Variation of Nanoantenna Characteristics Using Different Substrate Parameters

Gold nanostrips behaving as dipole nanoantennas (DNAs) do not resonate at the typical half-wavelength when subjected to optical frequencies. Scholars have found these DNAs to have an optical resonance which are linearly... [ view full abstract ]

Gold nanostrips behaving as dipole nanoantennas (DNAs) do not resonate at the typical half-wavelength when subjected to optical frequencies. Scholars have found these DNAs to have an optical resonance which are linearly dependent upon both thickness and aspect ratio as shown by Novotny. Bryant et al have also plotted the linear response of DNAs based on thickness and absolute length. The effect of substrates are often ignored during the theoretical study of optical resonant antenna (ORA) behaviour even though they are required for most practical applications. This work studies the effect of substrate parameters on the resonant length of ORAs using the simulation software CST Microwave Studios. Coupled gold strips are simulated and validated against previously published experimental results. A 250nm x 40nm x 40nm gold strip with a 20nm gap cut in the middle (effectively having a 115nm arm on each side) is simulated on a SiO₂ substrate and excited with a planewave perpendicular to the substrate. The Brendel-Bormann model for gold is used, fitted to experimental data for higher accuracy based on data by Rakić et. al. The gold strip with 40nm thickness and 20nm gap is maintained and placed on a substrate of 1500nm x 1500nm x 10nm while both the arm lengths and substrates permittivities are varied. The resonant wavelength is measured across the central gap. Substrate thickness and number of layers are also changed and the effects noted. It is found that substrate permittivity affects resonance wavelength, while substrate thickness and number of layers affect light enhancement. All variations show a strong effect that is a significant factor during fabrication of such optical antennae.