Bound States In The Continuum In The One-dimensional Plasmonic Grating

Bound states in the continuum(BICs),as the name suggests,are spatially localized non-radiative states with theoretically infinitely long lifetimes though their frequencies lie inside the continuous spectrum.Since the first proposal of this concept in the quantum system by von Neumann and Wigner in 1929,BICs have been revealed as a general wave phenomenon in many physical systems from quantum to classic waves.Different approaches have been applied to explore the physical mechanism of BICs.BICs have been confirmed experimentally in different ways.Due to the theoretically infinitely high Q factor,BICs are of great interest in the ultrasensitive optical absorption and strong light enhancement,leading to numerous application.This thesis is mainly about whether a dimerized plasmonic grating supports BICs and makes a systematic study on the physical mechanism of BICs generated by the above structure.Firstly,we investigate the multipole response of the simple grating and the BICs it produces.Then we investigate whether the dimerization isoperimetric gratings support BICs.We create a dimerized plasmonic grating structure system and excite it by plane wave,using TM wave,and adjust the structure parameters.In the frequency range we study,when the plane wave is incident normally,two resonance modes are found.According to the electric field distribution,they can be distinguished into symmetric mode and antisymmetric mode.In this frequency range,through the numerical simulation to calculate the transmission spectrum of the system,and can be found from the transmission spectrum has four bands,in which three bands appear the resonance characteristics disappear,shows that the existence of the BICs,two of the high frequencyΓ point BIC and a low frequency off-Γ(accidental)BIC.Usually,the BICs can be classified into two types: symmetry protected BICs and non-symmetry protected(or accidental)BICs.The former ones arise from the symmetry incompatibility between the confined mode and the external field,while the latter ones come from the destructive interference between the leakage channels of the system.At the same time,we obtain the band diagram by solving the eigenvalue problem,and calculate the Q factor corresponding to the band.The location of the Q factor divergence was consistent with the location where the resonance characteristics disappeared,further confirming the existence of BICs.Then,by adjusting the geometric parameters of the unit lattice of the dimerized plasmonic grating structure,we study the influence of the geometric parameters of the structure on the BIC,such as the lattice periodicity or the gap between the two bars in a unit cell,so that the accidental BIC can be adjusted easily.Both methods can adjust the off Γ point(accidental)BIC to Γ points,and the adjustment period and gap,the accidental BIC move to Γ point.The divergence of the Q factor of Γ point BIC has adjusted on the comparative study.Finally,by calculating the multipole moment of the system,we find that the dimerized plasmonic grating provides plentiful multipoles,and the interference between them can give rise zero-radiation directions.The accidental BIC results from the overlapping of the radiation channel of the system with the zero-radiation direction of the unit cell.The BICs on the simple grating and the dimerized grating are interpreted from the point of views of mode symmetry and multipole interference.