| Surface plasmon polaritons （SPPs） are electromagnetic surface waves resulting fromcoupling between an electromagnetic field and collective oscillations of free electron chargedensity at a metal/dielectric interface. Since the Ebbesen research group discovered theextraordinary transmission of light through subwavelength hole arrays at1988, there has beenconsiderable interest in the optical properties of SPPs for their novel applications and richunderlying physics purposes. SPPs have given rise to such significtant applications asmicroscopy beyond diffraction limit, light focusing, light bending and compact plasmonicmodulators. The study on light field manipulations of surface plasmon polaritons is a frontiertopic in nano-photonics. The excitations of surface plasmon polaritons by metal nano-slits andthe light wavefield pattern formations are the key factors.In this thesis, combining the far-field scattered imaging method with the digital Fouriertransform arithmetic, we experimentally study the excitations of surface plasmon polaritons bymetal nano-slits and the light wavefield pattern formations. In experimental studies,（1） weconstruct a Mach-Zehnder interferometer system with microscopic objective to image thescattered SPP field. The phase map is extracted by Fourier transform of the interference intensity.We produce the nanoscale phase vortices and singularity in plasmonic fields of silver ring slitand oil-immersed silver spiral slit under linearly polarized illumination.（2） With the L-shapedslit sample to achieve the transverse electric （TE） and transverse magnetic （TM） incidencesunder a linearly polarized illumination, the waves of the photonic and plasmonic modes arelaunched simultaneously and independently. We record the scattered image of their interferencepattern in the CCD. From the fringe data of the pattern, we obtain the quantities of the two modewaves and their dependency relationships on slit width and propagation distance. Thepolarization perpendicular relationship of the two mode waves are also achieved from thecontrast of the interference pattern.（3） We fabricate three ring-slit samples with the innerdiameter6μm and different slit widths on the Au film. Under the vertical polarization incidence,by adjusting the direction of the polarizer, we record the scattered image patterns of thex-polarization, y-polarization and total intensity, and observe the evolutions of the patterns withthe slit widths. In the theoretical studies,（1） based on the experimental data, we propose an empirical model for the ring-slit-excited SPP source field by trial calculations with theHuygens-Fresnel principle for SPP propagations to explain the formation of the phase vortices inthe central area of the silver ring slit and oil-immersed silver spiral slit.（2） With the relationshipsbetween the phase, intensity distributions and the energy current, we deduce the expressions ofthe momentum of a photon, the momentum density, the orbital angular momentum of thephotons and the orbital angular momentum density, and first apply them to analyze the propertiesof the plasmonic field.（3） By introducing the scattering theory under Kirchhoff’s approximationand hybridation of SPP wave and the quasi-cylindrical wave （CW） in plasmonic mode, we give abasic analytical description for scattering of photonic mode and the plasmonic mode wavefieldfrom the rough film surface, and explain the difference in the scattered wavevector componentsbetween the two mode waves.（4） By decomposing the electric vector of incident wave intoparallel and perpendicular components with respect to the slit of arbitrary direction, and usingthe obtained wavefield component quantities, we study the ellipsometric properties andpolarization conversion of the synthetic wavefiled due to the the amplitudes and initial phasesdifferences of the two mode waves. The polarization converter model of a single slit element isproposed. With the vector directional decomposition of lightfield excited by the single slitelement and its variation with the propagation distance, the Huygens-Fresnel principle of thevector form is also constructed for the description of converter effect. In the numericalsimulations, we calculate the SPP vortex field generated by ring-slits, and verify the proposedempirical expression of the ring-slit-excited SPP source. With the Huygens-Fresnel principle ofthe vector form, we also calculate the polarized patterns, and demonstrate the feasibility of theprinciple. The whole paper is divided into six chapters.In chapter1, we describe the research background of surface plasmon polaritons; the basicproperties and excitations of the surface plasmon polaritons; the experimental detection methods;the applications and developments of surface plasmon polaritons manipulated with nanoslits; thesignificance and content of the dissertationis.In chapter2, we report the tetrad phase vortex structure in the scattered surface plasmonpolariton （SPP） field produced by a silver nano-ring-slit with linearly polarized illumination. Inthe experiment, a Mach-Zehnder type interferometer is constructed in which a microscopicobjective （MO） is used to collect and image the scattered SPP field, and the phase map isextracted by Fourier transform of the interference intensity. To explain the formation of the tetradphase vortices in the central area of the ring, we propose an empirical model for thering-slit-excited SPP source field by trial calculations with the Huygens-Fresnel principle forSPP propagations. Detailed calculation and comparison with the experimental intensity patterns show that amplitude of the SPP wave excited by the ring-slit varies insin20on a constantbase, and the phase varies incos20with an amplitude0.9, slightly larger than/4. Thismeans that vertical wavelets have a phase delay about/2compared with horizontal wavelets.Finally, we deduce that the scattered SPPintensity pattern with its tetrad phase vortex structure isformed by interference of the excited SPP wavelets from the ring-slit due to their azimuthallyasymmetrical amplitude and phase distributions.The empirical expression the source SPPs isobviously helpful for understanding the basic properties of the excited SPP wave and useful forthe practical calculation of SPP field in the silver film. Moreover, the far-field scattered imagingmethod has been proved to be a powerful alternative for plasmonic wave detections.In chapter3, we experimentally investigate the singularity characteristics of the SPP wavesproduced by an oil-immersed silver spiral slit with linearly polarized illumination. We find sixnon-uniform vortices and a singularity