Resonance Characteristics And Dipole Emission Control Of Heterogeneous Optical Nanoantennas

Surface plasmon microcavity, nanorods and their clusters are the fundamental units in optical nanoantenna engineering. Plasmonic nanoantennas have been comprehensively studied as they could find wide applications in surface-enhanced Raman spectroscopy, solar energy utilization, optical sensor, non-linear optical enhancement and imaging. To this end, some theories have been developed to accurately predict the resonance wavelength for individual nanorod, but effective theory is still in lack for symmetric coupled nanorods and heterogeneous optical nanoantennas. In metal optical antennas, the plasmonic wave of radial polarization has an unconventional reflection phase at the antenna terminals, which is closely related to the structure, the end face morphology, the ambient medium, and the incident wavelength, etc. In the visible spectrum, existing theories on the reflection coefficient, reflection amplitude and reflection phase are no longer suitable due to the strong resonant coupling in the antennas. The conventional reflection coefficient, the calculation method of amplitude and reflection phase in low frequency would fail. Therefore, it is very necessary to build a concise and effective full analytical or semi-analytical theory to quantitatively describe the resonance characteristics for various nanoantennas.In such context, in this thesis we have developed a set of electromagnetic scattering theories which can be applied to different structures and coupling modes. We have mainly studied the resonant characteristics in single cylindrical metal nanoantenna, two-dimensional metal-dielectric-metal(MDM) structures of finite size, symmetric and asymmetric metal coupled nanorods with rounded ends. Based on the Sommerfeld surface wave theory and electromagnetic field distribution characteristics inside and outside the different structures, we explicitly define a complex reflection coefficient for the plasmonic waves at the nanoantenna terminals, taking advantage of the transmission line theory and the equivalent circuit method. We calculate the reflection phase at the antenna terminals through the complex reflection coefficient, which can be used to predict the resonance wavelength based on the Fabry-Pérot resonant phase matching conditions. By such theory, we strictly analyze and calculate the resonant wavelength for different structures with distinct parameters(such as nanoantenna materials, length and radius, different background mediums, etc.). The theoretical results are in good agreement with three-dimensional full-wave electromagnetic numerical calculations(based on COMSOL Multiphysics).By adjusting the antenna structure parameters, we can obtain specific wavelength resonance for different orders, which is called “commensurate resonance”, and we validate their enhancement effect on the radiation efficiency of an nearby electric dipole. Finally, we found a very special “toroidal mode” in coupled nanadisk antennas by further adjusting the ratio between the antenna length and the diameter. The results suggest that the field distribution in the resonant mode is highly matched to dipole radiation, and the radiative and nonradiative decay rate of the dipole source can be enhanced by nearly three orders of magnitude.The results in this paper would have strong guidance for the design of optical nanoantennas, and may be helpful for studying on surface spectroscopy of nanoantennas, luminescence properties, and other optical effects based on similar nano-microcavity.