**6. Design of nano-antenna**

From the various type of optical nano-antenna, the bowtie shape is the preferred choice in this study, as compared with a dipole [26]. Plasmonic BNAs is usually designed due to the confinement of the electric field in the gap region, working at higher frequencies, and keeping the whole size much smaller (nano-meter) [9]. Regarding the sharp tips of the two arms of the bowtie antenna, the group and phase velocities of surface plasmonic waves decrease with the distance of propagation and finally become zero [27]. BNAs are expected to possess a relatively broad bandwidth because they represent the two-dimensional analogue of a biconical antenna [28]. The localized plasmonic near-field, which is highly sensitive to the refractive index of its surrounding medium can be tuned by adapting the nano-structure shape. Geometrical parameters such as size, gap distance, height, and bowtie apex angle have a direct effect in the LSPR [29].
