| In recent years, with the deep research on the chemical synthesis technology and micro-and nano-machining technology, all-optical integrated optical devices developed rapidly at the small scale with metal structures. The diffraction effect that limits the resolution of focusing and imaging is becoming a major barrier in compact optical integrated devices. The particular characteristics of surface plasmon polaritons (SPPs), such as surface localization, shorter wavelength and extraordinary optical transmission offer a novel way to overcome the diffraction limit and accordingly attract significant attentions. The interference effect in the subwavelength metallic structures has been studied in this dissertation. Based on the interference effect of SPPs, the applications in nano-focusing and sensing have been investigated in the paper, such as beam focusing, unidirectional focus of SPPs on the metal surface and sensing.The main research works and conclusions are as following:1. Highly efficient plasmonic nano-focusing is proposed and demonstrated in a T-shape micro-slit surrounded by multi-slits. The subwavelength focusing phenomenon is achieved based on the multimode interference in the micro-slit, the constructive interference in the T-shape slit, and also the multiple-beam interference of the light radiated from the multi-slits and the transmitted light from the T-shape micro-slit. Due to the self focusing in the micro-slit aperture in the proposed structure, a large amount of light can pass through the micro-slit. This leads to a highly efficient nano-focusing. Meanwhile, the micro-slit means easily fabricating. In the experiment, the focusing phenomenon in the proposed structure was successfully demonstrated with a scanning near-field optical microscopy (SNOM) technology.2. A High efficient SPPs launcher is designed to unidirectionally generate SPPs on the metal surface, which consists of a high-index dielectric film and an asymmetric slit. The field intensity is first enhanced by the transmission resonance when passing through the dielectric film. Then, the SPPs fields could be undirectionally launched by the asymmetric semi-circular slit and accordingly enhanced. Base on the SPPs launcher, we designed an in-plan device to enhance the focusing intensity of SPPs fields on the metal surface. The structure consists of a subwavelength asymmetric semi-circular slit and a high-index dielectric film. The designing is calculated by the three-dimensional finite element method (FEM) solver Comsol Multiphysics. Because of the two successive enhancement processes in the dielectric film and the asymmetric slit, the focusing efficiency is enhanced more than4times. Therefore,the unidirectional enhanced focusing structure could have great potentials for applications in planar nano-photonic circuits.3. Using an ultracompact groove-slit-groove (GSG) structure, a refractive index sensor with a broadband response is proposed and experimentally demonstrated. Due to the interference of SPPs, the transmission spectra in the GSG structure exhibit oscillation behaviors in a broad bandwidth, and they are quite sensitive to the refractive index of the surroundings. Based on the above phenomenon, the characteristics of its refractive index sensing are demonstrated experimentally. In the experiment, the structure is illuminated with a bulk light source (not a tightly focused light source) from the back side. This decreases the difficulty of the experimental measurement and can protect the detection samples. Meanwhile, the whole structure of the sensor can be made more ultracompact without considering the influence of the incident waves.Highlights of the dissertation are as following:1. The designing of micro-scale slit and "T" shaped structures was firstly used in the subwavelength focusing and experimentally demonstrated a subwavelength focusing spot. The focusing effect is significant higher than the existing structures.2. The combining of a high-index dielectric film and an asymmetric slit is investigated for the first time to improve the generation efficiency of SPPs. Using such structure, a unidirectional subwavelength focusing is achieved on the metal surface with excellent properties of unidirectionality and high efficiency.3. An ultracompact SPP based sensor is proposed and experimentally demonstrated with light illuminating from the back side of the sample. This decreases the difficulty of the experimental measurement and can protect strong light sources from damaging the detection samples. |
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