平面复合金属微纳结构的圆二色性研究

Studies of circular dichroism of planar composite metal nanostructure arrays

圆二色性效应在圆偏振器、光调制器及光电器件等方面具有广泛的应用.为提高平面金属微纳结构的圆二色性,本文设计了由无限长纳米线和G形微纳结构组成的平面复合金属微纳结构,并应用有限元方法研究了该阵列微纳结构的圆二色性特性.数值计算结果显示,在圆偏振光的激发下,G形微纳结构和平面复合金属微纳结构均出现了电偶极子、电四极子和电八极子等共振模式.当G形微纳结构与无限长纳米线连接时,各共振波长均发生红移,并且无限长纳米线增加了不同圆偏振光激发下的局域表面等离激元共振强度,从而使得平面复合微纳结构的圆二色性信号明显增强.此外,还研究了平面复合微纳结构阵列的几何参数对其圆二色性特性的影响.这些结果为提高平面手性微纳结构的圆二色性信号强度提供一定的指导思路和方法.

Circular dichroism effects have been widely used in circular polarizers,optical modulators and optoelectronic devices.Periodically arranged artificial metal chiral nanostructures has a strong electromagnetic coupling effect with light,which can greatly increase the interaction between the light and matter.Threedimensional helix and helix-like chiral nanostructures show a larger circular dichroism effect due to the strong interactions between electric and magnetic resonance.The double-layer structures also can produce large circular dichroism,which signals also results from electric dipoles with different orientations between the two layers.Although the three dimensional plasmonic structures have shown large circular dichroism signals,however,three dimensional devices hold disadvantages in wide practical applications because of their complicated fabricating process,especially at micro-and nanoscales.Recent years,circular dichroism signals of planar nanostructures have been studied owing to their easy fabrication and wide potential applications.The resonance mode of planar metal nanostructures is sensitive to the shape,geometry,materials and surrounding environment of nanostructures,which provides a feasible technical approach for adjusting the circular dichroism signal of planar metal nanostructures.In this article,larger circular dichroism signals are realized through planar composite golden nanostructures,which composed of infinite long nano wire and G-shaped nanostructure.The absorption spectra,surface charge distributions at resonance wavelength of planar composite golden nanostructure are calculated by finite element method.For comparison,a circular dichroism signal with only Gshaped nanostructures is also studied.The numerical results show that under the illumination of right-handed polarized and left-handed polarized light,the planar composite golden nanostructure and G-shaped nanostructure exhibit electric dipole,quadrupolar,octupolar resonance modes,respectively.When the G-shaped nanostructure is connected to an infinitely long nanowire,all resonance peaks have a red shift and infinitely long nanowire increases the local surface resonance intensity under different circularly polarized light excitation.Therefore,it significantly enhances the circular dichroism signal of the planar composite golden nanostructure.At the same time,the influence of geometric parameters such as the different length of each nanorod of the Gshaped nanostructure and the thickness of the infinitely length nanowire on the circular dichroism modes are also studied.The findings may provide some guideline and methods for improving the circular dichroism signal of planar chiral nanostructure.