Apertureless near-field optical microscopy via local second-harmonic generation

We describe an apertureless scanning near-field optical microscope (SNOM) based on the local second-harmonic generation enhancement resulting from an electromagnetic interaction between a probe tip and a surface. The imaging mechanisms of such apertureless second-harmonic SNOM are numerically studied. The technique allows one to achieve strongly confined sources of second-harmonic light at the probe tip apex and/or surface area under the tip. First experimental realization of this technique has been carried out using a silver-coated fibre tip as a probe. The experiments reveal a strong influence of the tip–surface interaction as well as polarization of the excitation light on images obtained with apertureless second-harmonic SNOM. The technique can be useful for studying the localized electromagnetic excitations on surfaces as well as for visualization of lateral variations of linear and nonlinear optical properties of surfaces.     

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.     

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.     

Apertureless near-field scanning Raman microscopy using reflection scattering geometry

The combination of near-field scanning optical microscopy and Raman spectroscopy provides chemical/structural specific information with nanometer spatial resolution, which are critically important for a wide range of applications, including the study of Si devices, nanodevices, quantum dots, single molecules of biological samples. In this paper, we describe our near-field Raman study using apertureless probes. Our system has two important features, critical to practical applications. (1) The near-field Raman enhancement was achieved by Ag coating of the metal probes, without any preparation of the sample, and (2) while all other apertureless near-field Raman systems were constructed in transmission mode, our system works in the reflection mode, making near-field Raman study a reality for any samples. We have obtained the first 1D Raman mapping of a real Si device with 1s exposure time. This is a very significant development in near-field scanning Raman microscopy as it is the first demonstration that this technique can be used for imaging purpose because of the short integration time. In addition, the metal tips used in our set-up can be utilized to make simultaneous AFM and electrical mappings such as resistance and capacitance that are critical parameters for device applications.     

Field enhancement and apertureless near-field optical spectroscopy of single molecules

We report on fluorescence enhancement in near field optical spectroscopy by apertureless microscopy. Our apertureless microscope is designed around a confocal fluorescence microscope associated with an AFM head. First, we show that the confocal microscope alone allows single molecule imaging and single molecule fluorescence analysis. When associated with the AFM head, we demonstrate, for the first time to our knowledge, that single molecule fluorescence is enhanced under the silicon tip. We tentatively attribute this effect to field enhancement under the tip.     

Fibre-top atomic force microscope probe with optical near-field detection capabilities

We present a fibre-top probe fabricated by carving a tipped cantilever on an optical fibre, with the tip machined in correspondence of the fibre core. When approached to an optical prism illuminated under total internal reflection conditions, the tip of the cantilever detects the optical tunnelling signal, while the light coupled from the opposite end of the fibre measures the deflection of the cantilever. Our results suggest that fibre-top technology can be used for the development of a new generation of hybrid probes that can combine atomic force microscopy with scanning near field optical microscopy.     

Large-area topography analysis and near-field Raman spectroscopy using bent fibre probes

We present a method for combined far‐field Raman imaging, topography analysis and near‐field spectroscopy. Surface‐enhanced Raman spectra of Rhodamine 6G (R6G) deposited on silver nanoparticles were recorded using a bent fibre aperture‐type near‐field scanning optical microscope (NSOM) operated in illumination mode. Special measures were taken to enable optical normal‐force detection for control of the tip–sample distance. Comparisons between far‐field Raman images of R6G‐covered Ag particle aggregates with topographic images recorded using atomic force microscopy (AFM) indicate saturation effects due to resonance excitation.     

Apertureless scanning near-field magneto-optical microscopy of magnetic multilayers

We imaged magnetic domains in Pt/Co/Pt multilayers using an apertureless scanning near-field optical microscope operating in reflection mode. As the magneto-optical effects are weak for this kind of structure, a polarization modulation technique with a photoelastic modulator was used to reveal the contrast between magnetic domains. In the case of a Pt/Co/Pt trilayer structure, a strong improvement in lateral resolution is observed compared with far-field magneto-optical images and good sensitivity is achieved. In the case of a Pt/[Co/Pt]Pt multilayer structure, stripe domains of 200 nm width could be resolved, in good agreement with images obtained by magnetic force microscopy on the same structure.     

Scanning near-field cathodoluminescence investigations

By implementing a scanning near-field optical microscope into the analysis chamber of a scanning electron microscope, the light emitted due to cathodoluminescence can be locally detected in the near-field using tapered, coated optical fibers. In addition to the ability to perform contactless measurements of local diffusion lengths, the achievable spatial resolution can be increased to about 50 nanometers.     

Photocurrent near-field microscopy of Schottky barriers

We used a combination of internal photoemission and of near-field optical microscopy (SNOM) to study the lateral variations in solid interface properties such as energy barriers and electron–hole recombination. In particular we investigated the fully formed Pt–GaP, Au–GaAs, Au–SiN _x –GaAs and PtSi–Si Schottky barriers. Our approach enabled us to measure large lateral variations in the photocurrent with spatial resolution on the nanometric scale. Due to the ability of SNOM to supply parallel topographic information, we observed photocurrent variations from zone to zone that only correlated in a few cases with local variations in surface morphology. We assigned the uncorrelated fluctuations to local variations in the interface stoichiometry, the presence of interface states induced by the metallic overlayer and to defect states at the junction. Furthermore, by tuning the photon energy and applied bias we were able to measure the surface distribution of the diffusion length.     

