The Study Of Anomalous Scattering Properties And Underlying Mechanism Of Metallic Textured Structures

Metallic nanostructures support surface plasmon polaritons(SPPs),which can achieve localization and manipulation of light fields at subwavelength scales.It is a frontier research hotspot in the field of optics,and its optical properties are the core basis for applications such as nonlinear optics and high-performance detection.However,due to the inevitable loss of metal,field enhancement effect caused by localized surface plasmon resonance is greatly weakened,which affects the development of high-performance photonic devices based on nanostructures.In this thesis,we focus on the localized states in the subwavelength metallic nanostructures to reaval the underlying mechanism,and then study what’s the influence of absorption on their optical properties,such as scattering and localization of light fields.To reveal the influence of absorption and exclude the influence of other factors,we take the metal cylinder with artificial nanostructure of periodic subwavelength metal grooves on the surface working in the terahertz band as studied model,which has been widely studied in the field.The losses are introduced by controlling the lossy medium in partial grooves.The studies range from two-dimensional(2D)infinitely long structures to three-dimensional(3D)finite heights and subwavelength ultrathin disk structures.The specific research contents and the results obtained are divided into the following aspects:(1)Anomalous scattering and underlying mechanism of 2D subwavelength periodically textured metal cylinders.Previous studies have reported the existence of spoof local surface plasmon polaritons(SLSPs)in this structure.We revisited the structure and found that in addition to SLSPs,there is a class of bound states in the continuum(BICs)due to structural symmetry when the structure is lossless,and the electric field is sharply enhanced and confined to the grooves and surfaces of the cylinder.BICs modes and SLSPs are essentially two eigenmodes generated by the coupling of two SPPs in the same channel in the structure:one is the BICs mode without loss and the other is the SLSP mode with radiation loss.Temporal coupled-mode theory(TCMT)verifies this result,and the numerical simulation and theoretical calculation results correspond extremely well,confirming the reliability of the research results.The findings open up a new avenue for further enhancing light-matter interactions in plasmonic structures,with great potential for applications in sensors,nanoscale lasers,and nonlinear optics.(2)Anomalous scattering and underlying mechanism of 3D subwavelength periodically textured metal cylinders.Based on the above research,research contents are extended from2 D infinite length to 3D finite height,and the anomalous scattering properties and underlying mechanism of finite-scale metal cylinder nanostructures are studied.A series of 3D metallic nanostructure systems was constructed and studied,and the scattering cross sections of the structures were numerically calculated.The study found that the BICs found in the 2D system also exist in the 3D structure,with the same properties as the 2D case,proving the existence of BICs in the 3D case.This result breaks the common sense that BICs only occur in single particles with zero-refractive-index metamaterials or closed hard walls.(3)Anomalous scattering and underlying mechanism of lossy periodically textured metal cylinders.Based on previous research,by introducing loss in some grooves,the influence of system loss on the scattering properties of metal cylinder nanostructures is studied.It is found that at a certain frequency,no matter how the loss changes,the absorption cross section of the metal cylindrical nanostructure is always almost zero.Scattered field distribution and eigenmode analysis show that this loss-independent anomalous scattering is due to the existence of a lossless bound state in the nanostructure caused by symmetry breaking,the electric field of this bound state is mainly distributed in the grooves without loss,while there is essentially no electric field in the grooves withloss.This study provides a new method to facilitate the design of high-performance photonic devices with lossy metal cylinder nanostructures.