Study On The Resonance Characteristics In Plasmonic Cavities

Surface Plasmon Polariton(SPP) is the coupled excitation of electromagneticwave and collective oscillation of free electrons at the interface between metal anddielectric. For a system formed by multiple plasmonic resonators, the interactionamong those resonators directly determines the optical property of the system.According to the distance between those resonators, the interaction can bedistinguished as near-field and far-field interaction. Hybridized SPP modes can begenerated by near-field coupling. However, for far-field case, the resonators interactwith each other assisted by Fabry-Pérot(FP) cavity modes where the systemfunctions as a plasmonic cavity. Due to the strong field enhancement andsubwavelength confinement, plasmonic cavity offers potentials for variousapplications, such as surface enhanced Raman scattering, enhanced fluorescenceprocess, solar cells, bio-chemical sensing, subwavelength optical lithography,integrated optical circuit, etc. The study on plasmonic cavity is on its primary stagerecently and a further thorough investigation is required. At present, many problemsabout plasmonic cavity are still unsolved, such as the mechanism of the interactionsamong SPP, FP cavity, waveguide modes within a plasmonic cavity, the influence ofshape and material of SPP resonators on the modes generated within a plasmoniccavity, the dependence of spectral responses on the structure symmetry of aplasmonic cavity. Based on above mentioned problems, in this dissertation, we willstudy the resonance properties of several metal-dielectric plasmonic cavities, suchas mode hybridization between SPP modes, interaction among SPP modes, FP cavitymodes and waveguide modes, anticrossing behavior between those modes. Theunidirectional spectral responses and SPP nanojet effect within those plasmoniccavities is also studied.We theoretically study mode hybridization and interaction among SPP Blochwave mode, FP cavity mode, and waveguide mode within a plasmonic cavitycomposed by two parallel planar bimetallic gratings. Four hybridized modes resultfrom mode hybridization between SPP Bloch wave modes on the two gratings areobserved. By varying the dielectric environment, mode hybridization behavior canbe manipulated. Importantly, waveguide plasmon polariton modes due tohybridization between grating-supported SPP Bloch wave modes and cavitysupported waveguide modes are observed. According to the theories of modessymmetry and relative phase shifts, we demonstrate that SPP Bloch wave mode andFP cavity mode with the same mode symmetry can interact by presenting ananticrossing behavior, which can be controlled by laterally shifting one grating withrespect to the other that causes a phase shift difference of the two involving modes.The proposed plasmonic cavity offers potentials for subwavelength lithography,tunable plasmonic filter, and controllable light-matter interaction.Unidirectional spectral responses from a plasmonic cavity formed by two metalnanostripe arrays with different thicknesses are studied. Illuminated by a p-polarizedplane wave from the front or back side, the system functions as either an absorber ora reflector according to the illumination direction under FP cavity resonances.Treating the whole system as a cavity with two resonant mirrors, it is revealed thatthe phase shift of reflection at the metasurface determined by localized surfaceplasmon resonance of the nano-stripe plays a critical role in the formation ofunidirectional spectral response. Using multiple-interference and transmission linemodel, the unidirectional spectral response of the plasmonic cavity is well explained.This work explores an extra degree of freedom of manipulating the spectral responseand light-matter interaction within a microcavity and offers potentials fordirection-involved active plasmonic devices and unidirectional thermal emissions.A complex plasmonic cavity-like structure formed by a metallic thin film and adielectric micro-disk is designed. Nanojet effects excited by SPP at the shadow-sidesurfaces of dielectric microdisks positioned on gold films are reported. Similar tothe reported photonic nanojet, the SPP nanojet can propagate over several opticalwavelengths while still maintaining a subwavelength full-width at half-maximumtransverse. The electric field of the highly confined nanojet at metal-dielectricinterface is enhanced by about 30 times. The formation mechanism of SPP nanojetsand photonic nanojets are the same. However, by varying the thickness of thedielectric microdisk, the formation of SPP nanojet can be flexibly controlled eventhough the refractive index of the dielectric disk is not optimal. The SPP nanojetshows great promise for enhanced Raman scattering and integrated plasmoniccircuits. Furthermore, we study the mode excitation of dielectric-loaded surfaceplasmon waveguide(DLSSPW) assisted by SPP nanojet. It is found that for anoptimal gap distance between the edge of the dielectric disk and dielectric ridge, theDLSSPW mode can be efficiently produced due to the coupling of SPP nanojet tothe dielectric ridge.