Tuning circular dichroism by capping SiO₂ nanohelixes upon silver nanohelixes

Three-dimensional chiral metamaterials, plasmonic nanohelixes, with chiroptical activity have attracted attention for many potential photonic applications. In previous works, glancing angle deposition (GLAD) has been applied to deposit various Ag nanohelix arrays on smooth substrates and the structure dependent circular dichroism has been measured and analyzed. In this work, two-turn SiO₂ nanohelixes are grown upon Ag nanohelixes to form a SiO₂-Ag nanohelix array. For the bottom Ag nanohelices, the average pitch length (P) and the radii of curvature (R) are 231nm and 137nm, respectively. P and R are 174nm and 118nm for the top SiO₂ nanohelices, respectively. Under left-handed (righthanded) circularly polarized light illumination, the transmittance, reflectance, and extinctance of the SiO₂-Ag nanohelix array are measured. The circular dichroism described with g-factor is also presented here as a function of wavelength. According to the measurement, the g-factor exhibits a shift phenomenon as the SiO₂ nanohelixes are capped upon Ag nanohelixes. This phenomenon leads to flexible control over the circular dichroism of SiO₂-Ag nanohelix arrays with respect to the resonance wavelength and amplitude of g-factor.