Design Of Plasmonic Nanoantenna Based On Characteristic Mode Theory

In recent decades,characteristic mode theory has been widely applied in the design of various antennas.On the one hand,the characteristic mode can obtain the spectral resonances of various shaped antennas without the excitation source,on the other hand,the characteristic mode explains the radiation mechanism of antennas from the wave physics.Nowadays,in the radio frequency,the antenna designs with characteristic mode theory is well known and widely used,especially in metal characteristic mode theory.However,in the optical frequency,there are few studies on the characteristic mode,application of the characteristic mode to optical antennas.In this thesis,we develop the characteristic mode theory to analyze the plasmonic nanoantenna,the eigenmode of the plasmonic nanoantenna is obtained,the underlying mechanism of near-field enhancement is explained with the respect of eigenmode,and then a dual-band plasmonic nanoantenna is designed.The content of the work are listed as follows:(1)Analysis of plasmonic nanoantenna based on characteristic mode theory.The gold nanorods are compared with perfectly electrically conducting(PEC)nanorods.First,both the characteristic value and current are calculated,as well as the near-field and total surface current under different excitation sources.Then,we offer the explanations that why they have different resonance frequencies and near-field enhancement effects.Finally,we found that the eigenmode calculated by the characteristic mode theory can well match with the near-field enhancement.In order to excite the specific mode,the appropriate excitation source can be implemented under the guidance of mode orthogonality,and then the design procedure of plasmonic nanoantenna by characteristic mode theory is presented.(2)Design of a dual-band plasmonic nanoantenna based on characteristic mode theory.To engineer the broadband near-field enhancement,which is critical important to surface enhanced Raman scattering(SERS)that mismatched frequencies of Raman excitation and Raman radiation,the dual resonance respectively corresponding to excitation and radiation will favor the achievement of better near-field enhancement.Based on the principle of dipole antenna,this work proposes a plasmonic nanoantenna with the configuration of fan-shaped structure.According to characteristic mode theory,the designed nanoantenna has dual resonances that corresponds to the two single dipole modes.The location of the dual resonance can be independently manipulated,and the maximum enhancement of near-field intensity for the dual resonances can reach to 9000 and 5500 respectively.