Study On Surface Plasmon Polariton Of Conical Structure Assisted With Photonic Crystals

Today witnesses the rapid development of photonics in nanometer scale. However, we still have to face the problem of how to confine electromagnetic waves into sub-wavelength structures which are quite small compared to the scale of visible light. The emergence of SPPs (Surface plasmon polaritons) meets this problem thanks to its special property. Therefore, optical devices of small dimensions can be fabricated, which sheds new light on fields like data storage, integrated light circuit and cancer treatment, etc. Especially for conical metallic structure, we can realize strong convergence at the tip, which breaks traditional diffraction limits and hence helps the development of biology detection, imaging and other usages. However, single cone also has light distribution around the cone tip. We designed and test a new structure which comprises of cone and photonic crystals to cope with this problem. Also, we find radial polarization is appropriate for light focusing than normally applied plane wave light source.In the first part, we analyzed the concept and applications of SPPs, and introduced SPPs on infinite interface and localized SPPs on the surface of sub-wavelength structures.For SPPs on infinite interface, after deduction, we can obtain the dispersion relation which locates at the right side of light line. So we need adopt special methods to excite it like electron beam, prism and single or periodic bumps. For localized SPPs on the surface of sub-wavelength structures, the curvy surface of structure can offer retraction force. As result, direct light excitation is possible.We can use surface plasmon resonance technique to analyze chemical component or, through covering antibody material on metallic spheres, realize cancer treatment. We can also use SPPs to enhance solar battery efficiency.In the second part, we discussed the properties and applications of photonic crystals. For the structure we propose contains photonic crystals, it is necessary to analyze what properties PhCs have. Then we introduce the two applications we've finished which are thermal enhancement and spontaneous emission enhancement. Photonic crystals are sub-wavelength structures which compose periodic change of material. Similar to traditional electron energy band, if we organize materials with different refractive index periodically, we can obtain dispersion curves. Steep dispersion curve with positive slope can be used in negative refraction. Flat dispersion region can be used in slow light realization.Since higher state density can be found at the edge of photonic band, if we cover photonic crystals with metal, we can get higher thermal emission efficiency. Because of the high symmetry in the plane of photonic crystal we studied and the finite length in the axial direction, scattering matrix calculation is quite appropriate which is more precise than transmission matrix calculation if there is metal in the structures. We can also exploit the advantage of photonic crystals to enhance spontaneous emission. We use scattering matrix method, plane wave expansion method and FDTD method to do theoretical calculation. And the result matches the experiment.In the third part, we simulated and discussed localized SPPs on the surface of sub-wavelength cylinder and cone structure.In 1994, Kawata reported that metallic tip structure can be exploited in near-field scanning optical microscopy. This thought is used in tip enhanced Raman spectrum. Nie and Emory reported that single or congregated particles can offer strong enhancement in Raman signal. Here we use silver to calculate under Lorentz model. We discussed the modes of SPPs on sub-wavelength cylindrical structures. We find that the modes are different from normal two-dimension oscillation. We also find that for plane wave excitation, because of opposite phase at the two sides of bottom, interference at the top might affect the localization.In the fourth part, we simulated and discussed SPPs of conical structure assisted with photonic crystal.We design a new conical structure assisted with photonic crystals, which can effectively lower the surrounding electromagnetic field. We also find that radial polarization is satisfactory in exciting SPPs and realization of localization. We analyzed the performance of each component and discussed the difference of excitation with radial polarization and plane wave. We found that radial polarization is better in the excitation of SPPs on conical structures.For radial polarization obtained through optical fiber, we use two LP01 modes to couple. For radial polarization obtained through metallic cylinder, we couple dipole source to SPPs on the surface.We adopt photonic crystal with holes on the slab, which is made of Si3N4 material. We remove the hole in the middle to form a defect. Since light is prone to propagate in higher refractive region, we only need to try to a find a perfect mode in the low frequency region. The reason is also related to the high symmetry of radial polarization.We can observe that radial polarization can realize good localization and the field at the tip has high symmetry which is good for coupling afterwards. We can see the radial polarization source couples into defect mode of photonic crystal well.We also perform the simulation of the same structure except the source is plane wave this time. We can find the excitation is different. The localization at the tip is not satisfactory. But the blocking effect of photonic crystal can been observed. Also, we can see that the coupling of plane wave into defect mode of photonic crystal is not good.