| Metamaterial(MM)is usually defined as a new type of artificial material composed of sub-wavelength metal and dielectric with periodic arrangement of unit cells,which exhibits exotic physical properties that is not existed in nature,breaks the limitations of traditional material design,and greatly improves the combination property of designed material.Its precise shape,geometry,size,orientation and arrangement give smart properties capable of manipulating electromagnetic waves.Nowadays,MM has developed as a frontier interdisciplinary involving physics,material science,chemical,and so on.MM electromagnetic wave absorber possesses many advantages than the traditional absorber,such as perfect absorption,ultra-thin structure,adjustable working frequency,and flexible electromagnetic parameters and structure.It can be applied to the fields such as sensors,detectors,solar photovoltaic(PV),and so on.In recent years,MM electromagnetic wave absorber has become one of the research hotspots in the field of micro-nano photonics.In this thesis,the design and theoretical simulation of electromagnetic absorbers based on metamaterials are performed.The electromagnetic absorption characteristics of various metamaterial absorbers are calculated by using FDTD Solutions software based on the FDTD method.The working frequency band of the designed and simulated electromagnetic wave absorbers includes single band,dual-band,broadband and so on,and its working wavelength covers visible,near/middle infrared and terahertz wave band.The electromagnetic absorption mechanism of the metamaterial absorber is analyzed by impedance matching theory.The influence of the geometric parameters and material properties of the metamaterial on the electromagnetic absorption efficiency is discussed.At the same time,the influence of the incident angle for TM and TE polarization on the metamaterial electromagnetic absorber is further discussed.The main contents of this thesis are as follows:Firstly,a narrow-band metamaterial absorber based on metal grating structure is designed and simulated.The device can absorb electromagnetic wave in different frequency bands in the range of 600~1300 nm.Based on the distribution characteristics of the electromagnetic field and energy flux density,the electromagnetic wave absorption mechanism is analyzed in detail.The electromagnetic absorber can be considered as nearly perfect single band absorption when the grating width is less than 250 nm.However,the device exhibits dual-band absorption when the metal width is about 360 nm.Moreover,tuning of the resonant absorption wavelength can be achieved by adjusting the thickness of the metal grating and the dielectric layer.Therefore,the structural parameters in the unit cell can be designed according to practical needs.In order to overcome the shortcoming that the proposed grating structure shows sensitive to the polarization mode of the incident wave,we have further designed a new metamaterial electromagnetic absorber based on cylindrical array,which exhibits the characteristics of polarization insensitivity,and can achieve dual-band absorption in the visible range.The common features of these two types of electromagnetic absorbers are the properties of narrow band and high light absorption.Next,a broadband electromagnetic absorber based on the pyramidal array is designed and simulated in this thesis.The pyramidal unit structure is composed of an alternating metal(Al)and dielectric(Ge)multilayered thin films.The device exhibits ultra-broadband electromagnetic wave absorption in the range of 0.2~3.6 μm.The electromagnetic absorption mechanism of the metamaterial absorber is investigated based on surface plasmon resonance as well as slow wave modes,and the electromagnetic field distribution in the unit structure is explained.The research results show that the device has advantages of high absorption,wide-angle,polarization-insensitive and azimuthal-insensitive,and it has important application prospects in solar photovoltaic,thermal emitters and so on.Obviously,to fabricate the tapered multi-layer nanostructures will face technological challenge.In order to simplify the unit structure,a two-dimensional metamaterial electromagnetic absorber based on the metal-dielectric-metal(MDM)array is designed.Results show that,in the range of 600~1300 nm,the absorption efficiency is higher than 80% over a bandwidth of about 300 nm.Furthermore,the electromagnetic absorption mechanism of the simplified structure is analyzed,and the influence of the structural parameters on the absorption efficiency is discussed.The theoretical work present here provides a basis for the applications in solar photovoltaic and other fields.Terahertz wave is an emerging research hotspot and has many important applications,accordingly,we extend the working waveband of the metamaterial absorber to terahertz.Three metamaterial terahertz absorbers with narrow band or broadband absorption are designed and simulated in this thesis,to study the electromagnetic absorption in the terahertz waveband.For a narrow band terahertz absorber,a relatively simple structure based on the metal plate is designed to achieve resonant absorption,and the relationship between the structural parameters and the resonant frequency is obtained by using equivalent LC circuit model.The proposed structure is simple,easy to fabricate and with tunable working freqency,which is suitable to be applied in detectors and other fields.According to the methods of achieving broadband absorption,the terahertz absorber based on the stacked metal structure and the coplanar metal structure is proposed and simulated,respectively.It is found that broadband absorption in the terahertz regime can be achieved by overlapping the absorption peaks for the metals with different size.Based on the characteristics of electromagnetic resonance at the resonant frequency and the surface current distribution,the electromagnetic absorption mechanism of the metamaterials terahertz absorber is discussed,and the influence of the structural parameters on the absorption is analyzed.The research results of the broadband terahertz absorber obtained in this thesis provide a theoretical basis for the applications in electromagnetic stealth,broadband communications,security check and so on. |
