Research On The Dynamical Manipulation Technology Of Electromagnetic Absorption Based On Subwavelength Structure

Recently,subwavelength metamaterials have attracted great attention from all over the world due to their unique electromagnetic absorption ability,and have become a focus research in both domestic and overseas.Unlike traditional natural materials,the absorption ability of subwavelength metamaterials mainly depend on its unit structure.Nowadays,most of subwavelength metamaterials can only achieve static electromagnetic absorption,once designed and manufactured,have fixed absorption characteristics that cannot be adjusted to cope with complex electromagnetic environment.In order to overcome this limitation,tunable subwavelength metamaterials have been proposed to possess reconfigurable functions and change the way they manipulate electromagnetic waves.In the microwave region,there are mainly two approaches to realize the dynamic manipulation of electromagnetic wave.One is to embed electrically active lumped elements,such as PIN diodes and varactors,into subwavelength metamaterials to controll the frequency or amplitude responses of the incident electromagnetic waves.The other method to realize the dynamic manipulation of electromagnetic wave is to use active-controlled substrate materials,such as ferroelectric film,liquid crysta and graphene,and so on.However,there are still some bottlenecks to block their development in the real-application,such as narrow bandwidth,small tuning range,single tunable functionality and so on.This dissertation focuses on the dynamical absorption of electromagnetic wave using subwavelength metamaterials,and some constructive approaches have been proposed to solve the above issues.The main achievements include 1)The broadband polarization-insensitive tunable absorber has been proposed by using active frequency selective surface.2)The active absorption/transmission multifunctional electromagnetic device has been proposed based on the combination of the PIN diode switching and graphene tuning techniques.3)The multiband and tunable absorber has been invented by using graphene and active frequency selective surface.The detailed works and conclusions have been listed as follows:1.In order to expand the opreation bandwidth of tunable absorber,a broadband polarization-independent tunable absorber is proposed based on active frequency selective surface(AFSS).The AFSS is composed of periodic patterns of four-patch array with four PIN diodes loaded among them,and a simple bias network is designed behind the ground plane through metallized via-holes,which can effectively reduce the influence on the absorbing performance.By changing the bias voltage applied on the PIN diodes,the reflectivity of the proposed absorber can be dynamically adjusted from near-unity reflection to less than-10 d B absorptivity under normal incidence over a wide frequency band ranging from 8.5 to 18.2 GHz.Due to symmetrical structure of the AFSS,the proposed absorber still has polarization insensitivity.2.In order to solve the single tunable function of sub-wavelength metamaterials,a multifuctional AFSS structure is proposed based on the combination of the PIN diode switching and graphene tuning techniques.This hybrid design makes our AFSS have the capability of switching transmission and absorption responses and simultaneously tuning their amplitudes.The proposed structure is composed of a graphene capacitor layer and a metallic structure layer.By controlling the bias states of the PIN diodes loaded on the metallic structure,the AFSS can achieve the dynamical switching between transmission and absorption modes.The amplitude for the above two modes can be further tuned by changing the graphene resistance through the external bias voltage.We have provided the equivalent circuit model of the designed structure for well understanding its operation principle.In addition,our AFSS is also verified to possess the polarization-insensitivity and angle-stability properties.The experimental results have fully verified its great reconfigurable performances in both transmission and absorption modes.3.The multiband and tunable radar absorber is presented,which can achieve dynamical modulation of absorbing frequency,bandwidth and amplitude.Such a tunable radar absorber is composed of a graphene capacitor layer and an active metasurface layer.By controlling the impedance characteristic of each layer through the external bias voltages,our absorber can not only realize the frequency-agile absorbing,but also can be switched to the ultra-wideband absorbing mode.In each operation mode,the absorption amplitude can be also dynamically tuned.Both simulated and experimental results have demonstrated that the-10-d B absorbing bandwidth ranging from 4 to 20 GHz can be realized,and the average absorbing amplitude can be dynamically tuned between-4 and-12 d B.In addition,by changing the bias voltages,our absorber can also respectively switch its-10 d B absorbing bandwidth to 4-11.5 GHz and 10.1-19.3 GHz,where the larger dynamic range of amplitude modulation is obtained.The equivalent circuit model is employed to explain the tunable absorption mechanism by analyzing the impedance matching characterisitcs.This hybrid design approach can effectively expand the electromagnetic reconfigurable functionalities of the current tunable absorber,which may further open a novel way of the graphene application in microwave frequencies.