Near-field scanning optical microscopy imaging of multimode interference

By use of a near-field scanning optical microscope (NSOM) in collection mode, the intensity distribution along a 2 x 2 multimode interference coupler was directly imaged as a function of wavelength. Although calculations can predict the general trend of wavelength dependence and the approximate positions of multiple images in the coupler, the accuracy is poor because of uncertainties in the waveguide width. We show that direct imaging using a NSOM bypasses calculational uncertainties and proves to be a powerful technique for studying these waveguide devices.     

Nanostructured optical fiber with surface-enhanced Raman scattering functionality

A powerful method for the production of reproducible surface-enhanced Raman scattering (SERS) substrates is described based on the scaling properties of glass rods when drawn into fibers. The fabrication process involves chemically eroding the cleaved tips of drawn silica imaging fibers and then coating them with silver. For an appropriate choice of final diameter the drawn and eroded tips show clearly defined and regular triangular formations on a scale of approximately 80 nm. The favorable SERS properties of these structures have been demonstrated by the observation of enhancement factors of approximately 10(6).     

Near-field scanning optical microscopy local luminescence studies of rhodamine dye

This article presents results of near-field scanning optical microscope measurement of local luminescence of rhodamine 3B intercalated in montmorillonite samples. We focus on how local topography affects both the excitation and luminescence signals and resulting optical artifacts. The Finite Difference in Time Domain method (FDTD) is used to model the electromagnetic field distribution of the full tip-sample geometry including far-field radiation. Even complex problems like localized luminescence can be simulated computationally using FDTD and these simulations can be used to separate the luminescence signal from topographic artifacts.     

Near-field scanning optical microscopy of single molecules by femtosecond two-photon excitation

We present a demonstration of near-field scanning optical microscopy of single molecules based on ultrafast two-photon-induced fluorescence. Measurements were performed by use of 100-fs pulses at 800 nm from a Ti:sapphire laser to excite the two-photon transition in Rhodamine B molecules. Although near-field probes are normally metal coated to achieve superresolution, we used uncoated tips to achieve sufficiently high optical powers to generate acceptable fluorescence emission rates. Images of single molecules demonstrate a resolution of ~175nm(< lambda/4) on a topographically smooth surface, which surpasses the apparent lambda/2 resolution limit for uncoated tips operating in the linear response regime.     

Near-field scanning optical microscopy studies of thin film surfaces and interfaces

In this article, the results of the modeling of topography related artifacts appearing in near-field scanning optical microscopy measurements are presented. The results obtained for near-field scanning optical microscope operation in reflection mode with off-axis far field detector position are compared with experimental results. It is shown that the chosen numerical method - Finite Difference in Time Domain method (FDTD) - can be used for efficient modeling of main topography related artifact. It is also seen that the far field detector position can have large influence on the resulting reflection mode optical images.     

Near-field effects of focused illumination on periodic structures in scanning tunneling optical microscopy

The influence of a focused polarized Gaussian beam on image formation was studied. We show that the position of the tip with reference to the center of the beam involves asymmetry in the intensity map. A comparison between s and p polarization can be made, owing to the definition of both a three-dimensional polarized Gaussian beam and a three-dimensional object. This result implies that the best way to scan a sample consists in moving it and not the tip; moreover, focusing the incident light to get a higher signal-to-noise ratio must be done carefully with respect to the sample period.     

Combined taper-and-cylinder optical fiber probes for highly sensitive surface-enhanced Raman scattering

A combined taper-and-cylinder optical fiber fabricated by simple tube-etching and modified with silver nanoparticles is developed for surface-enhanced Raman scattering (SERS) sensing. It has the advantages of high light transmission efficiency and large interaction areas for light and silver nanoparticles. The detection of rhodamine 6G in remote mode indicates that the sensitivity could realize 10^?13 M. The fiber SERS probe with high flexibility and sensitivity shows great potential for molecule detection in various sensing applications.     

Raman and photon scanning tunnelling microscopy of optical waveguides

Recent results from our laboratories on the design and characterization of optical waveguides are reviewed. Modelling of the design of channel optical waveguides is discussed and two means of experimental characterization that provide spatially local information are presented. Raman microprobe spectroscopy is used to explore the role of stress induced by a thin dielectric overlayer used to form channel waveguides in GaAlAs, and thus the role of photoelastic effects on waveguide behaviour. Photon scanning tunnelling microscopy is used to investigate local surface and index variations by probing the evanescent waveguide field.     

