| In the area of the cutting-edge fundamental research and aerospace,electromagnetic shielding is currently the most challenging and strategic issue.To achieve this goal,as the necessary information channel for all optoelectronic instruments,viewing windows with high transparency must have strong electromagnetic shielding capabilities to reduce the overall radar characteristic signal and harmful electromagnetic signals that interferes with the precise instruments in the cabin.With the continuous development of electromagnetic wave technology,as well as the advancement of radar and precision photoelectric instrument detection technology in recent years,the ultimate challenge to achieve highperformance and ultra-wide band shielding performance is proposed for transparent electromagnetic shielding technology.Metal mesh filtering technology is extremely advantageous due to its high visible transmittance and favorable shielding capability in the low-frequency band.However,it still suffers from the following problems,such as the highly concentration of high-order diffraction energy which degrades the imaging quality,the limitation of the shielding bandwidth and electromagnetic reflection.Solving the above problems to achieve the goal of “high light transmission,broadband strong electromagnetic shielding and low electromagnetic reflection” involves a series of key issues in the field of science and technology,and has become a frontier topic in this field,which has not been solved at home and abroad.This Ph.D.thesis studies the contradiction between electromagnetic shielding performance and shielding bandwidth of metal mesh filtering technology and the reasons for the concentration of stray light distribution.For different applications,the strong shielding method with ultra-wide band based on ultra-thin doped metal film(< 10 nm)and the metal mesh based on multi-period micro-ring were proposed respectively and the perfect absorption of electromagnetic wave in the specific band was realized by constructing an asymmetric Fabry-Perot cavity.The thesis has carried out in-depth theoretical analysis and experimental verification on the sc ientific issues and major technical issues involved in the above aspects.The main contents and results are as follows:First,in order to solve the problem of the limited electromagnetic shielding bandwidth of metal mesh filtering technology,a stong electromagnetic shielding method with ultra-wide band based on ultra-thin metal film is proposed.According to the metal co-deposition growth process,a low concentration of copper is added during the growth of the silver film,which suppresses the island growth mode of ultra-thin silver film at the initial stage.The doped silver film with a continuous surface at the thickness of only 8 nm and a very low roughness and good electrical conductivity is obtained,which overcomes the problem of discontinuous of pure silver film under ultra-thin thickness(< 10 nm).Furthermore,the conductive medium is used as an optical anti-reflection film to construct an indium tin oxide-doped silver-indium tin oxide multilayer film structure,which realizes high-transmission,ultra-wide suppression band,strong electromagnetic shielding effectiveness,and high-performance transparent electromagnetic shielding.Based on the principle of multi-beam interference and transmission line theory,the optical transmission and electromagnetic transmission models are established respectively.The experimental results show that the indium tin oxide-doped silver-indium tin oxide multilayer nano-film has the transmittance of 96.5% in the visible range,and the electromagnetic shielding effectiveness is 26 dB in the range of 8 to 40 GHz,without any downward trend.Compared to the traditional square metal mesh structure,the visible transmittance is increased by 1.5%,and the shielding efficiency is improved by 8 dB,with the shielding bandwidth increased by 26 GHz.Second,in order to solve the problem of electromagnetic reflection in metal-based transparent electromagnetic shielding technology,a transparent perfect microwave absorption method based on the graphene and doped silver films is proposed.The asymmetric Fabry-Perot cavity is comprised of graphene and doped silver films,and the conditions for the perfect absorption of electromagnetic wave in specific band are proposed,which greatly improve the absorption of the structure,overcoming the electromagnetic reflection of the doped silver film.The electromagnetic transmission model of the cavity is established according to the transfer matrix method.Based on the coupling mode theory,the electromagnetic absorption characteristics of the cavity are described,and the microwave absorption characteristics of the cavity are obtained.Experiments show that under the condition of the graphene’s Fermi level of 0.3 eV and relaxation time of 20 ps,when the dielectric thickness of the asymmetric Fabry-Perot cavity dielectric layer is 3 mm,the near-perfect absorption of electromagnetic waves is realized by this cavity structure at 13.75 GHz.At this stage,the electromagnetic absorption was 99.5%,and the electromagnetic transmittance was 0.3%.At the same time,in the visible light band,the transmittance of the cavity was 87.0%,and the relative transmittance was 93.0%.Third,in order to solve the problem of the concentration of high-order diffracted energy distribution in the existing metal mesh filtering technology,the metal mesh structure based on multi-period micro-ring arrays is proposed.Furthermore,the multiperiod micro-ring metal mesh is combined with the doped silver film to obtain a twolayer structure,which alleviates the contradiction between light transmittance and electromagnetic shielding performance.Introducing sub-ring units in a basic triangular ring distribution array,the basic ring period is enlarged and its diffraction is thus reduced,and the diffraction energy of the sub-rings is not superimposed with that of the basic rings.Furthermore,making the adjacent units rotate at the specific angles from the perspective of “symmetric breaking” to break the symmetry of the original structure,which makes the distribution of the multi-period ring elements more evacuated,and finally homogenizing the diffraction distribution in-depth.From Huygens-Fresnel principle,the Fraunhofer diffraction intensity distribution model based on multi-period micro-ring metal mesh is established,and the influence of mesh design parameters and sub-ring rotation on diffraction distribution is revealed.Based on the complex equivalent refractive index and the equivalent electromagnetic parameter models,an electromagnetic transmission model based on multi-period micro-ring metal mesh is established.Experiments show that the metal mesh based on multi-period micro-ring structure can greatly homogenize the higher-order diffraction energy distribution and solve the problem of concentrated diffraction distribution in traditional metal mesh structure.When the light transmittance is 95.2%,the maximum normalized high-order diffraction energy is 0.0167%,which is nearly 90% lower than that of the square metal mesh at the same visible transmittance,and there is a 3 dB improvement on electromagnetic shielding effectiveness in the Ku-band.The transmittance of multi-period micro-ring metal mesh and ultrathin doped Ag film double-layer structure is 92.0%,and the shielding effectiveness is ~45 dB in the Ku band.The new transparent electromagnetic shielding technology based on ultra-thin metal film proposed in this paper can achieve high visible transmittance,low imaging quality impact,ultra-wide band strong electromagnetic shielding and low electromagnetic reflection.These technical indicators are listed in the best level of related reports at home and abroad,and laid the foundation for the new generation of transparent electromagnetic shielding technology. |
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