Design And Research On Optical Properties Of Metallic Compound Nanostructures Based On Surface Plasmons

Surface plasmons(SPs) are electromagnetic surface waves, which are excited on the interface between metals and dielectrics due to the interaction between the photons and free-electrons in the metals. SPs include two kinds of modes, surface plasmon polaritons(SPPs) propagating along the surface between metals and dielectrics, localized surface plasmons(LSPs) localized at metallic nanoparticles and surface mirco-structures of metals. The radiation energy of light in free space can be coupled into metal nanostructures due to the excitation of surface plasmons, which results in local intensified electric fields at nanoscale near the metal surface, and thus enhances many optical effects, such as Raman scattering, optical transmission and photochemical catalysis. Many interesting physical phenomena, like enhanced optical transmission, Fano resonances and subwavelength light localization can be obtained based on SPs. SPs have become the most promising information carrier in integrated photonic devices because they have the speed of photonics and the scale of electronics, and can overcome the diffraction limit and control the light in subwavelength structures.SPs mainly depend on the structure geometrical shapes, materials and dielectric surrounding environments, which contribute to the manipulation of optical properties of metal nanostructures. With the development of nano-precision machining technology, many novel nano-devices and metal nano-structures can be designed and fabricated, which have widely potential applications in biophotonics, subwavelength integrated optics, bio-molecule chemical detection and other fields. In 1998, Ebbesen et al found extraordinary optical transmission(EOT) through thin metallic structure perforated with a subwavelength hole array. Then, the enhanced optical transmission phenomenon based on the excitation and coupling of SPs resonances has attracted great interest. These characteristics of metallic nanostructures contribute to realize miniaturized filters, transparent conductors, sensor, photocell and other optoelectronic devices.In this paper, we designed six kinds of metallic compound nanostructures, studied their optical properties, and explored potential applications in optical sensing. The main work and research results are listed below:1 SPs are introduced, mainly including the development history, categories, physical mechanisms, some physical characteristics like dispersion relations, excitation methods, propagation ways and applications, which are theoretical basis of this paper.2 Numerical simulation methods of metal film nanostructures are introduced, including the finite-difference time-domain(FDTD) and electromagnetic field finite element method. And FDTD is employed to calculate the optical properties and electronic field intensity distribution patterns of metallic nanostructures in this paper.3 We designed and studied two kinds of thin metal structures with air slits. Firstly, double-level cylindrical structure is obtained by adding one air loop into simple holes array. It is discovered that two EOT peaks occurred in optical and near infrared ranges due to the resonances and coupling of SPs, modes hybridization and optical waveguide cavity modes effects. The transmission spectra could be effectively tuned by changing the size of inner and outer holes. Secondly, with the combination of metal cubes and holes in the thin metal film, we designed a “日” style nanostructure. The air holes with same size could contribute to the selection of single wavelength. The optical transmission behavior can be regularly tuned by changing the size of inner cubes and the period of structure. And this structure shows perfect EOT effect due to the localized SP resonances and coupling. In addition, the regular change of transmission spectra to the environment dielectric constants indicates that it has potential applications in sensors.4 The compound hole arrays have the advantages of light enhancement and convenient tunability, and the disadvantage of difficult realization due to complex construction. We designed two simple compound thin metal structures. The compound rectangular nanoholes array(unit cell) consists of a large square hole with two small rectangular holes symmetrically distributed at its both sides. EOT is obtained in this structure, which is larger 12% than the sum of those of the two structures(the large holes array or the small holes array). The EOT in the optical regime mainly results from the excitation and coupling of SPs in inner and outer holes. Similarly, the compound circular holes structure was designed to obtain great EOT, including one big circular hole surrounded by some small circular holes in a unit. By studying the structures with zero, four and eight small holes, we found that the added outer small holes could obviously enhance the transmission property; and the more outer small holes(eight) don’t show a large advantage on EOT, but the transmission spectra show linear increase with the size of holes increasing in both structures. Finally, it is concluded through both designs that the introduction of outer small holes can enhance the excitation and coupling effects of SPP between inner and outer holes, contributing to a larger EOT. And they show highly sensitivity to environment, which have potential applications in sensors.5 The triangular hole has a unique advantage due to that intense localized SPs can be excited at its corners of the three angles of 60 degree. So, we presented a plasmonic nanostructure consisting of a thin metallic film perforated with an array of compound equilateral triangular holes. By changing the arrangement or size of triangles and the period of structure, a tunable EOT in optical region can be realized.6 The proposed “sandwich” structure consists of two flat dielectric films inserted with double parallel metal and dielectric(SiO2) nanosphere arrays. By tuning the resonance modes of LSPs and the modes hybridization and coupling effects between adjacent nanospheres, a broadband and strong enhanced transmission phenomenon in the invisible and near-infrared regions is achieved.