Braodband Composite Metamaterial Absorbers Bytailoring Substrates

High performance microwave absorbing materials should have such advantages as broad bandwidth,strong absorption,light weight and thin thickness.Compared with the metamaterial absorbers,till now traditional microwave absorbers still maintain the advantages to achieve ultra-thin,wideband absorption in the high frequency band,like 8 ~ 18 GHz.However,inthe low frequency range,like L and S wavebands,the absorption performance of traditional microwave absorbers are so poor unless greatly increasing the weight or thickness,which not only deteriorate the high frequency absorption but also restrict their applications.For the metamaterial absorbers,it is easy to reachstrong absorption with thin thickness in extremely low frequency.However,only one or several narrow absorption peaks can be obtained by metamaterial absorbers.Considering the respective properties of the two types of materials,we propose a composite of them to achieve strong absorption at both high and low frequency with an ultra-thin thickness.In order to realize this idea,we carry out the research works as follows:First,according to the microwave absorbing mechanism and the concentration effect for electromagnetic energy of the metamaterials,we put forward a viewpoint that the dielectric substrate of the metamaterial can be divided into two areas,the energy concentration area,where most incident energy is focused in,and the redundant area,where less energy distribution located in.As an example,we analyze the field distribution and the energy loss in the square ring metamaterial absorber.It is revealed that the energy of the incident electromagnetic wave is aggregated in the area near the metamaterial metal structure so that there are much redundant area appearing in the substrate.Tailoring of the redundant area do not change evidently the field distribution in the metamaterial at low frequency.Since the redundant area is a result of the field concentration of the meta-structure,it should be a common phenomenon in metamaterials,which is further confirmed by field analysis on several different metamaterial absorbers and polarization conversion metamaterials.However,with frequency increasing to high band,the high ability of field concentration by the meta-structure greatly decrease.The energy tends to distribute evenly in the substrate of metamaterials.Second,we propose and verify an embedded method of a metamaterial absorber by combining multiple absorbers in structure and in absorption.By covering the redundant area of the metamaterial with another patch-like meta-structure,it is verified that the absorption bands of them can be maintained by the composite,revealing that the essence of this widespread method to achieve multiband metamaterial absorbers appearing in literature is the utilization of the redundant area.Furthermore,we use the traditional microwave absorbers and the dielectric square ring metamaterial absorber to build another type complex metamaterial absorber.This composite metamaterial absorber achieve perfect absorption at 1.17 GHz and more than 90% absorption in a broad bandwidth of 7.8 ~ 18 GHz.Through the equivalent circuit model of square ring metamaterial absorbers and the effective medium theory of the traditional microwave absorbers,we give the quantitative analysis for the absorption performance of complex metamaterial absorbers.The agreement on the simulation and measurement of the composite metamaterial absorber indicates that this method is feasible.In addition,more examples of composite metamaterial absorbers by using different metamaterials and traditional material including a resistive sheet show the generality of the principle and versatility of the method.Last,the founding of redundant area may provide large potentiality for assemble metamaterials,which can be used to lighten the weight of absorbers or improve the utilization of the substrate.This has a profound meaning in diverse fields,such as,military stealth and camouflage,tunable metamaterial absorbers,electromagnetic radiations and pollutions.On the other hand,the embedded method,based on tailoring their substrates,can effectively realize combination in a plane so that it can receive a broad absorption band with a thin thickness.It will provide a new and universal methodology for the ultra-thin,ultra-wide absorbers in the near future.