Research On The Resonant And Coupling Behavior Of Optical Antennas

Optical antennas can serve as an effective tool for efficient and directive conversion between localized energy and far-field radiation at optical frequency. Influenced by the Localized Surface Plasmon Resonance(LSPR), optical antennas exhibit great differences with microwave antennas in the case of feeding, working wavelength and coupling characteristics. Based on these, optical antennas have important applications ranging from antenna assisted fluorescence enhancement, nano scale light beaming and direction control, optical imaging beyond diffraction limit and is believed to be a promising direction in future developments in the field of optics. This thesis mainly focuses on the resonant and coupling phenomena inside optical antenna system, with a special emphasize on the antenna-assisted fluorescent enhancement, the control over the emission direction using optical antennas and quasi-quantum effect inside the optical antenna system.The main research achievements of this thesis can be summarized as follows:1. The radiation enhancement as well as radiation directional control are studied using different optical antenna systems. In specific, an ultra-compact optical antenna aims at spatial isolation of emission and excitation is developed. The enhancement of the directivity using antenna arrays is also studied.2. Based dyadic Green’s function method, the interaction between dipoles of different polarizations as well as different materials is studied in detail. The asymmetry of induced field pattern by vertically and horizontally oriented dipoles is considered to design an optical antenna structure with dynamically controlled emission direction.3. Through analysis of the angular spectrum of circularly polarized dipole, a mechanism of unidirectional excitation of guided mode is developed. An optical antenna is designed to have a similar function.4. Fano-like resonance in plasmonic system is explained in terms of the interaction of bight mode and dark mode. The effect of coupling strength on the lineshape, sensitivity is studied to develop a plasmonic sensor to monitor local changes in refractive index change both spatially and spectrally.5. Strong coupling between magnetic plasmons and Farbry-Perot cavity plasmons is studied in detail. A polariton theory is applied to describe the coupling behavior. A field enhancement of845times and280meV Rabi splitting is observed in numerical simulation.The highlights of the thesis are as following:1. A new way of controlling the far-field emission pattern is presented in this thesis. By utilizing the interference between different dipole components, the emission of the circularly polarized dipole can be actively controlled. Also, an optical antenna is designed to mimic the role of circularly polarized dipole as a tool to unidirectional launching of guided modes.2. A two-resonance optical antenna is proposed. This antenna utilizes the two plasmonic resonances covering both the excitation and the emission of the fluorescent molecule to control the direction of the excitation laser beam as well as the emission of the fluorescent molecule independently.3. Plasmonic analog of two quantum effects are discussed using optical antennas based on similarity between modes in optical antennas and energy levels in atom structure. A plasmonic sensor to monitor refractive index changes both spectrally and spatially is proposed. A large Rabi splitting of280meV is achieved. This offers a new platform to study traditional quantum effects.