Study On The Excitations And Coherence Of Surface Plasmon Polaritons

Surface plasmon polaritons (SPPs) are electromagnetic excitations, which are located at the interface of dielectric media and metal and coupled to surface collective oscillations of free electrons in the metal. SPPs techniques are capable of manipulating and routing light in the nanometer scale with the unique properties of nanoscale metallic structures and have become the candidate of the next generation information transport technologies. Plasmonics is dealing with the excitations, propagation, modulations and detections of surface plasmon polaritons, which proposes a new solution to optical interconnecting, high performance computing and novel sensors. Fabrications of plasmonic devices are metallic nano-structure preparation in essential. Many different metallic structures have been designed to carry out various functional elements in plasmonics. Hence, the researches in the properties and preparation of metallic nano-structures have attracted a lot of attentions.Considering the advantages towards the traditional optical communication, we made efforts to study on the coherence of the SPPs propagation in the metal/dielectric structures. A four-layered system consisted of a prism, a silver film, air and an iron-doped lithium niobate crystal was constructed. In order to get the best coupling efficiency, we calculated the minimum of the total reflectivity, the corresponding incident angle and transmissivity of the four-layered system for different thickness of silver film and air layer, and found out the optimal setup. As the SPPs dispersion is related to the air layer, we deduced the dispersion in the four-layered system and calculated the grating period with the SPPs wave-number theoretically. In the experimental aspect, two coherent671nm p polarized beam symmetrically entered the isosceles prism through the two legs and intersected on the hypotenuse at the excitation angle of SPPs. SPPs were excited at the interface of silver and air according to the Kretschmann configuration. The SPPs propagated for a distance and were decoupled into light by the iron-doped lithium niobate for the high refractive index. Two decoupled waves met in the crystal and interfered. A holographic grating was recorded in the crystal after long time illumination due to the photorefractive effect. A weak488nm lasers beam was utilized to read the grating out. Only when the incident angle was around48.40°, one diffraction (transmission) order was found. From the Bragg angle, we figured out the period of the grating, which agreed well to the theoretical one. This confirmed that the grating was induced by the two coherent beams. To find out the mechanism in the grating formation, we analyzed the rocking curve of the holographic grating and got the grating thickness and the modulation of the refractive index. The thick grating character showed that the grating was written by the interference of decoupled waves. After light-SPPs-light transformation, the two beams remained coherent.Metallic nano-structure fabrications with traditional methods usually need extremely high vacuum conditions, expensive equipment and complex processes. Here we provide a novel method for fabricating large area metallic gratings which are capable of coupling light into SPPs efficiently. The photochemical reduction was used to synthesize silver nano-particles in the aqueous solution containing sodium sitrate and silver nitrate. In the experiment, a488nm laser served to irradiate the solution. Silver nanoparticles were deposited on the glass substrate and finally a nanopaticle film was formed. From the XRD and AFM results, we knew that most of the particles were tetrahedron, octahedron or truncated octahedron. This was because the silver nitrate was sensitive to488nm light which led to the reaction undergoing fast. Single-crystal seeds were dominated in the process of the nanocrystal growth. From the experiments, the thicknesses of silver nanoparticle films prepared by this method showed a dependance on the wavelength and the intensity of the light. Meanwhile, the film had a strong SERS effect. Silver was not stable in the air and the reaction rate was not suitable to use the light to control the shape and size of nanoparticles. Hence gold nanopartcles were generated by the photochemical reduction of Au3+with488nm excitation in ethylene glycol. The PVP was implemented as the capped material. A lot of gold nanoparticles existed as decahedron and icosahedron. Some of the seeds even grew into a plate-like shape. The features of the nanoparticles got from the XRD and SEM tests indicated that many multiple-crystal seeds grew into nanocrystals in the low-rate reactions because of the weak absorption of488nm. The preparation of the nanoparticles showed that the shapes and the sizes of the nanoparticles could be controlled by the wavelengths, the intensities or even the intensity distributions of the laser.We managed to fabricate metallic particle gratings with two methods. First, a periodic space-charge field was established in the iron-doped lithium niobate crystal with two waves coupling method. When the colloidal solution flew on the crystal slowly, the metallic particles were trapped on the surface via electrophoresis or dielectrop effect and a metallic particle grating at hundreds of micro-meter scale was formed in this process. The transition between dielectrophoresis and electrophoresis effect was also observed. Second. two coherent beams were set to meet and interfere on the surface of the substrate. The intensity distribution on the substrate was periodic, which induced the formation of Au nanoparticle grating via photochemical reduction. This process was determined by the hydroxyl, the intensity distribution and the diffusion of the Au nanoparticles. When the optical gradient force balanced by the diffusion effect, a grating was formed which had the same period as intensity distribution of light. According to the interference theory of light, the surface feature could be adjusted by the two beams. Finally we produced a grating coupler for SPPs under proper modulation and illumination time. The SPPs excitation with the gold nanoparticle grating was the result of propagating SPPs and localized SPPs. After annealing treatment, the surface of the grating became smoother and the dielectric constant approached to the bulk materials. Therefore the excitation angle of SPPs got closer to the theoretical value.Fast electron beams were used to excite SPPs modes of silver nanowire pair. We studied on the mode size and propagation length of the Gap SPP mode in different cross-section shapes. The calculating results showed the electric-magnetic field was mainly distributed in the dielectric area between silver nanowires. The Gap SPP had a strong localization of the field and an ideal propagation length at the same time. The dispersion of Gap SPP had little dependence on the cross-section shapes. When the cross-section shape changed, only the propagation performance and the excitations efficiency were changed apparently. This was a remarkable character for the structure fabrications.