Research On The Near-field And Far-field Characteristics Of Nano-metallic Optical Antenna

Nano-metallic optical antenna is a kind of nano-metallic photonic device which can break the diffraction limit and couple the optical frequency electromagnetic wave to nano scale space efficiently.Because of its support for surface plasmon,nano-metallic optical antenna has many properties such as highly field localized,highly field enhancement and modify emission directivity,these new and unique optical properties make it popular in many fields,for example high-resolution near-field optical microscope,high-efficiency solar cells,nanolithography,etc.This paper designed a new type of nano-metallic optical antenna,double butterfly-shape nano-metallic optical antenna.The optical properties of double butterfly-shape nano-metallic optical antenna are researched based on finite difference time domain method and surface plasmon theory,including near-field local field enhancement,near-field electric field distribution and far-field directivity.The main contents are as follows :1.This paper studied the near-filed characteristics of rectangle,trapezoid,butterfly-shape three basic symmetry dipole structure of nano-metallic optical antenna.The study found that: butterfly-shape symmetry dipole has the largest field enhancement factor,but the field enhancement is extremely unstable when changing the polarization direction of the incident light.2.This paper studied the influence to the field enhancement and electric field distribution of the double butterfly-shape nano-metallic optical antenna caused by the gap between antenna arms and the different polarization direction of the incident light.The study found that: double butterfly-shape optical antenna has the stronger field enhancement than butterfly-shape symmetry dipole,the electric field enhancement can reach 390 times by selecting the appropriate structure parameters;with the decrease of the gap,the distribution of field enhancement gradually transferred from the antenna's center to the edge,and at the same time,the electric field intensity of the center of the antenna decreases;different from the butterfly-shape symmetry dipole,the field enhancement of double butterfly-shape optical antenna is very stable when changing the polarization direction of the incident light.3.The influence to the far-field directivity of the double butterfly-shape nano-metallic optical antenna caused by the change of structure parameters was researched in this paper.The study found that: the decrease of the gap between antenna arms can make the far-field pattern shows apparent side lobes and the main lobe gain increase;far-field pattern will show apparent side lobes when the dipole is placed a appropriate distance beneath the antenna structure vertically;the increase of length is the main factor to make the side lobes appears in the far-field pattern;the increase in antenna thickness also can causes side lobes in the far-field pattern and the main lobe gain first increases then decreases.The results show that,double butterfly-shape nano-metallic optical antenna designed by this paper has the strong local field enhancement and is very stable when changing the polarization direction of the incident light.The antenna coupling with dipole source can change the radiation directivity of the dipole source by adjusting structure parameters and has a more flexible regulation to the far-field directivity.This paper verified the effectiveness of the design through simulation experiments,the research work of this paper provides a reference of experience for the design of nano-metallic optical antenna.