line segment in the central part of the slit. We propose theexperiment method of assistive circular ring-slit to obtain the expression of the excited secondarySPP source and use the Huygens-Fresnel principle for the SPP wave propagations to explore theformation of vortices. From the experimental and the theoretically computed results, we find thatthe topological charge of each vortex is non-uniform despite of its unity mean value. The zerolines of the real and the imaginary parts intersect at the vortex cores, and the density of phasecontours surrounding a vortex point is inversely proportional to the intersection angles betweenthe zero lines of the real and imaginary parts. For singularity line segment, the coincidence of thezero lines corresponds to two nearly sudden phase changes of/2. The distribution of themomentum and the orbital angular momentum near a phase vortex can also be obtained from thephase of the plasmonic wave field. The non-uniform distribution of momentum and the orbitalangular momentum appears as:（1） the momentum magnitude is larger in the area closer to thevortex core and vice versa, where the phase varies more quickly, the magnitude is also larger.The momentum direction appears in that at a point close to the vortex core may deviateobviously from the perpendicular of the position vector relative to the core, while farther fromthe core, the direction is closer to the perpendicular.（2） The momentum density at a point isproportional to its distance to the vortex core.（3） The variation of the angular momentum of asingle photon versus the azimuth is not uniform, but remains constant in the radial direction.（4）The orbital angular momentum density at a point is proportional to the square of its distance tothe vortex core. In addition, the momentum of the single photon on the two sides of thesingularity line crosses the segment roughly perpendicularly, and while in the area a little farther,the momentum is very small. Outside the two ends of the ingularity line, the momentum densities “rotate” around each end. They may produce an angular momentum densityLz in thez-direction, the momentum densities in the upper-right and bottom-left areas of ingularity linemay produceL zopposite to z-direction, and two angular momenta may cancel. The studies ofthe vortex and the singularity structures in SPP wave would be important for the manipulationsin nanoscales and related application.In chapter4, based on the experimental results of an L-shaped metal nanoslit, we extract thequantities of the phtonic and plasmonic mode waves, and conduct the expressions of the twomode waves. In the experiment, the scattered pattern imaging setup is embedded in aMach-Zenhder interferometer and an L-shaped slit of width300nm on Au film is employed asthe sample. The two arms of slit provide the TE and TM incidence simultaneously and to launchthe two mode waves independently. The quantities needed in the solution including theamplitudes, wavevectors and initial phases of the two waves under the same condition areobtained from the interference fringe pattern. The wavevector components of the photonic andplasmonic modes are k1=8.67×10-3nm-1 and k2=8.86×10-3 nm-1, respectively. The differencein the wavevector components gives rise to an additional phase delay versus the propagationdistance. We introduce the scattering theory under Kirchhoff’s approximation to metal slitregime and explain the wavevector difference reasonably. The experimentally solved ratio of theinitial amplitudes for the photonic and the plasmonic modes is0.81.Using the reconstructedphase maps from the interference patterns, we finally obtain the phase difference0=2.34.Then we fabricate a sample of ring-slit with inner diameter6μm, and width300nmon the Au film as an example. Comparing the experimental pattern with that calculated with theHuygens-Fresnel principle, we find the experimental intensity patterns and phase maps are bothin good agreement with the calculated results.The applicability of these results are demonstrated.The solution of the quantities is a comprehensive reflection of excitation, scattering andinterference of the two waves and would be an advance in facilitating the design of metal slitstructure for manipulating the nanoscale wavefields.In chapter5, we investigate the evolutions of the scattered imaging wavefields with slitwidths, and construct the model of polarization conversion of the wavefields under linearlypolarized illumination. From the interference patterns of photonic and plasmonc mode lightwaves excited by the L-shaped nanoslit of different slit width, the parameters of waves of bothmodes are acquired. From the contrast of the interference pattern, the polarization perpendicularrelationship of the two mode waves is also achieved. We found that the parameters of plasmoncand photonic mode waves are different for nanoslits with different widths:（1） for smaller width of slit arm, the quasi-CW decreases, and the wavevector component of plasmonic mode increase.In contrast, the single wavevectork0in photonic mode causes its wavevector to be almostindependent of slit width.（2） The initial amplitude of plasmonic mode is well linearly increasedwith the slit widths, and only when slit width is greater than or equal to200nm, the initialamplitude of photonic mode is approximately increased linearly.（3） The initial phases of theplasmonic modes lead those of the photonic modes, and the narrower the slits, the more they lead.The initial phases of the photonic mode generated by wider slits lead those by narrower slits. Bydecomposing the electric vector of incident wave into parallel and perpendicular componentswith respect to the slit of arbitrary direction, we demonstrate that polarization of the excitedwavelet deviates from that of incidence, and the slits work as polarization converters. Based onthe obtained wavefield quantities and the polarization relationship, we construct theHuygens-Fresnel principle of vector form to describe the interference of the excited waves.Samples of Au ring-slits of different slit widths are fabricated, and the experimental evolutionsof scattered imaging patterns of the x-polarized, y-polarized and total intensities with the slitwidths are studied systematically for the first time. The patterns are highly consistent with theexperimental results, and the evolutions indicate that a complicated slit function as a converter ofpolarized SPP patterns.In chapter6, we sum up the main conclusions and the innovations of the dissertation, andbriefly introduce the in-depth researches we will conduct. |
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