Photonic nanopatterns of gold nanostructures indicate the excitation of surface plasmon modes of a wavelength of 50–100 nm by scanning near-field optical microscopy

Scanning near‐field optical microscopy images of metal nanostructures taken with the tetrahedral tip (T‐tip) show a distribution of dark and bright spots at distances in the order of 25–50 nm. The images are interpreted as photonic nanopatterns defined as calculated scanning near‐field optical microscopy images using a dipole serving as a light‐emitting scanning near‐field optical microscopy probe. Changing from a positive to a negative value of the dielectric function of a sample leads to the partition of one spot into several spots in the photonic nanopatterns, indicating the excitation of surface plasmons of a wavelength in the order of 50–100 nm in metal nanostructures.     

Applications of field-enhanced near-field optical microscopy

Metal nanostructures such as sharp tips can enhance emission yields through shape-induced local field enhancement. The enhancement originates from two mechanisms: surface plasmons and electrostatic lightening rod effects. We present fluorescence imaging using the strong local field created at the apex of a gold tip and demonstrate optical resolution of 25 nm. The enhancement effect gives also rise to photoemission from the tip itself. Measured spectra of the tip emission show a broad band continuum together with a second-harmonic peak. Both continuum and second-harmonic are confined at the apex of the tip. We find that, depending on the spectral position, the photoluminescence originates either from intraband or from interband transitions. The nonlinear response can be described by a single dipole oscillating at the second-harmonic frequency and oriented along the tip axis. These unique properties can be used to map focal fields distributions.     

Fabrication of near-field optical apertures in aluminium by a highly selective corrosion process in the evanescent field

A simple, one‐step process to fabricate high‐quality apertures for scanning near‐field optical microscope probes based on aluminium‐coated silicon nitride cantilevers is presented. A thin evanescent optical field at a glass–water interface was used to heat the aluminium at the tip apex due to light absorption. The heat induced a breakdown of the passivating oxide layer and local corrosion of the metal, which selectively exposed the front‐most part of the probe tip from the aluminium. Apertures with a protruding silicon nitride tip up to 72 nm in height were fabricated. The height of the protrusion was controlled by the extent of the evanescent field, whereas the diameter depended on the geometry of the probe substrate. The corrosion process proved to be self‐terminating, yielding highly reproducible tip heights. Near‐field optical resolution in a transmission mode of 85 nm was demonstrated.     

The role of tip plasmons in near-field Raman microscopy

The enhancement in electric field strength in the vicinity of a metal tip, through the excitation of plasma modes in the tip, is investigated using the finite difference time domain method; such tip enhancement has significant potential for application in scanning near-field Raman microscopy. To represent an experimentally realistic geometry the near-field probe is described by a conical metal tip with a spherical apex, with radii 20 nm and 200 nm considered, in close proximity to a glass substrate. Illumination through the substrate is considered, both at normal incidence and close to the critical angle, with the polarization in the plane of incidence. By modelling the frequency dependent dielectric response of the metal tip we are able to highlight the dependence on the scattering geometry of the nature of the electromagnetic excitations in the tip. In particular, the strongest electric field enhancement with the greatest confinement occurs for the excitation of modes localized at the tip apex, excited only for off-normal incidence. Bulk modes excited in the tip also produce enhancement, although over a larger area and with significantly less enhancement than that of the localized modes; however, the excitation of bulk modes is independent of the angle of incidence.     

Theoretical study of transient phenomena in near-field optics

A time-domain study of the propagation of a light pulse is made by the finite-difference time-domain method. This method is described briefly and then two applications are presented: creation and diffraction of surface plasmons in the time-domain, and propagation of a light pulse through two tip models, a dielectric one and a metal-coated one. Results on propagation speed of surface plasmons, spatio-temporal shape and spectral study of the field emitted by a tip are presented.     

