Michael R. Beversluis

Plasmon‐coupled tip‐enhanced near‐field optical microscopy

Near the cut‐off radius of a guided waveguide mode of a metal‐coated glass fibre tip it is possible to couple radiation to surface plasmons propagating on the outside surface of the metal coating. These surface plasmons converge toward the apex of the tip and interfere constructively for particular polarization states of the initial waveguide mode. Calculations show that a radially polarized waveguide mode can create a strong field enhancement localized at the apex of the tip. The highly localized enhanced field forms a nanoscale optical near‐field source.

Characterization of nanoplasmonic structures by locally excited photoluminescence

A method is presented for the characterization of locally enhanced fields at laser-irradiated metal nanostructures. Excitation with 120 fs laser pulses gives rise to photoluminescence mediated by two-photon absorption. A metal tip used to locally scatter the photoluminescence renders a map of regions with high field strengths. Near-field photoluminescence images of particle clusters reveal the dipole nature of the electromagnetic field surrounding the particles. Spectra acquired with and without the presence of the tip show no significant shift of the surface plasmon resonance of the particle clusters, confirming that the tip acts as a passive probe.

Programmable vector point-spread function engineering

We use two nematic liquid crystal spatial light modulators (SLMs) to control the vector point spread function (VPSF) of a 1.3 numerical aperture (NA) microscope objective. This is achieved by controlling the polarization and relative phase of the electric field in the objectives pupil. We measure the resulting VPSFs for several different pupil field polarization states. By using single fluorescent molecules as local field probes, we are able to map out the focal field distributions and polarization purity of the synthesized fields. We report the achieved field purity and address the experimental issues that currently limit it.

Tip-enhanced optical spectroscopy

Spectroscopic methods with high spatial resolution are essential for understanding the physical and chemical properties of nanoscale materials including biological proteins, quantum structures and nanocomposite materials. In this paper, we describe microscopic techniques which rely on the enhanced electric field near a sharp, laser–irradiated metal tip. This confined light–source can be used for the excitation of various optical interactions such as two–photon excited fluorescence or Raman scattering. We study the properties of the enhanced fields and demonstrate fluorescence and Raman imaging with sub–20 nm resolution.

Near-field scattering of longitudinal fields

Longitudinal fields created in strongly focused laser beams are investigated by near-field optical microscopy. Sharp metallic and dielectric tips are raster scanned through the focus of these modes. It is found that regardless of the tip material, the signal scattered by the tip is a measure for the strength of the local longitudinal field. A surprising contrast reversal is observed between the images obtained with a metallic tip and the images obtained with a dielectric tip. The contrast reversal originates from a non-negligible tip–sample interaction.

Analysis of copper incorporation into zinc oxide nanowires.

ZnO nanowires (NWs) are grown on a bulk copper half-transmission electron microscopy grid by chemical vapor deposition in a high temperature tube furnace. Photoluminescence (PL) microscopy revealed band gap emission at 380 nm and a more intense visible emission around 520 nm due to defect states in these NWs. High-resolution transmission electron microscopy shows that the ZnO NWs are single crystalline with hexagonal structure. Auger electron spectroscopy (AES) and energy dispersive X-ray spectroscopy reveal that copper atoms are present along the length of the NW. AES also found that the surface of the NWs is oxygen rich. The surface concentration of zinc increases moving from the tip toward the base of the NW while the concentration of oxygen decreases. The copper in this system not only remains at the tip of the growing NW but also acts as a dopant along the length of the NW, leading to a decrease in the intensity of the band gap PL of these NWs.

Near-field optical spectroscopy with 20 nm spatial resolution

A near-field optical method is introduced that makes use of the strongly enhanced electric field close to a sharply pointed metal tip under laser illumination. The tip is held a few nanometers above the sample surface so that a highly localized interaction between the enhanced field and the sample is achieved. The method has been successfully combined with vibrational spectroscopy by making use of the well-known effect of surface enhanced Raman scattering (SERS). We mapped out the vibrational modes of individual single-walled carbon nanotubes (SWNT) with a resolution better than 20 nm. The technique has great potential for becoming a routine tool for the chemical analysis of surfaces at high spatial resolution.

Nanoscale optical spectroscopy and detection

We use a laser-irradiated metal tip to create a locally enhanced field at the tip apex. The tip acts as an optical antenna and is held a few nanometers above the sample surface so that a highly localized interaction between the enhanced field and the sample is achieved. The method has been successfully combined with vibrational spectroscopy by making use of the well-known effect of surface enhanced Raman scattering (SERS). We mapped out the vibrational modes of individual single-walled carbon nanotubes (SWNT) with a resolution down to l0nm.

Nearfield optical interactions and spectroscopy

The locally enhanced optical field near a laser-irradiated metal tip is used as a local photon source for spectroscopic studies on nanoscale materials. Resolutions better than 20nm are achieved on individual single-walled carbon nanotubes.