Optical Imaging and Spectroscopy of Metal Nanostructures

Doctoral thesis, 2003

Noble metal structures with size in the sub 100 nm range exhibit extraordinary optical properties due to collective oscillations of conduction electrons, the surface plasmon resonances (SPR). The most remarkable feature of these structures is the ability to redistribute electromagnetic radiation and concentrate strong fields near the surface. The resonant scattering cross-section of nanoparticles exceeds their geometrical cross-section. Resulting field "amplification" has been exploited in surface enhanced spectroscopy techniques for many years. Future applications may include new biochemical sensors and high density data storage. This thesis describes various experimental studies of metal structures that support SPR. A near-field scanning optical microscope (NSOM) was used to probe the phase of the SPR coupling in colloidal nanoparticles. The phase of the complex particle polarizability changes rapidly near resonance and is shown to have a major impact on the contrast formation in NSOM. The NSOM was also used to perform surface-enhanced Raman spectroscopy (SERS) of Rhodamine 6G adsorbed on colloidal silver nanoparticles. The data showed that SERS spectra do not change under near-field illumination conditions and that only certain particle aggregates provide a high enhancement factor. These two studies highlight the importance of optical near-field interaction between metal structures. This phenomenon was further studied using nanofabricated Ag particle arrays. The data demonstrated a large shift in the particle SPR due to radiative interaction. Using dark-field (DF) optical microscopy, elastic light scattering by nanostructures can be studied with virtually no background. By comparing the scattering spectra of individual nanoparticles and nanoholes in thin films we found evidence for localized dipolar SPR in holes. However, the hole plasmons are greatly affected by delocalized surface plasmons in the thin gold film, as shown by NSOM data. Finally, by combining DF microscopy with optical tweezers, we could trap single colloidal silver nanoparticles in water, and use them as a local probes. We observed a SPR red-shift when two nanoparticles were brought in a near-field interaction, as expected from theory. We anticipate that the technique will lead to various novel applications in nanoparticle spectroscopy and sensing.