Alexandre Bouhelier

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.

Apertureless scanning near-field optical microscopy: a comparison between homodyne and heterodyne approaches

In coherent homodyne apertureless scanning near-field optical microscopy (ASNOM) the background field cannot be fully suppressed because of the interference between the different collected fields, making the images difficult to interpret. We show that implementing the heterodyne version of ASNOM allows one to overcome this issue. We present a comparison between homodyne and heterodyne ASNOM through near-field analysis of gold nanowells, integrated waveguides, and a single evanescent wave generated by total internal reflection. The heterodyne approach allows for the control of the interferometric effect with the background light. In particular, the undesirable background is shown to be replaced by a controlled reference field. As a result, near-field information undetectable by a homodyne ASNOM is extracted by use of the heterodyne approach. Additionally, it is shown that field amplitude and field phase can be detected separately.

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.

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.

Surface plasmon rainbow jets

A new method for optically exciting and visualizing surface plasmons in thin metal films is described. The technique relies on the use of a high-numerical-aperture objective lens to locally launch a broad wavelength spectrum of surface waves and to detect the leaky radiative modes associated with them. We used this approach to obtain a direct visualization of the plasmon intensity distributions, e.g., rainbow jets, and to quantify their propagation lengths throughout the visible spectrum.

Longitudinal anisotropy of the photoinduced molecular migration in azobenzene polymer films

The effects of tightly focused, higher-order laser beams on the photoinduced molecular migration and surface deformations in azobenzene polymer films are investigated. We demonstrate that the surface relief is principally triggered by longitudinal fields, i.e., electric fields polarized along the optical axis of the focused beam. Our findings can be explained by the translational diffusion of isomerized chromophores when the constraining effect of the polymer-air interface is considered.

Photoluminescence from sharp gold tips

The properties of photoluminescence from gold nanostructures is studied with one and two-photon excitation. By using a tip, the signal is studied spectroscopically under well-defined conditions.

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.

Plasmonic heterostructures for addressable nanophotonics

Plasmonics applications will benefit if reliable means to alter plasmon absorption and damping properties via external inputs are found. We are working towards this goal by functionalizing noble metal films with polarizable, excitonic molecular films. Examples include molecular j-aggregates, whose excitonic absorptions can be photobleached to modify plasmon absorption properties. We report two developments in this area. The first is the observation of coherent polarization coupling between the exciton of a molecular J-aggregate and the electronic polarization of noble metal nanoparticles. The second is a new far-field method to directly observe surface plasmon propagation, demonstrating that the lateral intensity decay length is affected by a change of the interface property. The method relies on the detection of the intrinsic lossy modes associated with plasmon propagation in thin films. We also uniquely introduce a method to excite a broad spectral distribution of surface plasmon simultaneously throughout the visible spectrum allowing surface plasmon based spectroscopy to be performed.