Light Field Modes And Properties Of Long-range Surface Plasmons Based On Modulated Symmetric Structures

Long-range surface plasmon polaritons(LRSPP)are a type of surface electromagnetic wave mode that propagates at the metal-dielectric interface,featuring long propagation distance,easy planar integration,high wave vector,and high sensitivity.They have a wide range of applications in sensors,nanophotonic devices,integrated circuits,long-distance communication,and signal processing.In the early 21 st century,research on LRSPP has achieved deeper understanding and development.This article elaborates on the research background,development history,and main characteristics of LRSPP,with a focus on their application advantages in sensor technology.A study of the perfect absorption phenomenon of light by continuous metal film symmetric gratings has been conducted,and sensors have been designed based on the mode characteristics,with some optimization of the material structure of the sensors.A symmetric grating structure with a continuous metal film was designed,and numerical simulation research using the finite element method was conducted to achieve perfect absorption of incident light.With a grating period of 400 nm,a dielectric medium of silica(refractive index of 1.46),and a silver film thickness of 20 nm,the maximum absorption rate of 99.47% was achieved through optimization of structural parameters.The corresponding absorption spectrum linewidth was 2.53 nm,and the quality factor was 296.06.The results indicate that LRSPP play a decisive role in perfect absorption,with their electric fields symmetrically distributed on both sides of the silver film and forming a standing wave pattern,allowing for longer transmission distances.As the silver film thickness decreases,the absorption spectrum linewidth decreases,and the quality factor increases;the minimum linewidth of 0.98 nm and the maximum quality factor of 760.0 are obtained when the silver film thickness decreases to 12 nm.By adding a substrate to the continuous metal film symmetric grating,the structure can be designed as a refractive index sensor,still capable of exciting LRSPP.The substrate material is set as silicon,and the grating materials are silica,titanium dioxide,and silicon nitride.By optimizing the structural parameters,the overall sensitivity size relationship is S(Si O2)> S(Ti O2)> S(Si3N4).Based on this,silica is selected as the grating material,and when the gas refractive index changes from 1.000 to 1.001,the highest sensitivity can reach531.2 nm/RIU.Furthermore,in methane concentration detection,the structure sensitivity is maximized within the range of 0% to 4.9% methane concentration,reaching 544.21 nm/RIU.An asymmetric grating structure with a continuous metal film was designed,and the evolution of the absorption spectrum from symmetric grating structure to asymmetric grating structure was studied.At the same time,the electric field modes of asymmetric grating material structure,asymmetric grating width structure,and completely asymmetric structure were analyzed.In particular,the completely asymmetric structure is discussed,with a maximum absorption rate of 90.8%.In addition,a refractive index sensor was designed based on the completely asymmetric structure.With alumina as the grating material in the completely asymmetric structure,the sensor’s sensitivity is optimal.As the hydrogen concentration increases from 75.6% to 100%,the sensitivity reaches 334.78 nm/RIU;as the oxygen concentration increases from 19.5% to 23.5%,the sensitivity reaches 333.62 nm/RIU.