Chemical contribution to surface-enhanced Raman scattering

We present a new mechanism for the chemical contribution to surface-enhanced Raman scattering (SERS). The theory considers the modulation of the polarizability of a metal nanocluster or a flat metal surface by the vibrational motion of an adsorbed molecule. The modulated polarization of the substrate coupled with the incident light will contribute to the Raman scattering enhancement. We show that for a metal cluster and for a flat metal surface this new chemical contribution may enhance the Raman scattering intensity by a factor of approximately 102 and approximately 104, respectively. The new SERS process is determined by the electric field parallel to the surface of the metal substrate at the molecular binding site.     

Long-range manifestation of surface-enhanced Raman scattering

The spatial scale at which the effect of surface-enhanced Raman scattering by planar Ag nanostructures manifests itself is investigated experimentally by direct measurements of the dependence of the enhancement factor on the distance between the surface of the Ag nanostructure and a layer of test organic molecules. It is found that the enhancement factor remains almost constant up to distances as large as 30 nm and drops abruptly at larger distances. The obtained dependence is universal for all kinds of organic molecules investigated. The fact that the surface enhancement of Raman scattering manifests itself on such a long spatial scale sharply contradicts the broadly accepted model assuming that the surface-enhanced Raman scattering decreases rapidly at distances as short as 2–3 nm.     

Single-site surface-enhanced Raman scattering beyond spectroscopy

Recent progress in the observation of surface-enhanced Raman scattering (SERS) is reviewed to examine the possibility of finding a novel route for the effective photoexcitation of materials. The importance of well-controlled SERS experiments on a single molecule at a single site is discussed based on the difference in the information obtained from ensemble SERS measurements using multiple active sites with an uncontrolled number of molecules. A single-molecule SERS observation performed at a mechanically controllable breaking junction with a simultaneous conductivity measurement provides clear evidence of the drastic changes both in the intensity and in the Raman mode selectivity of the electromagnetic field generated by localized surface plasmon resonance. Careful control of the field at a few-nanometer-wide gap of a metal nanodimer results in the modification of the selection rule of electronic excitation of an isolated single-walled carbon nanotube. The examples shown in this review suggest that a single-site SERS observation could be used as a novel tool to find, develop, and implement applications of plasmon-induced photoexcitation of materials.     

Near-field optical microscopy of nonlinear susceptibilities

Local probing of second-order susceptibilities with a near-field optical microscope is demonstrated for the first time. Using an uncoated fiber tip as a light source, near-field images of a surface of y-cut LiNbO₃ crystal and of a multilayer Langmuir–Blodgett film of 2-docosylamino-5-nitropyridine are obtained at the fundamental pump and second harmonic wavelengths while simultaneously recording surface topography. It is shown that optical second-harmonic images for different polarizations of the pump light reflect the difference in magnitude of the corresponding components of the second-order susceptibility tensor. Various degrees of correlation in contrast of optical (fundamental and second harmonic) and topographical images are observed and discussed.     

Near-field optical microscopy using an integrating sphere

A scanning near-field optical microscope using an integrating sphere is demonstrated. The images from a usual near-field optical microscope often include contrasts caused by the sample structure because the reflection and transmission angles of signal waves depend on the size and shape of the sample. The observation angle dependency of signals can be avoided by using an integrating sphere. Background signals resulting from using the sphere can be reduced by using lock-in detection synchronously with modulation of tip–sample distance. The whole detection system is possibly useful for observing the distribution of the refractive index and/or the absorption coefficient. Received: 27 October 2000 / Final version: 29 August 2001 / Published online: 7 November 2001     

Sensitive surface-enhanced Raman scattering substrates based on anti-pyramidal gold architectures decorated with silver nanoparticles

Abstract

A micro-structured gold surface, consisting of a periodic square–based anti–pyramidal array (Klarite) with a smooth boundary surface on which silver nanoparticles (diameter: 60?nm) were deposited, produced an active surface enhanced Raman scattering substrate. With p-aminothiophenol as a probe molecule, the Raman activity of the micro–structured surface was compared before and after deposition of the silver nanoparticles. Experimental results show that the Raman spectra on the silver/p-aminothiophenol/Klarite structure is stronger than that on the silver/p-aminothiophenol/gold film and the Raman spectra on the silver/p-aminothiophenol/gold film is stronger than that on silver/p-aminothiophenol, p-aminothiophenol/Klarite structure, p-aminothiophenol/gold film, which is confirmed by numerical simulations. A similar result is obtained with crystal violet as test molecule.     

