Research On Bound States In The Continuum In Metallic Grating

The most well-known bound states in the metallic reflection grating structure is surface plasmon polarization(spoof SPP).In recent years,the research on spoof SPP in metal reflection grating structures has been very sufficient,and there have been many applications in this area.But through our research,we found the bound states in the radiation region in the metal reflection grating structure for the first time,that is,bound states in the continuum(BICs).BIC is a non-radiative state in the continuum spectrum,historically,the concept of BIC was first proposed by von Neumann and Wigner in quantum systems.In recent years,with the development of micro-and nano-technology,the photonic system has become an extremely attractive platform for experimental observation and practical application of BIC.BIC's quality factor is theoretically infinite.Because of this feature,it has great potential applications in optical devices such as lasers and cavities.This thesis mainly studies the BIC in the simple metallic reflection grating structure,and briefly introduces the BIC in the one-dimensional metallic cylindrical chain.In order to realize the BIC in the metal reflection grating system,two types of grating structures are considered to suppress the radiation leakage and hence generate bound states.The first one is a simple grating with broad grooves in which multiple cavity modes are accommodated.Due to the symmetry incompatibility and the destructive interference mainly from the TM₀ and TM₁ modes,BIC at the(38)point and at off-(38)points are both realized.The second one is a dimerized grating with two grooves in each unit cell.The destructive interference between the modes in the two grooves can suppresses the radiation and BIC at the(38)point are observed.In this thesis,a mode expansion method is mainly used in the theoretical calculation,by which the electromagnetic field mode of the metallic reflection grating is calculated and the dispersion relation of the structure is solved.By comparing the results of theoretical calculations with the results of FEM(Finite-element Method)simulation,we have found that although the high-order electromagnetic field modes in the metallic reflection grating do not determine whether BIC can be generated,it can only affect the location of the BIC point.This thesis also analyzes the mechanism of the formation of BIC in metallic reflection gratings.These research results have extended the understanding of the simple but important optical device metallic reflection gratings,and a new perspective of the formation of BIC in such systems,which are important for the design of optical devices such as lasers and sensors with better performance.