Near-field imaging of surface plasmon on gold nano-dots fabricated by scanning probe lithography

We obtained scanning near‐field optical microscopy images to study the excitation of surface plasmons on metallic dots fabricated using scanning probe lithography. Gold nano‐dots were fabricated by applying electric voltages to conducting probes installed in an atomic force microscope using the mechanism of field‐induced diffusion and nano‐oxidation plus Au‐coating. High spatial resolution of scanning near‐field optical microscopy revealed a ‘bifold’ pattern of surface plasmon mode on fabricated Au dots in the polarization direction of incident light. We found that scanning near‐field optical microscopy imaging combined with scanning probe lithography is able to provide a systematic study of surface plasmon excitation on nano‐metallic structures.     

Apertureless scanning near-field optical microscopy for ion exchange channel waveguide characterization

We report the characterization of an integrated Ag⁺/Na⁺ ion exchange waveguide realized in a silicate glass substrate using apertureless scanning near‐field optical microscopy. Our experimental set‐up is based on the combination of a commercial atomic force microscope with an optical confocal detection system. Thanks to this system, the topography and evanescent optical field at the waveguide top surface are mapped simultaneously. Also, the process of apertureless scanning near‐field optical microscopy image formation is analysed. In particular, fringe patterns appearing in the image reveal the intrinsic interferometric nature of the collected signal, due to interference between the field scattered by the tip end and background fields related to guide losses. We give a quantitative interpretation of these fringes. Evanescent intensity mapping on the sample surface allowed us to extract physical waveguide parameters. In particular, it shows an unambiguous multimode beat along the waveguide propagation axis. Furthermore, we show that analysis of this intensity profile reveals back‐reflection effects from the waveguide exit facet. The resulting standing waves pattern allows us to evaluate the eigenmode propagation constants.     

The interaction of surface plasmon polaritons with a silver film edge

A prism coupling arrangement is used to excite surface plasmons at the surface of a thin silver film and a photon scanning tunnelling microscope is used to detect the evanescent field above the silver surface. Excitation of the silver/air mode of interest is performed at lambda1 = 632.8 nm using a tightly focused beam, while the control of the tip is effected by exciting a counter-propagating surface plasmon field at a different wavelength, lambda2 = 543.5 nm, using an unfocused beam covering a macroscopic area. Propagation of the red surface plasmon is evidenced by an exponential tail extending away from the launch site, but this feature is abruptly truncated if the surface plasmon encounters the edge of the silver film--there is no specularly reflected 'beam'. Importantly, the radiative decay of the surface mode at the film edge is observable only at larger tip-sample separations, emphasizing the importance of accessing the mesoscopic regime.     

用有限元方法计算X光源场发射阴极电流

针对场致发射阵列建立了有效的三维有限元模型来分析单个尖锥的发射电流.考虑到场致发射阵列的周期性和尖锥的轴对称性,仅对一个尖锥单元的1/4进行分析.对模型的表面施加电压边界条件,计算得到尖锥表面电场强度分布,电场强度在尖锥顶点最强,场发射电流在此处也最强.由Fowler-Nordheim函数可得到尖锥表面的场致发射电流密度分布,对整个尖锥表面进行积分后得到了单个尖锥的场致发射电流约为7μA.计算了在100V门电压下不同顶端半径的场致发射电流.结果显示,场发射对顶端半径有很强的依赖性.计算了100个顶端半径为8nm的尖锥在不同门电压下的总场致发射电流,发射电流与开启电压与实际测量值符合得很好.     

Evanescent fields - direct measurement, modeling, and application

The evanescent field surrounding an exposed planar waveguide in silica is accurately measured using scanning near field optical microscopy (SNOM) and compared to models of the field distribution. Distortions in the field due to edge effects and the proximity of the mode to the surface are all detected. The characterized field is use to quantitatively explore the difference in collection efficiency between contact mode SNOM and intermittent contact mode SNOM. A strong correlation between tip oscillation amplitude and detection efficiency is determined.