Dielectric nanostructures with high refractive index and low optical loss have attracted considerable attention as an alternative to plasmonic nanostructures. We experimentally demonstrated to control the visible electromagnetic resonances of Si-based core-shell nanostructures by thermally varying the core-shell ratio. We also found a Fano resonance which was generated by the interference between the electric and magnetic dipole moments excited in the core-shell nanostructures. The all-dielectric nanostructures realized low energy loss and high electromagnetic field enhancement comparable with that exhibited by plasmonic nanostructures. These unique optical properties would enable us to demonstrate effective field-enhanced spectroscopy and imaging with low heat generation.
Tip-enhanced Raman spectroscopy (TERS) offers one of the best techniques for analysis and imaging of molecule structures at nanoscale spatial resolution. An important issue in TERS is to improve the detection sensitivity of inherently weak Raman scattering so as to observe varieties of materials. For enhancement of the Raman signal, fully metallized tips are utilized in TERS, which enhance signals through plasmon oscillation at the tip apex. However, length of metal along the tip axis is on the order of a few to a few tens of micrometers, which means the plasmon resonant wavelength is much longer than the wavelength of the visible light used in TERS. From that point, if the tip has a metallic nanostructure on the apex, it would give better enhancement in the visible range compared with fully metallized tips. In this research, we employed photoreduction as a new fabrication method to grow a metallic nanostructure at the tip apex. We found a particular property of photoreduction that it occurs selectively at sharp corners, such as the tip apex of silicon cantilevers. Through this property, we succeeded in growing silver nanoparticles selectively at the tip apex. One of the advantages of the photoreduction is that the size of metal nanostructures is well controlled by optimizing various parameters. We controlled the size of silver nanoparticles from 100 to 400 nm by changing the laser exposure time. Furthermore, we obtained an order of magnitude higher enhancement from our fabricated tip compared with fully metalized tips through TERS measurements.
When Raman scattering is excited from the evanescent light field created by illuminating the apex of a sharp metallic
nano-tip, it achieves new aspects with strong enhancement of scattering efficiencies and super resolving capabilities. The
primary mechanism of tip-sample interaction is electromagnetic, which is based on the excitation of localized surface
plasmon polaritons. However, when the tip is close enough to the sample, typically at molecular distances, the chemical
interactions between the tip and the sample become important. Strong temporal fluctuations of Raman scattering,
including fluctuations of peak frequencies and peak intensities, together with extraordinary enhancement of several
peaks, were observed. These temporal fluctuations, which are typical signature of single molecule detection, were
attributed to the changes of molecular orientations of the sample molecules in the upper layer of the nanocluster, which
got chemically adsorbed at the tip molecules.
Near-field Raman scattering has been successfully utilized to study the interaction between a metal-coated nano-tip and
carbon nanostructures, such as carbon-60 molecules and single walled carbon nanotubes. The enhanced and localized
light field in the vicinity of the tip apex provides high resolution imaging as well as the detection of weak vibrational
modes. Apart from the electromagnetic and chemical interactions, a mechanical interaction between tip molecules and
the sample molecules has also been investigated.
A light microscope capable to show images of molecules in nanometer scale has been a dream of scientists, which, however, is difficult due to the strict limitation of spatial resolution due to the wave nature of light. While there have been attempts to overcome the diffraction limit by using nonlinear response of materials, near-field optical microscopy could provide better detecting accuracy. In this paper, we present molecular distribution nano-imaging colored by Raman-scattering spectral shifting, which is probed with a metallic tip. The metallic probe tip has been used to enhance the optical field only in the vicinity of probe tip. The effect is similar to the one seen in the detection of molecules on the metal-island film, known as surface-enhanced Raman spectroscopy (SERS), while in this case a single metallic tip works for the field enhancement in nanometer scale.