Design And Control Method Of Near Perfect Absorbers Based On Two-dimensional Materials

In recent 10 years,the research work related to the new two-dimensional materials has aroused great concern of the researchers.This is because the two-dimensional material has excellent physical and chemical properties,such as high electron mobility,high thermal stability,good high-temperature stability,chemical stability,and so on.At present,the most attention is paid to the two-dimensional materials,such as graphene,boron nitride,transition metal sulfides and black phosphorus,etc.Compared with the traditional absorbers,the use of two-dimensional materials can be designed for dualband,broadband,ultra-thin and adjustable perfect absorbers.In this paper,we mainly study several perfect absorbers based on two-dimensional materials,and get some meaningful results.The main work of this paper can be summarized in four parts,as follows:1.The infrared absorber based on the boron nitride(hBN)crystals:This section is mainly that we use the hBN crystals to replace the traditional film absorption layer,which can achieve full absorption in the mid-infrared band.Firstly,hBN is a new kind of two-dimensional material with many excellent physical and chemical properties.More importantly,in the mid infrared band,hBN has the characteristic of anisotropic hyperbolic,and one of its dielectric constant tensors is different from the other two components.The excellent physical properties provide the possibility for us to use the hBN crystals to replace the traditional absorption layer to design the critical coupling resonator.It is found that not only the total absorption in the mid-infrared band can be realized,but also the generation of total absorption can be controlled by changing the incident angle,the thickness of the boron nitride and the intermediate layer.Then,we analyze the physical mechanism of the critical coupling starting from the principle of coherent perfect absorption.Finally,we also discuss the coherent perfect absorption phenomenon based on the hBN crystals.We find that unlike the general condition of different absorbers to achieve perfect absorption,it does not need to satisfy the reflectance and the transmittance equal to zero,as long as the two coherent incident light have equal amplitude and their initial phase difference is equal to zero.2.Tunable mid-infrared absorbers based on graphene/hBN superlattices:this part mainly discuss that we can achieve the dynamic control of full absorption through the design of the absorber based on graphene/hBN superlattices.Graphene,due to its excellent physical properties,especially electronic control,is often used to control the production of some physical phenomena.hBN,on the other hand,can be used as an excellent substrate for the formation of graphene/hBN heterostructures.Therefore,when we use graphene and hBN to form the graphene/hBN super crystal,the crystal will also show the tunable and hyperbolic characteristics,and the equivalent dielectric constant of the super crystal can be described by the equivalent medium theory.By deriving the reflection and absorption of our structure,we obtain the conditions for the perfect absorption.For the TM waves,the total absorption can be achieved at two different wavenumbers and incidence angles,but for the TE waves,it is possible to achieve near-perfect absorption because the imaginary part of the dielectric constant is not equal to zero.However,both for the TM waves and TE waves,the dynamic control of total absorption can be achieved by adding the static bias and changing the chemical potential of graphene.3.Dual band absorbers based on FP resonance and surface phonon polarization:This section is mainly that we can achieve dual band near-perfect absorption by designing a simple hBN/medium/hBN three layer structure.In the design of perfect absorbers,the structure is often too complex or difficult to fabricate,so it is very important to design a simple and easy manufacturing perfect absorbers.In this chapter,we have designed a simple hBN/medium/hBN three layer structure to study the generation of total absorption.Research shows that,for the TE waves,with the Fabry Perot(FP)resonance of the middle layer and the hBN's surface phonon polarization effect,we can achieve near-perfect absorption at two different bands.These two mechanisms are very sensitive to the change of the thickness of the top hBN layer and the middle layer.The surface phonon polarization of hBN is related to the thickness of hBN,and increases with the increase of thickness of hBN.The FP resonance of the structure is also affected by the thickness of hBN and the thickness of the interlayer.When we choose the appropriate thickness of the intermediate layer and the top hBN layer,both the FP resonance and the surface phonon polarization will exist,and they can be coupled with each other to influence the reflection of the structure.4.Ultra-thin and broadband perfect absorbers:The two work of this section is to design ultra-thin absorbers and broadband absorbers.In the past,when we design the perfect absorbers,the absorber material is often too thick or the absorber material has high loss.The very thick perfect absorber structure is not suitable for the needs of the application and the high loss of the absorber material is not easy to find,therefore,it is interesting to work on how to design the ultra-thin absorber structure.In this chapter,we use the hBN's near zero dielectric constant characteristics to design the absorber structure.The structure can be very thin and the loss absorber material itself is not very high,and the thickness of the designed structure is 1/1000 times of the wavelength.Another work in this chapter is to design broadband absorbers.We use the high refractive index of BN to design a single layer cell structure:BF11-SiO2-Graphene/BN/Graphene-SiO2-BF11.When the structure is symmetrical,that is,the thickness of the two sides of the SiO2 is equal,we can optimize the parameters of the structure to achieve the full transmission phenomenon,and we can also achieve the full absorption phenomenon by changing the symmetry of the structure.Therefore,by changing the thickness of the two sides of the SiO2,the full transmission phenomenon can be changed to the full absorption phenomenon.We also find broadband absorption can be achieved with the use of a cascade approach,increasing the number of layers of the cell structure.