表面增强拉曼光谱的研究进展

本文从提高表面拉曼光谱检测灵敏度和空间分辨率两个方面的发展叙述表面增强拉曼光谱和针尖增强拉曼光谱的原理、方法、特点以及最新进展。对利用表面增强拉曼光谱和针尖增强拉曼光谱研究金属表面上分子吸附等方面的应用进行总结 ,并对他们的应用前景做了预测     

近场光学显微镜及其在生物膜方面的应用

本文首先介绍近场光学显微镜的基本原理,然后介绍近场光学显微镜与传统光学显微镜、原子力显微镜、扫描隧道显微镜相比,在生物膜研究方面的优势。并在此基础上着重介绍近场光学显微镜在生物膜方面的应用。     

A novel fabrication method for fluorescence-based apertureless scanning near-field optical microscope probes

We report a novel method for the fabrication of probes with localized sub-wavelength fluorescing media at their extremities. We present our first results and discuss future plans to extend this technique to the systematic fabrication of fluorescent probes for apertureless scanning near-field optical microscopy.     

Evanescent-wave scattering in near-field optical microscopy

Extended Mie theory is used to investigate the scattering and extinction of evanescent waves by small spherical particles and aggregates of such particles. Metallic, dielectric and metal-coated dielectric particles are taken into consideration. In contrast to plane-wave excitation, p - and s -polarized spectra differ in the case of evanescent waves due to the inherent asymmetry of both polarizations. Furthermore, contributions from higher multipoles are strongly enhanced, compared with plane-wave excitation, and the enhancement factors are polarization dependent. The corresponding changes in the scattering and extinction spectra are most pronounced in cases where higher multipoles exhibit resonances in the spectral range considered. This applies, for example, to morphological resonances of dielectric particles with size parameters > 1. The effect of the surface, where the evanescent wave is generated by total internal reflection, on the scattering and extinction spectra is investigated via numerical field calculations employing the multiple multipole method. In an application to apertureless near-field optical microscopy, the variation of the scattered power is calculated when a silicon particle is scanned across a silver particle in the evanescent field.     

A single gold particle as a probe for apertureless scanning near-field optical microscopy

We report on the fabrication, characterization and application of a probe consisting of a single gold nanoparticle for apertureless scanning near-field optical microscopy. Particles with diameters of 100 nm have been successfully and reproducibly mounted at the end of sharp glass fibre tips. We present the first optical images taken with such a probe. We have also recorded plasmon resonances of gold particles and discuss schemes for exploiting the wavelength dependence of their scattering cross-section for a novel form of apertureless scanning near-field optical microscopy.     

Non-optical tip–sample distance control method for scanning near-field optical microscopy using a piezoresistive micro cantilever

A piezoresistive micro cantilever is applied to monitor the displacement of an optical fibre probe and to control tip–sample distance. The piezoresistive cantilever was originally made for a self-sensitive atomic force microscopy (AFM) probe and has dimensions of 400 µm length, 50 µm width and 5 µm thickness with a resistive strain sensor at the bottom of the cantilever. We attach the piezoresistive cantilever tip to the upper side of a vibrating bent optical fibre probe and monitor the resistance change amplitude of the strain sensor caused by the optical fibre displacement. By using this resistance change to control the tip–sample distance, the two-cantilever system successfully provides topographic and near-field optical images of standard samples in a scanning near-field optical microscopy (SNOM)/AFM system. A resonant characteristic of the two-cantilever system is also simulated using a mechanical model, and the results of simulation correspond to the experimental results of resonance characteristics.     

Non-optical bimorph-based tapping-mode force sensing method for scanning near-field optical microscopy

A non‐optical bimorph‐based tapping‐mode force sensing method for tip–sample distance control in scanning near‐field optical microscopy is developed. Tapping‐mode force sensing is accomplished by use of a suitable piezoelectric bimorph cantilever, attaching an optical fibre tip to the extremity of the cantilever free end and fixing the guiding portion of the fibre to a stationary part near the tip to decouple it from the cantilever. This method is mainly characterized by the use of a bimorph, which carries out simultaneous excitation and detection of mechanical vibration at its resonance frequency owing to piezoelectric and anti‐piezoelectric effects, resulting in simplicity, compactness, ease of implementation and lack of parasitic optical background. In conjugation with a commercially available SPM controller, tapping‐mode images of various samples, such as gratings, human breast adenocarcinoma cells, red blood cells and a close‐packed layer of 220‐nm polystyrene spheres, have been obtained. Furthermore, topographic and near‐field optical images of a layer of polystyrene spheres have also been taken simultaneously. The results suggest that the tapping‐mode set‐up described here is reliable and sensitive, and shows promise for biological applications.     

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.     

Mid-infrared scanning near-field optical microscope resolves 30 nm

We explore the performance of a scanning near-field infrared microscope, which works by scattering tightly focused CO₂ laser radiation (λ = 10  μ m) from the apex of a metallized atomic force microscope tip. The infrared images of test samples prove a spatial resolution of 30 nm and are free of topographical and inertial artefacts, thus they should be of great interest for practical applications. We also observe that the infrared contrast vanishes when the input beam polarization is orthogonal to the tip axis, in accordance with theoretical expectations for a mechanism of longitudinal field interaction.