Research On The Characteristics Of Optical Antenna Based On Broadband Nanostructure

Antenna is a tool used to transmit or receive electromagnetic energy, which has a widespread applications. Optical antenna is defined as a device designed to efficiently converse free-propagating optical radiation to local energy and vice versa. Metal optical nanoantenna is generally built up by two metal nanoparticles. At optical frequencies, nanoantenna allows unprecedented control of light within the nano scale. The typical characteristics of nano optical nanoantenna based on plasma is that it could constraints free space electromagnetic wavelength on subwavelength scales and exhibits strong absorbing and scattering properties at special frequencies. Because of the special properties, nanoantenna attracts wide attention and are well applied in surface-enhanced Raman scattering, fluorescence phenomenon, chemical biological sensor, nano lithography, plasmonic solar cells and optical microscope. However, the properties of nanoantenna are strongly shape, material and medium environment dependent owning to surface plasmon resonance.The thesis bases on bowtie and toothed log-periodic nanostructure aiming at optimizing the optical properties of the nanoantenna, studying the multi-resonant field enhancement of the log-periodic nanoantenna and the tunable field enhancement properties of graphene log-periodic nanoantenna. The properties studied of the nanoantenna includes Purcell factor and multi resonant field enhancement and so on.Especially, the dissertation provides a theoretical basis for applications of wideband nanoantenna in plasmonic solar cell and multiple harmonic generation. In addition, it will contribute to the wideband nanoantenna design. For this purpose, it studies the internal relations between the graphene dielectric and the graphene chemical potential, the field enhancement and the radar cross section of the graphene log-periodic nanoantenna. Then the adjusted mechanism law between the field enhancement and the graphene chemical potential are obtained. The main contents of this dissertation is as follows:(1) Based on the Drude dispersion model, The simulation and design platform were established by using finite difference time domain method. Through the research on the mechanism and the structural property of the nano optical antenna, approaches to improve the properties and the related theory basis were explored.(2) For the bowtie nano structure, its Purcell factor、electric field distribution and antenna efficiency were calculated, the influence of the structure parameters on the its optical properties ware analyzed. And the relation between the bowtie nanoantenna and the structure parameters were investigated by changing the structure parameters.(3) According to the broadband characteristics of log-periodic nano structure, through the calculation of field enhancement and Purcell factor for the log periodic nanoantenna with different structure parameters, the relationship between the multi-resonant field enhancement and the structure parameters was studied. Specially, the physical mechanism of the multi-resonant field enhancement properties for the log periodic nanoantenna was explained by the conformal transformation and the electric field distributions calculated at corresponding resonant frequencies of log-periodic nanoantenna. Last, the field enhancement and Purcell factor of the log periodic nano antenna was compared with that of the bowtie antenna with the same structure parameters.(4) Modeling and Simulation of Log periodic nanoantenna based on monolayer graphene was completed. The dielectric constant of graphene was fitted by using the Drude model. After that, the influence of the chemical potential on the dielectric constant was analyzed. Then, the field enhancement and the radar cross section of log-periodic toothed nanoantenna based on graphene with different chemical potentials were investigated by using the FDTD method. According to different chemical potentials of graphene, the resonance characteristic of multi-resonance field enhancement was analyzed and concluded their law.The work of this dissertation is to design a broadband nanoantenna, analyze and optimize the parameters using electromagnetic numerical method and classic theory.The main innovations are listed as follows:Ⅰ. A bowtie nano-antenna has been shown and analyzed via FDTD simulations. Purcell factor and near field enhancement of bowtie nano-antenna was studied in great details placing special focus on geometrical configuration. The thickness, flare angle, curvature radius, and length were systematically varied in a series of FDTD simulations. A combination of values was found resulting in very high theoretical Purcell factor and electric field enhancement. Furthermore, the internal mechanism for the Purcell factor enhancement and optical bowtie nano-antenna structure parameters was described. These simulation calculation results and corresponding analysis have important significance in designing and optimizing of optical bowtie nano-antenna.Ⅱ. A log periodic nano-antenna has been designed and analyzed via FDTD simulations. The nano-antenna is investigated in great detail with particular emphasis on field enhancement and the Purcell factor. The parameters such as the outer radius, the bow angle, the tooth angle, the number of teeth, and the size ratio of the tooth and anti-tooth, are systematically varied in a series of simulations which allowed us to identify their influence on antenna performance. Also, conformal transformation and electric field distribution are used to explain the broadband characteristic of log-periodic nanoantenna. The simulation results prove that the multi-resonant field enhancement can be modified through variations in the geometric parameters of the log-periodic nanoantenna. Finally, the field enhancement and Purcell factor of log-periodic nanoantenna are compared with that of the bowtie nanoantenna. The results shown in this article establish the important theoretical foundation to improve the optical properties of wideband nanoantenna.Ⅲ. Basing on the metal log-periodic nanoantenna analyzed above, a log-periodic toothed nanoantenna based on graphene is proposed. The surface conductivity of graphene is calculated by Kubo formula. And the dielectric constant of graphene is calculated by Drude fitting in the frequency of 4-24 THz. According to the simulation results, the relation between the graphene dielectric constant and the chemical potential was concluded. The field enhancement and radar cross-section of the antenna for different chemical potentials are calculated, In addition, effect of the chemical potential on the resonance frequency is analyzed. The effects of chemical potential on the multi-resonant field enhancement and radar cross section were obtained. It is shown that large modulation of resonance frequency and resonance intensity in log-periodic toothed nanoantenna can be achieved via turning the chemical potential of graphene without any structure modification.In this dissertation, the structure parameters of the bowtie and log periodic nanoantenna are optimized, respectively. The influence of the structure parameters on the multi-resonant property of the log periodic nanoantenna is studied. And on this basis, a graphene log periodic nanoantenna is proposed and provides very important theoretical reference value in the design and fabrication of broadband nanoantenna.