Tip-enhanced Spectroscopy Research Articles

先端増強近接場分光法

The synergy of scanning probe microscope and optical microscope realizes a promising sensing technique in surface characterization in the nanoscale with a high chemical sensitivity, which is not attainable by a simple topographic contrast. This paper introduces one of the synergy tools, that is, near-field scanning optical microscope particularly the use of a sharp metallic probe tip. While optical microscope exhibits a high chemical sensitivity, the spatial resolution of conventional optical microscopes using lens is diffraction limited due to wave nature of light. The key technique to break through this limitation is surface plasmon polariton excitation at the tip, which is promising for both high spatial resolution and high sensitivity. This technique has been recently recognized as tip-enhanced near-field spectroscopy.

Advances in Tip-Enhanced Spectroscopy from Optical Nano-Crystallography to Ultrafast Spectroscopy

Extended abstract of a paper presented at Microscopy and Microanalysis 2011 in Nashville, Tennessee, USA, August 7–August 11, 2011.

Numerical investigations of a multi-walled carbon nanotube-based multi-segmented optical antenna

Motivated by the fabrication potential of multi-walled carbon nanotube structures, we numerically investigated a paired structure consisting of two metallic spheres each grown on one end of a multi-walled nanotube. The paired two-segmented structure is capable to convert free-space radiation into an intense near-field, and, hence, acting as an optical antenna. Vice versa the presence of the two nanotubes enable a current source at the antenna feed to more efficiently energy into the radiation modes, resulting e.g. in correspondingly altered luminescence lifetimes when an excited single molecule is placed in the feed point. Furthermore, the structure represents a mean to localize light on a sub-wavelength scale within different materials, which is interesting in the context of a fabrication technology for integrated nanophotonic components with different material combinations. The optical properties of the nano-antenna are analyzed by means of numerical simulations using the finite element method. Our investigations have revealed that the field enhancement, the resonances, and the radiation patterns can be easily tuned since all these quantities strongly depend on the size of the nanotubes and the metallic spheres, as well as on their material properties The structure we propose here carries a great potential for bio-sensing, for tip-enhanced spectroscopy applications, and for interfacing integrated photonic nano circuits.

Highly efficient nano-tips with metal – dielectric coatings for tip-enhanced spectroscopy applications

We studied how a protection layer influences the performance of an Ag-coated dielectric tip in tip-enhanced Raman spectroscopy using 3D simulations. Two typical chemically inert materials, Au and SiO2 were considered in this work. Neither of them introduces a significant shift of the resonance frequency; the field enhancement as well as the spatial resolution can also be preserved with a thin protective layer. Surprisingly, a 5nm layer of SiO2 can even improve the field enhancement of the tip. This provides a new method to prevent Ag tips from undesired physical and chemical damages without losing their original performance for TERS.

Focused Excitation of Surface Plasmon Polaritons Based on Gap-Mode in Tip-Enhanced Spectroscopy

We propose a novel method to improve field enhancement factor in tip-enhanced spectroscopy techniques using gap mode configuration. In gap-mode, a silver coated tip is placed just above the focus at silver thin film deposited on a dielectric substrate. The enormous electric field enhancement generated at the focus is due to coupling of surface plasmon polaritons at the gap between the 48 nm silver film and silver coated tip. The proposed technique was demonstrated in both radially polarized and linearly polarized beam. Radial polarization was found to be better than linear polarization in gap mode because only p-polarized component can contribute to the excitation of surface plasmon of the silver film. The electric field distributions at the gap below tip are calculated using finite difference time domain method and experimentally visualized by scanning probe microscope. Numerical results are in good agreement with the empirical data.

Near-field photonics: tip-enhanced microscopy and spectroscopy on the nanoscale

Any detailed study of the interaction of electromagnetic radiation with nanoscale systems is limited by diffraction effects in classical optical systems. Near-field microscopy extends conventional imaging beyond this self-imposed barrier and is used to perform microscopy and spectroscopy with ultra-high spatial resolution. In this article we will discuss the use of the enhanced electric field created at the apex of a sharp laser-irradiated metal tip as a means of producing a truly nanoscale light source. This confined light source can be used to excite locally vibrational modes along carbon nanotubes or to investigate surface charge oscillations in optically resonant nanoparticles. We report the use of such a technique to demonstrate localized photofluorescence and Raman imaging with sub 20 nm spatial resolution.

Production of gold tips for tip-enhanced near-field optical microscopy and spectroscopy: analysis of the etching parameters

In this paper, we analyze in detail the etching parameters that are used for manufacturing gold tips for tip-enhanced near-field optical microscopy and spectroscopy. From the current variation versus time, we first demonstrate that the sharpest tips are obtained if the etching is stopped at a specific time where the curve presents an inflexion point. We then analyze the influence of the concentration of the etching solution and the applied voltage on the roughness and the tip apex. As a result, we propose a set of parameters allowing for reproducible production of gold tips with 20 nm radius of curvature.

チップ増強型近接場ラマン散乱分光によるナノイメージング

A metallic probe having a nano-scaled tip strongly enhances the optical field in the local vicinity of the tip apex. Using this effect, which is analogous to the surface enhanced Raman scattering, we can excite Raman scattering in a nanometric region, and consequently realize nano-identification, nano-analysis and nano-imaging of molecules and crystals. In this article, we introduce the mechanism and recent results of the tip-enhanced near-field Raman scattering spectroscopy. In particular, unique spectral shapes observed in the tip-enhanced Raman scattering spectra are discussed via a quantum chemical calculation. We also demonstrate that tip-enhanced spectroscopy has been applied to nonlinear Raman scattering and has achieved molecular nano-imaging with a high resolution and a high sensitivity.