| Summary: | 博士 === 國立交通大學 === 光電工程研究所 === 105 === In this study, we propose a novel mirror-image nanoepsilon (MINE) structure resembled the combination of two face-to-face nanoscale ϵ-shaped structures to achieve highly localized and enhanced near field at its gap and systematically investigate its plasmonic behaviors. The MINE can be regarded as the combination of two fundamental plasmonic nanostructures: a nanorod-dimer and nanoring. By adapting a nanoring surrounding a nanorod-dimer structure, the nanorod is regarded as a bridge pulling the charges from the nanoring to nanorod, which induces stronger plasmon coupling in the gap to boost local near-field enhancement. This MINE with mode coupling between the nanorod-dimer and nanoring leads to the inducement of hybridized plasmon modes. Among these plasmon modes, the symmetric mode in the MINE structure is preferred because its charge distribution leads to stronger near-field enhancement with a concentrated distribution around the gap. In addition, we investigate the influence of geometry on the optical properties of MINE structures by performing experiments and simulations. These results indicate that the MINE possesses highly tunable optical properties and significant near-field enhancement at the gap and rod tips. Besides, MINE dimers arrays with varying dimer gap are also explored. As a result, a coupled symmetric (CS) mode can be excited in small dimer gap under longitudinal polarization, which provides the practicality of a high density hot spot and uniform pattern in MINE dimers array. The results improve the understanding of such complex systems and are expected to guide and facilitate the design of optimum MINE structures for various plasmonic applications. Finally, the application of the particle trapping is numerically and experimentally performed using the unprecedented MINE structure. It is shown that the proposed MINE structure can achieve stable trapping on tiny particles for future bio-chemical applications.
In addition to the novel MINE structure, we also focus on another topic about a novel plasmonic transparent conducting oxide (TCO) material in this dissertation. It is well known as conventional metals suffering from large losses in optical frequencies because of the electron transition and electron scattering losses, which limits the feasibility of plasmonic applications, such as transformation optics (TO) devices and epsilon-near-zero (ENZ) devices. Thus, searching low-loss alternative materials for plasmonic applications is an important topic. In the study, we systematically investigate the plasmonic behaviors of aluminum-doped zinc oxide (AZO) thin films and patterned AZO nanostructures with various structural dimensions under different annealing treatments. We find that AZO film can possess highly-tunable, metal-like, and low-loss plasmonic property and the localized surface plasmon resonance (LSPR) characteristic of AZO nanostructure is observed in the near-infrared (NIR) region under proper annealing conditions. Finally, environmental index sensing is performed to demonstrate the capability of AZO nanostructure for optical sensing application. High index sensitivity of 873 nm per refractive index unit (RIU) variation is obtained in experiment. Taking with the advantages of excellent sensing ability on gas or biochemistry using AZO material, we believe that highly sensitive and responsive optical nano-sensor can be expected in the future.
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