An effective surface-enhanced Raman scattering template based on gold nanoparticle/silicon nanowire arrays

A large-scale Si nanowire array （SiNWA） is fabricated with gold （Au） nanoparticles by simple metal-assisted chemical etching and metal reduction processes. The three-dimensional nanostructured Au/SiNWA is evaluated as an active substrate for surface-enhanced Raman scattering （SERS）. The results show that the detection limit for rhodamine 6G is as low as 10-7 M, and the Raman enhancement factor is as large as 105 with a relative standard deviation of less than 25%. After the calibration of the Raman peak intensifies of rhodamine 6G and thiram, organic molecules could be quantitatively detected. These results indicate that Au/SiNWA is a promising SERS-active substrate for the detection of biomolecules present in low concentrations. Our findings are an important advance in SERS substrates to allow fast and quantitative detection of trace organic contaminants.     

银岛膜表面胸腺嘧啶吸附行为的表面增强拉曼光谱和表面增强红外光谱研究

利用激光刻蚀法制备了具有化学纯净表面的银岛膜,该岛膜有很好的表面增强特性。利用表面增强拉曼光谱和表面增强红外光谱对胸腺嘧啶分子在银岛膜表面的吸附状态进行了对比研究。表面增强拉曼光谱中CN和C—O伸缩振动模式的出现表明胸腺嘧啶分子由原来的酮式结构变成了烯醇式结构;C(4)O伸缩振动谱带明显增强和N(3)的去质子化异构体特征峰的存在证明胸腺嘧啶分子是通过O(8)和N(3)的共同作用倾斜地吸附在银岛膜表面。对10-5 mol.L-1胸腺嘧啶在银岛膜表面上的红外光谱利用欧米采样器进行了反射法测量,发现其红外吸收增强了200倍。红外信号分析的结果支持了胸腺嘧啶分子通过O(8)与银表面发生相互作用的论断,同时也可得出胸腺嘧啶倾斜地吸附在银岛膜表面的结论。     

Silver fractal networks for surface-enhanced Raman scattering substrates

Based on diffusion-limited aggregation process, a convenient nanotechnique is demonstrated to obtain large silver fractal networks for a surface-enhanced Raman scattering (SERS)-active substrate. The silver fractal networks are of high SERS enhancement factor, large dynamical range. The observed SERS efficiency can be explained in terms of strongly localized plasmon modes relative to the single particle plasmon resonance.     

Silver nanoparticle coverage dependence of surface-enhanced Raman scattering

We report surface-enhanced Raman scattering (SERS) from 4-mercaptopyridine adsorbed on nanotextured silver surfaces as the coverage of silver is varied. The degree of surface enhancement is strongly dependent on silver coverage and correlated to the extinction of the surface at the Raman excitation wavelength, that extinction being determined by multiparticle surface plasmon resonances. The coverage dependence of the Raman intensity is consistent with signals being dominated by molecules at junctions inside nanoparticle aggregates where electromagnetic energy is localized into “hot spots” by interactions of the incident and scattered fields with the surface plasmons. The Raman intensity drops precipitously near the conductivity percolation threshold because these hot spots are destroyed when conducting paths allow plasmons to propagate. Our approach to substrate preparation provides clean surfaces with average enhancements ≥10⁷, an order of magnitude larger than typical for SERS. PACS 78.67.-n; 78.68.+m; 33.20.Fb     

双酚A拉曼光谱的密度泛函理论计算及表面增强拉曼光谱研究

以密度泛函理论(DFT),RB3LYP/6-311G(d)方法计算得到的双酚A(BPA)分子振动光谱为依据,对BPA分子常规拉曼光谱进行了详细的指认,对其振动模式进行了归属.研究了BPA在金胶体系中的表面增强拉曼光谱,对其吸附方式进行了分析:BPA分子在酸性pH下,分子以=CO-吸附到金溶胶上,-OH键的振动消失,苯环以直立方式垂直于金胶上.