Research On The Mechanism Of Abnormal Diffraction Characteristics Based On Phase Gradient Meta-grating

Electromagnetic metasurface is a very important branch of metamaterials,it is generally composed of artificial units on the sub-wavelength scale through periodic arrangements.These periodically arranged units can flexibly adjust the phase,amplitude and polarization of the incident electromagnetic wave.Because of its simple structure,flexible control and easy processing,it has gradually become one of the hot topics.In recent years,researchers have found that it is possible to introduce phase changes in the phase range of 2π on the interface of the electromagnetic metasurface,which makes it easier to manipulate electromagnetic waves.This provides a new dimension for the study of electromagnetic metasurfaces,that is,phase gradient metasurfaces.However,because the thickness of the phase gradient metasurface is extremely small,it has a limitation of inefficient control of electromagnetic waves.Studies have proved that the phase gradient meta-grating can be used to greatly improve the control efficiency;however,because the phase-graded meta-gratings control the phase periodically,this makes the research of phase-graded meta-gratings have complicated diffraction phenomena.The mechanism of this kind of diffraction is different from the traditional grating diffraction mechanism.This unclear diffraction mechanism has brought considerable difficulties to the study of phase gradient metasurfaces.In addition,because of the appearance of surface plasmons,sub-wavelength scale metal gratings have good application prospects in the fields of optics and acoustics.Therefore,on the one hand,this paper studies the diffraction mechanism of phase gradient metasurfaces;on the other hand,discusses the asymmetric transmission and diffraction phenomena and physical mechanisms that occur when SPPs are introduced.The main content of the paper includes the following aspects:1.Anomalous diffraction characteristics related to parity in phase gradient metasurfacesThis article first discusses the transmission and reflection characteristics of the phase gradient metasurface.After simulation,it is found that under the premise of large angle incidence,the phase gradient metasurface exhibits very good abnormal transmission and reflection characteristics.In terms of physical mechanism,this article uses multiple reflections as a starting point to try to explain the higher order diffraction phenomenon of phase gradient metasurfaces.Our conjecture can well explain the higher order diffraction phenomenon related to the parity of the unit cell number m on the phase gradient metasurface.Simulation and research have found that it is possible to switch between abnormal reflection and abnormal transmission of incident waves by changing the parity of the unit cell number m constituting the phase gradient metasurface.Through multiple simulations of multiple angles and wide frequency bands,the simulation results prove that this characteristic has very good stability.2.Asymmetric diffraction characteristics caused by loss in the phase gradient metasurfaceIn the process of broadband simulation,it is found that in the visible light band,as the incident angle changes,the absorption of the incident electromagnetic wave by the phase gradient metasurface is also changing — asymmetric absorption,and this phenomenon is not only related to the incident angle,but also It is related to the number of identity cells m.For this phenomenon,we suspect that because electromagnetic waves are reflected multiple times in the phase gradient metasurface,the difference in the number of reflections will cause the loss of electromagnetic waves to be different,which results in asymmetric transmission and diffraction.In higher order diffraction,two factors,the phase gradient of the phase gradient metasurface and the number of unit cells m,play a decisive role.After the qualitative analysis,this article makes a quantitative analysis of the influence of the two on asymmetric absorption,and puts forward an expression about m,trying to quantitatively analyze the changes in the degree of absorption.