Study On The Plasmon Mode Of Gold Nanoring

Localized surface plasmon resonances in metallic nanostructures have the characteristics of subwavelength localization and near-field enhancement,which is widely used in many fields.In particular,the metallic nanoring structures,because of its large cavity volume,can form a uniform and stable electric field inside their cavity,so they are extremely suitable for sensing,detection,solar cell,etc.Therefore,the study of metallic nanoring structures which supports plasmon resonance is one of the research hotspots.In this thesis,finite-difference time-domain(FDTD)algorithm and photo-emission electron microscopy are used to comprehensively study the plasmon mode distributions of gold nanoring.The optical response of gold nanoring with different sizes is simulated with the software of FDTD solutions,the generation conditions of the bright and dark modes of plasmon are analyzed from the incident angle of light and so on.The simulation results show that the plane wave source irradiating gold nanoring at oblique angle can excite the bright mode and dark mode simultaneously,while only bright mode can be excited by vertical irradiation of light.The results also show that the plasmon resonance peaks are very sensitive to the ratio of thickness and outer diameter of the nanoring(T/D).As the ratio of T/D decreases,the plasmon resonance peaks appear redshift.It is further found that the charge distribution in gold nanoring is abnormal under the p-polarized incident excitation.We attribute this abnormal distribution to the existing of z-directed electric component under p-polarized light illumination.This component will induce the electric charges oscillating between the top and bottom surfaces of nanoring,and between the bottom surfaces of nanoring and the surface of the substrate.This charge oscillation results in the abnormal charge distribution in the monolayer surface of the nanoring.On the other hand,the dynamic evolution relationship between different excitation points is revealed.The results show that the charges oscillation of the two hotspots with the strongest intensity is synchronous,but with opposite phase under the excitation of s-polarized light;For the p-polarized excitation,the electric field enhancement of the position which is far from light source is much higher than that of near light source position.Moreover,the oscillations in the position which is close to the light source start earlier than that of the farther one,and the oscillations will be in phase after the disappearance of the external light source.Finally,high-resolution in-situ imaging of the dark-mode near-field distribution in single gold nanoring is realized by photo-emission electron microscope(PEEM).The experimental results show an obvious quadrupolar distribution under the s-polarized light illumination.For the p-polarized light excitation,a hot spot can be observed at the farther light source position.The experimental results are in good agreement with theoretical simulation results.Gold nanoring owes the characters of extremely high sensitivity and quality factor,which fully embodies excellent sensing property.It is of great significance for the development of sensing application.