Study On Unidirectional Reflectionlessness And Non-reciprocal Perfect Absorption Properties In Metamaterials

In recent years,with the advent of metamaterials,it has exhibited extraordi-nary phenomena that are not available in nature and has broad application prospects in negative refraction,invisible cloaks,perfect lenses and perfect absorption.Thus researchers have drawn great attention on it.Metamaterial perfect absorber as one of the most important applications can control the electromagnetic response by engi-neering the metamaterial geometry and size.Unlike conventional perfect absorbers,the mechanism of metarnaterial perfect absorber is to properly operate the electrical and magnetic responses so that the impedance of the structure and the free space are matched,so the reflection is minimized to zero and at the same time utiliz-ing the dissipation of metamaterial structure response to cancel the transmission,thus achieving a perfect absorption on the verge of 1.Another recent application of metamaterials is the Parity-Time(PT)media,and the related theory is called PT symmetry theory,which was proposed by Bender and colleagues in 1998.At first,PT theory was considered to be an interesting mathematical discovery without practical application.In recent years,with the rapid development of PT media,it has been gradually applied to optical waveguides,perfect cavity absorber lasers,mi-crowave cavity absorbers,and resonators.In particular,it is widely used in various systems such as metamaterials.We will propose a scheme to achieve the narrow-dual-band perfect absorption plasmonic sensor in metamaterials based on the coupling of two resonators.In the scheme,The absorptance of dual resonant peak all exceeding 95%at l1 = 270 nm and 98%at l1 = 290 nm,respectively,where the full absorption width at half-maximum is about 3%(low frequency)and 5%(high frequency).In addition,we demonstrate that the metamaterial perfect absorber can work as a sensitive sensor for refractive index sensing.The figure of merit FOM*reaches 197.7(low frequency)and 274.8(high frequency)when l1 = 290 nm.On this basis,we upgraded the plane structure to stacked structure,and pro-pose a scheme for achieving unidirectional reflectionlessness and perfect nonrecip-rocal absorption based on neax-field coupling.The numerical calculation is carried out using a commercial finite-difference time-domain software package,and the the-oretical analysis is accomplished by using the two-harmonic oscillators model.The result of theoretical analysis showed good agreement with the numerical simulation,which proved the feasibility of our scheme.Finally,we propose a scheme for switching the unidirectional refectionlessness by polarization in double crosses structure based on the phase coupling.In the scheme,the unidirectional refectionlessness could be efectively controlled only by adjusting the phase coupling of the two resonators,and our system is non-PT-symmetric.At the exceptional point,we realized the unidirectional refectionless phenomenon.Besides,unidirectional perfect absorber occurs in the vicinity of ex-ceptional point.