Near-field Optical Microscopy Tip Research Articles

Degradation of silver near-field optical probes and its electrochemical reversal

Deterioration of the outstanding optical properties of elemental silver due to atmospheric corrosion compromises its use in the field of plasmonics. Therefore, more chemically inert, but more lossy, metals (e.g., gold) are often used as a compromise. Silver tips for near-field optical microscopy are only utilized by specialized laboratories with in-house tip production facilities. This article presents a time-dependent study of the effect of atmospheric corrosion on the electromagnetic enhancement of solid silver tips. It was found that chemical degradation renders them unusable for tip-enhanced Raman spectroscopy (TERS) within the first two days after production. Furthermore, we present a simple electrochemical method for recovering the enhancing effect of corroded silver tips, as well as for storing freshly prepared probes, for example, for easy shipment. The present work greatly simplifies the experimental aspects of near-field optical microscopy, which should make near-field optical techniques, and, in particular, TERS, more accessible to the scientific community.

CdSe single-nanoparticle based active tips for near-field optical microscopy

We present a method to realize active optical tips for use in near-field optics that canoperate at room temperature. A metal-coated optical tip is covered with a thin polymerlayer stained with CdSe nanocrystals or nanorods at low density. The time analysis of theemission rate of the active tips and the analysis of their emission spectra reveal that a verysmall number of particles—possibly down to only one—can be made active at the tip apex.This opens the way to optics with a single inorganic nanoparticle as a light source.

Efficient Raman Enhancement and Intermittent Light Emission Observed in Single Gold Nanocrystals

This paper reports two fundamental observations on the size-dependent optical properties of colloidal gold nanoparticles. First, faceted gold nanocrystals in the size range of 63 ± 3 nm have been found to be highly efficient for surface-enhanced Raman scattering (SERS). These nanocrystals are identified from a heterogeneous population for large optical enhancement at 647-nm laser excitation. Second, spatially isolated single gold particles emit Stokes-shifted Raman photons in an intermittent on-and-off fashion. In contrast to population-averaged studies, blinking surface-enhanced Raman scattering is a signature of single-particle (or even single-molecule) behavior. By directly measuring optical enhancement and time-resolved emission on single nanoparticles, this work opens new possibilities in studying the mechanisms of SERS, in developing metal tips for near-field optical microscopy, and in designing new nanostructured materials.

Chemically etched fiber tips for near-field optical microscopy: a process for smoother tips

An improved method for producing fiber tips for scanning near-field optical microscopy is presented. The improvement consists of chemically etching quartz optical fibers through their acrylate jacket. This new method is compared with the previous one in which bare fibers were etched. With the new process the meniscus formed by the acid along the fiber does not move during etching, leading to a much smoother surface of the tip cone. Subsequent metallization is thus improved, resulting in better coverage of the tip with an aluminum opaque layer. Our results show that leakage can be avoided along the cone, and light transmission through the tip is spatially limited to an optical aperture of a 100-nm dimension.