| Electromagnetic absorption plays a crucial role in many applications such as solar cells,biosensors,electromagnetic stealth and thermal emission.The metamaterial based on terahertz waves has a large loss at the resonance frequency,which can be used to regulate the amplitude,phase and polarization state of electromagnetic radiation.Therefore,it has displaced traditional materials to become a must-have for making absorbers.However,since the absorption efficiency and operating wavelength of metamaterial absorbers are determined by their structure and cannot be easily controlled,it is difficult to further improve them in such areas as modulation and optical switch.In this context,researchers have discovered that graphene,a type of metasurface material,can change optical property by the methods like electrical doping.Specifically,the plasma frequency of graphene can be changed by changing its Fermi level.This feature endows graphene with a unique advantage in terms of dynamic absorption.In recent years,the coherent perfect absorption technology brings forth new ideas for realizing the tunability of light absorption.Coherent perfect absorption is usually interpreted as the technology that absorbs all lights in lossy material due to the interference of two back-propagating coherent waves.So far,a large number of structures,including metal-insulator-metal structure and all-dielectric metasurfaces,have achieved coherent perfect absorption.However,many coherent perfect absorbers are only able to operate at one frequency band,thus greatly limiting their applications in broadband areas like solar cells.For this reason,it is critical to investigate the application of coherent perfect absorption in multiple bands.By focusing on the regulation of Fermi level,size and insulation layer of grapheme,the present study intends to investigate the coherent absorbability of single insulation layer,multiple insulation layers and a structure with different sizes of single insulation layers.The specific studies are as follows:1.The effect of regulating graphene's Fermi level through applied electric field on the super-surface of the graphene layer-insulation-graphene layer on the peak frequency of coherent absorption was studied.Numerical simulation was conducted to discuss the absorption characteristics of the structure under different applied electric fields;the absorption spectrum in the process of transitioning from a symmetric coherent source incidence to an antisymmetric coherent source incidence was showed in such cases.When the Fermi level of the graphene decreased,the resonance frequency of the relevant absorption peak showed blue shift;conversely,it showed red shift.When two graphene patches of a cell were controlled by different applied electric fields,dual-frequency absorption took place,and the control range was from 7.6Hz to 12.5Hz.In addition,the resonance frequency of an absorption peak was changed while ensuring that the position of another absorption peak was unchanged.By adjusting the phase difference of the two beams of incident light,the absorptivity changed from 0% to 98%.This can be effectively applied to the field of light modulation.Considering the symmetry of the structure,the absorption effect did not change significantly when the TM and TE polarized waves were incident.The research data in this stage provide a considerably significant reference for the expansion of single-frequency and dual-frequency coherent absorbers.2.In order to achieve a better wave-absorption effect,a super surface structure model with multiple-unit graphene patches under multiple-layered and multi-voltage control was designed by means of overlaying dielectric layers.First,this thesis analyzed the spectrogram of the absorber under a constant applied voltage.Due to different composite states of the Fermi level of the graphene patches,double peaks or four peaks might appear on the super surface,providing an experimental basis for the physical mechanism of such super surface.Next,taking multiband absorption as an example,the positions for resonant frequency were monitored under the precondition of keeping some absorption double peaks unchanged,to flexibly adjust the remaining resonant positions within the scope of 6.7THz-18.2THz,which is much larger than the monolayer super surface stated in the previous chapter.Based on the electromagnetic theory and a massive numerical simulation dataset,the analysis of the electromagnetic field distribution conducted under the resonant frequency in this study suggested that,apart from the fact that the dielectric layer between graphene layers has some constraint on light,the main absorption mechanism could take place because the single absorption peak in each graphene group triggers superposition and resonance,both of which are not mutually correlated.Through phase adjustment of the two incident light beams,the adjustment of absorptivity in the multiband absorption of such super surface was realized.The depth of the adjustment was also found to be superior to most similar absorbers.In addition,by virtue of adjusting the applied voltage,this study was able to make the multiband resonant absorption peaks to gradually form a broadband with a scope reaching 7.2THz.Under such circumstance,the absorber could flexibly switch from ON(absorptivity >0.9)to OFF(absorptivity<0.2).The research results indicated that when the angle of incidence is less than 60°,the average absorptivity of such structure will not be lower than 90°.This super surface wave-absorption trait guaranteed a better absorption effect by adding the number of graphene group or further increasing the Fermi level.Therefore,the proposed method shows that the application of these traits in the realm of optical modulation is feasible,and that the realization of such application is worth anticipating.3.A multilayered and multi-sized super surface was designed to study the effect of the patterned graphene size as well as the dielectric constant and thickness of insulating layer material on the absorption peak.The absorption performances of coherent incident lights at normal incidence in the same direction were calculated and compared through numerical simulation.First,it perfectly absorbed dual band coherence.The calculation results and electromagnetic field distribution indicated that the horizontal axis and the vertical axis of the square annular graphene pattern would respectively incite a characteristic frequency.The two points of resonance had a linear superposition and finally achieved a perfect absorption effect.Subsequently,the wave-absorption performance at an oblique incidence was observed.When the angle of incidence was less than 75°,the absorptivity of both points of resonance became higher than 99%.Under the size effect,we simulated three conditions,i.e.,to adjust the vertical axis while keeping the horizontal axis width constant;to adjust the horizontal axis while keeping the vertical axis height constant;and to simultaneously adjust both the horizontal axis and the vertical axis.The spectrogram results all complied with the previously made speculation,proving the possibility of keeping the features of one absorption peak constant while adjusting the other absorption peaks.After improving the structure of the cell,the sizes of the absorption peaks generated in multiband resonance were adjusted to draw them closer and achieve the broadband absorption.Afterwards,the effects of dielectric constant and thickness of the insulating layer material on the absorption performance were further explored.The analysis results indicated that the insulating layer material determines the characteristic frequency.The confirmation of the relationship between them can largely alleviate the influence of error in the actual preparation,and also give full play to the adjustability of frequency.By controlling the two parameters,the resonant frequency can be shifted to any desired position.The proposed absorber does not rely on complex patterned graphene,and thus it will become an excellent reference for the making of dual band structure. |
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