorption peak position and peak intensity dependent on temperature.In addition,we also introduced the amorphous-crystalline phase change of chalcogen phase change materials e.g.Ge₃Sb₂Te₆?GST?and In₃Sb₁Te₂?IST?.The combination of GST or IST with VO₂ respectively empowered the smart absorber with the ability of multiple optical modulations.To be specific,the structure with GST and VO₂ has high transmittance at temperatures lower than 50oC,and the transmittance decreases with the crystallization of GST.However,the structure has high absorptance at temperatures higher than 65oC,and the absorption peak redshifts with the crystallization of GST.The structure with IST and VO₂ can switches between high reflection,high absorption and high transmission,which depends on the phases of both IST and VO₂.The main research contents and conclusions obtained in this dissertation are as follows:?1?VO₂ was prepared by reactive sputtering and atomic layer deposition.A stable,repeatable deposition process of pure VO₂ was obtained for the two methods,respectively.The crystalline structure,film composition,morphology,optical and electrical properties of the VO₂ films were studied.The phase transition temperature of the obtained VO₂ is about 67oC.In order to reduce the phase transition temperature and make VO₂ a better applicant in future use,Mo-doped VO₂ thin films were fabricated by alternatively depositing Mo O3/VO2 nanolaminates via atomic layer deposition method followed by post-annealing process.The V1-x Mox O2 films with different Mo contents can be obtained by controlling the Mo O3 cycle ratio?PMo?.When PMo?5%,the phase transition temperature of the films can be reduced by about 11oC for every1%increase of PMo.?2?A smart MIR absorber based on VO₂ metasurface was simulated and fabricated.The absorber is composed of an upper patterned VO₂ metasurface,a middle dielectric spacer layer,and a bottom reflective metal layer.The absorber has a classic metal-dielectric-metal?MDM?structure.When temperature is below 67oC,VO₂ is semiconducting with high MIR transmittance.The absorber has a high absorption at a certain wavelength due to the anti-reflection effect on the underlying metal layer provided by semiconducting VO₂ and dielectric spacer.When the temperature is higher than 67oC,VO₂ is metallic,with low transmission and high reflection in the MIR.Resonance absorption can be achieved via the excitation of the electric dipole and magnetic dipole in the metamaterial unit.Besides,the absorption peak position can be adjusted by varying the size of the VO₂ pattern.Therefore,the structure has a smart function of shifting absorption peak position in MIR at low/high temperatures?selective absorption?.?3?A smart absorber based on VO₂ and GST was fabricated.The absorber is composed of 80 nm Zn S:Si O₂?protective layer?/150 nm Ge₃Sb₂Te₆/250 nm VO₂.The bottom VO₂ layer is used as a smart reflector:high reflection at high temperatures while low reflection at low temperatures.The upper GST layer has the function of shifting the absorption peak position or adjusting the intensity of transmittance.When temperature is higher than 65oC,the MIR absorption peak redshifts from 4.1?m to 5.8?m via the phase change of GST from the amorphous to the crystalline state.At temperatures lower than 50oC,the transmittance of the film stack at 3.2?m decreases from 0.91 to 0.24 with the crystallization of GST.Besides,the transmittance of the film stack can be continuously regulated from the highest value to zero by increasing the temperature.In addition,the experimental simulation results show that the absorption peak position of the film stack at high temperatures can also be linearly redshifted by increasing the thickness of GST.?4?A smart MIR reflection/absorption/transmission modulator based on VO₂ and IST was fabricated with the structure of 80 nm Zn S:Si O₂?protective layer?/100 nm In₃Sb₁Te₂/250 nm VO₂.The film stack can switch between three modes:reflection?R?,absorption?A?,and transmission?T?.When temperature is higher than 65oC,the phase transition of IST from amorphous to crystalline state facilitates the film stack switching between A?A=0.99 at 2.8?m?and R mode?R=0.85 at 2.8?m?.At temperatures lower than 50oC,the film stack with amorphous In₃Sb₁Te₂ shows T mode?T=0.99 at2.7?m?.In addition,the high transmittance in T mode can be continuously reduced to zero by increasing the temperature.?5?A smart absorber based on the IST metasurface and VO₂ was designed and simulated.The IST metasurface can be obtained by laser writing method to crystallize IST in specific regions,thereby avoiding complicated lithography methods for micro/nano structures.The characteristics of this absorber are:first,it has temperature-regulated absorption?high absorption at high temperatures while low absorption at low temperatures?;second,even if the fabrication of the structure has finished,the absorption peak position can still be shifted by laser writing in post process.