Norihiko Hayazawa

Metallized tip amplification of near-field Raman scattering

We have observed the amplification of near-field Raman scattering by using an apertureless near-field scanning optical microscope the tip of which is a 40 nm silver-layer-coated cantilever of an atomic force microscope. Localized surface plasmon polaritons are excited at the metallized tip apex of the cantilever thereby producing a field enhancement effect. We have unambiguously observed the amplification of the near-field signal due to the field enhancement by measuring the Raman spectra of Rhodamine 6G molecules adsorbed on 10 nm silver islands film when the metallized tip approached the surface of the sample. In addition, due to the nature of near-field optics, spectral mapping of Raman Scattering was attained with a 50 nm super-resolving power.

Detection and characterization of longitudinal field for tip-enhanced Raman spectroscopy

We characterized the longitudinal field formed at a tightly focused spot by a high numerical aperture objective lens using a tip-enhanced near-field microscope. The longitudinal field efficiently excites the localized surface plasmon polaritons at the metallic tip apex resulting in an electric field enhancement. Radially polarized light generated by a combination of four half-waveplates successfully increases the longitudinal field resulting in higher sensitivity for tip-enhanced Raman spectroscopy of adenine nanocrystals.

Detection of an individual single-wall carbon nanotube by tip-enhanced near-field Raman spectroscopy

A tip-enhanced near-field Raman microscope has been applied to the detection of an individual single-wall carbon nanotube (SWNT). The detected information is a color (Raman-shift) image of molecular distribution without having to resort to staining non-fluorescent molecules of interest. In addition to nanometric-sensing and -imaging capability, local field-enhancement of the metallic tip has been utilized to detect a weak Raman scattering from nanometer region, which cannot be observed by conventional micro-Raman configuration (far-field Raman). The experimental results are shown with analysis of distinct vibration modes of both radial breathing mode and G-band.

A 1.7 nm resolution chemical analysis of carbon nanotubes by tip-enhanced Raman imaging in the ambient

Surface morphology of materials is routinely analysed by an atomic force microscope and scanning tunneling microscope (STM) down to subnanometer precision. However, it is still challenging to investigate the surface chemistry simultaneously, which requires specific capability of force or tunneling spectroscopy in ultrahigh vacuum environment and liquid Helium temperature. Here we demonstrate the simultaneous chemical and structural analysis of individual carbon nanotubes (CNTs) by STM-based tip-enhanced Raman spectroscopy (STM-TERS) with 1.7 nm spatial resolution in the ambient. Raman contrast over different types of CNTs, local defects, diameters and bundling effect are all visualized in real space. Disengaging from ultrahigh vacuum and cryogenic environment, our ambient STM-TERS imaging is powerful in analysing local chemistry for CNTs and also suitable for analysing as-made and soft materials, which cannot be seen with general electron microscopy techniques.

Nanoscale characterization of strained silicon by tip-enhanced Raman spectroscope in reflection mode

We observe localized strains in strained silicon by tip-enhanced near-field Raman spectroscope in reflection mode. The tip-enhanced Raman spectra show that the Raman frequency and intensity of strained silicon were different within a crosshatch pattern induced by lattice mismatch. Micro-Raman measurements, however, show only uniform features because of averaging effect due to the diffraction limit of light. Nanoscale characterization of strained silicon is essential for developing reliable next generation integrated circuits. This technique can be applicable not only to strained silicon but also to any other crystals.

Near-field enhanced Raman spectroscopy using side illumination optics

We demonstrate near-field enhanced Raman spectroscopy with the use of a metallized cantilever tip and highly p-polarized light directed onto the tip with side illumination optics using a long working distance objective lens. The highly p-polarized light field excites surface plasmon polaritons localized at the tip apex, which results in the enhanced near-field Raman scattering. In this article, we achieved an enhancement factor of 4000 for Rhodamine 6G molecules adsorbed on a silver island film. The side illumination is also applicable to an opaque sample and to near-field photolithography.

Evanescent field excitation and measurement of dye fluorescence in a metallic probe near-field scanning optical microscope.

We introduce a method of dye fluorescence excitation and measurement that utilizes a near‐field scanning optical microscope (NSOM). This NSOM uses an apertureless metallic probe, and an optical system that contains a high numerical aperture (NA) objective lens (NA = 1.4). When the area which satisfies NA < 1 is masked, the objective lens allows for the rejection of possible transmitted light (NA < 1) through the sample. In such conditions, the focused spot consists of only the evanescent field. We found that this NSOM system strongly reduces the background of the dye fluorescence and allows for the measurement of the fluorescence intensity below the diffraction limit of the excitation source.

Towards atomic site-selective sensitivity in tip-enhanced Raman spectroscopy

Depending on each nitrogen atom of adenine molecule to which a silver atom of a metallic tip approaches, tip-enhanced near-field Raman spectroscopy may show a potential to achieve atomic site-selective detection sensitivity. Molecular vibrational calculations show that silver atoms and adenine molecule create several isomers generating specific vibrational modes of each isomer that are shifted or not observable in isolated adenine molecule itself. Here, the authors observe the specific vibrational modes and spectral shifts of isomers experimentally and are in good agreement with their calculations.

Amplification of coherent anti-Stokes Raman scattering by a metallic nanostructure for a high resolution vibration microscopy

On the basis of the mechanism of surface enhanced Raman scattering, it is shown that coherent anti-Stokes Raman scattering (CARS) of molecules attached to isolated gold nanoparticles are strongly enhanced and the signal from each particle is well localized. In addition to well-known advantages of CARS, the surface enhanced CARS combined with a scanning system of metallic nanoprobe tip can realize high spatial resolution CARS microscopy beyond the diffraction limit of light by locally enhancing the weak signals from the small sample volume. This concept is realized by tip-enhanced coherent anti-Stokes Raman spectroscopy using a metallic nanoprobe of near-field scanning optical microscope.

Near-field scanning optical microscope using a metallized cantilever tip for nanospectroscopy

We have developed a NSOM which has a metallic probe tip and a highly focused evanescent light field spot. Evanescent illumination effectively rejects the background light, e.g. the stray light from the shaft of the probe. By suppressing the stray light and utilizing the field enhancement generated by the metallic probe, a sudden increment of the fluorescence was observed in the near-field region. We have used this for near-field Raman scattering detection of molecules vibrations with the aid of surface enhanced Raman scattering. One specific stokes-Raman-shifted lines was observed by near-field excitation together with several other lines that were excited by the far-field light.