Studies of circular dichroism of planar composite metal nanostructure arrays

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. Three-dimensional 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 nanowire 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 G-shaped 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 G-shaped 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.