Optically Transparent Microwave Absoption Modulation Method Based On Patterned Vanadium Dioxide

With the increasingly complexity space electromagnetic environment,high-performance electromagnetic shielding technology have an increasingly critical demand.Absorption-type electromagnetic shielding that can convert electromagnetic waves into thermal energy is the ideal electromagnetic shielding method.In particular,the transparent tunable microwave absorption method that can realize optically transparent,tunable microwave absorption amplitude and peak frequency is a challenging and forward-looking topic in the frontier of basic science research,cutting-edge instrumentation and aerospace equipment and other areas.However,there are few reports on this topic.Vanadium dioxide（VO₂）is a promising candidate for optically transparent microwave absorption tunable field owing to its temperature-dependent drastic insulator-to-metal phase transition.However,VO₂ film is opaque in visible band and is difficult to be directly applied.To solve the above problems,this study patterns VO₂ film and reveales a optically transparent microwave absorbing thermally tunable mechanism based on patterned VO₂ film,the absorber based on this mechanism can simultaneously achieve high optical transmittance and thermally tunable microwave peak absorption.On the basis,this study develops a mechanistic study and experimental verification of the optically transparent microwave absorption frequency-tunable method based on double Fabry-Perot cavities resonance and tunable equivalent resistance percentage.The main contents and results are as follows:1.To address the lack of technical basis for optically transparent microwave absorption modulation,an optically transparent microwave absorption tunable mechanism based on patterned VO₂ film is revealed.The absorber is constructed from patterned VO₂ film,transparent substrate and transparent reflective layer,and the transmission line model is built for impedance matching degree analysis.The equivalent impedance of patterned VO₂ film can be changed by controlling the temperature and duty cycle.Patterned VO₂ film as the impedance layer of the absorber enables the absorber to achieve different free space impedance matching degrees.Thus,the absorber achieves tunable peak absorption and near-unity absorption by controlling temperature.Simultaneously,the optical transmittance,temperature tuning ranges and temperature to achieve near-unity absorption can be tuned by varying the patterned VO₂ film duty cycle.The experimental results show that the maximum optical transmittance and the reflection loss modulation depth of the absorber are 84.95%and 55.92 d B,respectively.And a peak absorption of99.99%is achieved at 15.06 GHz.2.For optical windows light and thin load requirements,an optically transparent ultra-thin microwave absorption frequency-tunable method based on tunable equivalent resistance percentage is proposed.The parameters of patch-type VO₂ film are designed to make the LC electric resonant frequency lower than the Fabry Perot cavity resonant frequency.Then,thermally variable sheet resistance of VO₂ film enables different percentage of equivalent resistance in the equivalent impedance of patch-type VO₂ film,which drives the peak absorption frequency caused by FP cavity resonance to that caused by both LC electrical resonance and FP cavity resonance.The absorber achieves good free space impedance matching with the free space at a lower frequency,tunes the peak absorption frequency to low frequencies,reduces the relative thickness of the absorber to satisfy the ultra-thin thickness characteristic.Based on the equivalent circuit method,the prediction model of the frequency modulation range of the absorber is established.The experimental results show that the average normalized transmittance of the absorber in 380 nm-6000 nm broadband is better than 82.94%.By controlling the temperature,the peak absorption frequency can be tuned from 15.42 GHz to 12.99GHz.The thickness of the absorber is ultra-thin（only 7.1%）compared with the center wavelength of the frequency modulation range.3.For the need of wide range of frequency modulation,an optically transparent microwave absorption frequency-tunable method based on double Fabry-Perot cavities is proposed.By introducing the front dielectric layer and a VO ₂ mesh interlayer,a double Fabry-Perot cavities absorber is constructed.Based on the multi-beam interference theory,reflection characteristics models of two Fabry-Perot cavities are established respectively.The absorber changes from one Fabry-Perot cavity dominant to another dominant by thermally controlling the VO₂ film sheet resistance.Due to the different optical thicknesses of the two Fabry-Perot cavities,the peak absorption frequencies are different,which enables the tunable peak absorption frequency of the absorber.Simultaneously,because of the small duty cycle of VO₂ mesh,the absorber has high optically transparency.The experimental results show that the peak absorption frequency of the double cavities absorber can be tuned from 12.51 GHz to 16.71 GHz,achieving a broadband modulation of 4.20GHz,and the average normalized transmittance in broadband（380 nm-4200 nm）is better than 72.48%.This paper explores the mechanism of microwave absorption thermally modulation based on patterned VO₂ film and proposes a variety of optically transparent microwave absorption tunable methods based on patterned VO₂ film,which provides the basis for the application of a new generation of optically transparent microwave absorption tunable technology based on VO₂ film.