DESIGN AND ANALYSIS OF A CROSS STRUCTURE BOWTIE NANOANTENNA

Abstract

Optical nanoantennas are a developing concept in physical optics. They enhance the interaction of light with nanoscale matter because of their ability to convert the freely propagating optical radiations into localized optical fields and vice-versa. They can be used in ultraviolet (UV), infrared (IR) and visible region. Optical nanoantennas are analogous to the radio antennas but there is slight difference in their scaling properties. This is because of the fact that at optical frequency, metals do not behave as perfect conductors. The most claimed advantage of optical nanoantenna is that it can be used for a wide range of wavelength as its tunability depends on the dimensions of the nanoantenna. A metallic resonant optical antenna of a four-fold symmetric ‘cross’ geometry consisting of two perpendicular nanosized gold bowtie antennas with a common feed gap in between them has been designed. It is able to convert the freely propagating fields of any polarization state into localized field at the feed gap of the nanoantenna. The designed metallic cross bowtie nanoantenna exhibits high radiation enhancement and localization of the field in the feed gap at the resonant wavelength of 500 nm. This nanoantenna has been made using gold, and its properties have been enhanced through geometry optimization to provide highly localized radiation enhancement at the resonant wavelength. The geometric parameters that are used for optimization of the designed nanoantenna are length of the antenna arm and the flare angle of the bowtie. The reason behind using wavelength of 500 nm is that at this wavelength solar radiation is maximum. Thus, the designed nanoantenna can be significantly used for solar energy harvesting. The antenna is modeled and its spectral analysis is carried out through finite element method (FEM) simulations using the ‘COMSOL Multiphysics’ software package in the visible regions. The effect of antenna arm length and flare angle of bowtie on resonance wavelength, scattering cross-section, field enhancement, far-field pattern, directivity and radiation efficiency are analyzed in detail. Thus, the proposed nanoantenna can be utilized for applications requiring strong radiation localization along with optional polarization control in the visible region. So, the designed nanoantenna has immense scope in the techniques such as surface enhanced Raman spectroscopy (SERS), fluorescence spectroscopy, and solar energy harvesting.