Symmetry Breaking Silver Nano-dimer Surface Plasmon Oscillation And Induction Transparency

Noble metal nanostructures can generate surface plasmons under the irradiation of the incident light. As Surface plasmon can break through the diffraction limit, support for near field, and be sensitive to environmental influences. It can be applied to many areas, such as chemical and biological sensing, negative refraction materials, waveguide, surface enhanced spectroscopy and solar-thermal resection of tumor cells, ect. Therefore, metal micro-nano structure system that support surface plasmons has gained increasing attention. The artificial nano structure materials shows unique optical properties due to strong coupling to the electromagnetic field, such as plasmon-induced transparency, transmission enhancement, and negative refraction. And it can manipulate light in some unique ways.In this paper, we designed an "artificial molecule" consists of two symmetry broken nanoring that have the same geometry size. We studied the absorption spectrum of this structure under different conditions based on the finite element method. The thesis is divided into two parts. In part one, the effect of plasmon coupling on absorption spectrum of symmetry breaking nano-ring dimer under different conditions is investigated. In part two, plasmon-induced transparency of symmetry breaking nano-ring dimer is investigated and a new analogy of the physical mechanism of PIT is explained in the form of analytic solution.Part one:the absorption characteristics of symmetry broken nano-ring dimer.The absorption characteristics of dimer are investigated by the finite element method. We systematically studied the effects of the polarization direction of the incident light and the structure of symmetry on this dimer by changing the polarization direction of the incident light and the structure of symmetry. The results shown that the plasmon resonance modes and the number of resonance peaks are different due to the difference of the polarization direction of the incident light. When the incident light polarizing along the x axis direction, multiple resonance peaks appeared on the absorption spectrum due to the strong coupling between two rings. The hot spot formed in the junction between nano-rings when the two rings are arranged chiral molecule resulting in the strong local electric field enhancement. It can be applied to the surface enhanced Raman spectroscopy. When the incident light is polarized along y axis direction and the right side of the ring rotated around z axis counterclockwise, the resonance peak splits in infrared wavelengths. When ?= 90°,the resonance peak splits into three peaks. We can regulate the spectral lines by changing the angle of rotation and it can be applied to optical switch based on surface plasmons.Part two:Plasmon-induced transparency in symmetry-broken nanoring dimer.The absorption characteristics of dimer are investigated by the finite element method. Plasmon-induced transparency appears in the absorption spetrum when the offset and the spatial arrangement of the dimer are specific. We can control the effect of plasmon-induced transparency by changing the gap between two rings and the offset of the ring. In this chapter, We achieved a analogy of the mechanism of Plasmon induced transparency by introducing a phase factor in lorentz harmonic oscillator coupling model. And the calculation results agreed with the results of numerical simulation. It further proves that the mechanism of Plasmon induced transparency is the dipole mode of the left ring stimulated by the quadrupole mode of the right ring. And the absorption of light of dipole mode is supressed because of the interaction between the quadrupole mode and the dipole mode. It can be understood as the coupling mechanism of quadrupole-dipole. These results have a certain guiding significance to profoundly understand the mechanism of Plasmon induced transparency and design artificial atoms and molecule.