Research On The Design And Mechanism Of A Novel Ultra-broadband Optically Transparent Absorber

In the field of modern military technology,metamaterial absorbers are developing towards a multifunctional trend.To meet the development needs of stealth fighter cockpits,military ship command cabins,and flexible electronic devices,the absorber may satisfy several important characteristics,e.g.,thin thickness,light weight,wideband absorption,and high absorption strength.Additionally,the better transmittance in the visible light spectrum may also be needed.Furthermore,the broadband absorption performance and high angular stability of the absorber have certain restrictions discovered through literature research.As the incident angle increases,the degree of mismatch between the absorber and the free space gradually increases,which causes problems such as weakening of the absorption intensity and narrowing of the absorption bandwidth.The focus of this paper is to solve the problems of insufficient angular stability of planar broadband transparent absorbers and insufficient research on the angular stability of conformal absorbers.Our work is as follows.Firstly,an ultra-wideband transparent absorber with high angular stability based on a square spiral element is proposed.The absorber is mainly composed of a compensation layer made of polyvinyl chloride(PVC)sheet,an impedance layer designed with indium tin oxide(ITO)conductive films,a substrate layer composed of polyethylene terephthalate(PET)and PVC sheets,and a bottom floor layer.To obtain broadband and high angular stability,the compensation layer is used to solve the impedance mismatch between the free space and the absorber at higher incident angles,and the compact square spiral unit cells are then used to expand the absorption bandwidth of the absorber.Secondly,in view of the insufficient research on the angular stability of the existing cylindrical conformal absorbers,a conformal model was designed and processed and measured.Results show that the absorber has good light transmittance(average light transmittance of 69.4%),broadband absorption performance(the fractional bandwidth is greater than 85.7%)and high angular stability(60°)under the conformal state,including different curvatures.In addition,a miniaturized is designed with the wave spiral unit cells,which may achieve an angular stability of 60° while expanding the absorption bandwidth.Secondly,to further expand the absorption bandwidth,a broadband transparent absorber with high angular stability based on hexagonal combined unit cells is proposed.The absorber is prepared by a double-layer transparent TPX layer and the ITO films.Firstly,to achieve broadband absorption at high angular incidence,using the principle of slot resonance,the reflection response is adjusted by designing a novel ”cobweb-like” structure interwoven by a central connecting line and a hexagonal spiral unit cell.Secondly,to achieve broadband and high angular stability,a poly-4-methylpentene-1(TPX)substrate with lower relative permittivity is used as the optimal substrate of the compensation and substrate layer.Experimental results show that the absorber exhibits good light transmittance(average light transmittance is 70.7%),ultra-wideband absorption performance(the fractional bandwidth exceeds 106.6%)and high angular stability(60°).In addition,its absorption bandwidth gradually increases with the increase of the incident angle,and its fractional bandwidth reaches 124.6% at incident angle of 60°.Finally,to further extend the application of transparent absorbers to non-planar structures such as aircraft cockpits,a spherical conformal broadband transparent absorber is proposed.The absorber is a single-layer absorber composed of double-layer ITO films with PET sheets sandwiching an air substrate layer.The good light transmittance of the absorber in the planar state(average light transmittance is 68.7%),the angular stability of 40° and the broadband absorption characteristics(the fractional bandwidth reaches109.8%)are all obtained through experiments.However,the research on the absorption performance of the spherical conformal absorbers has not been reported,and the conformal absorber with a central bending angle of 45° has then been measured.Results show that the fractional absorption bandwidth of the absorber is 96.8%.Overall,this paper realizes the compatible design of broadband absorption and high angular stability of the absorber by designing a novel tightly coupled impedance layer and selecting a suitable compensation layer.The proposed various absorbers exhibit important characteristics such as broadband absorption,high angular stability,cross-polarization reflection suppression,and polarization insensitivity for various incident angles,which makes them widely applicable in flexible materials,optics and radar absorbing fields.