2013 Volume 86 Issue 4 Pages 397-413
Studies on physical and chemical properties of noble metal nanostructures, mainly by near-field optical microscopy and spectroscopy, are described. Near-field optical microscopy provides optical observation methodology with a spatial resolution in nanometer regime beyond the diffraction limit of light. We developed near-field imaging systems equipped with various light sources including ultrashort pulsed lasers, which enables advanced nonlinear and ultrafast near-field measurements as well as conventional near-field imaging. In particular, near-field two-photon excitation imaging was shown to provide a convenient tool to visualize the enhanced optical fields in the vicinities of metal nanostructures. With these methods we demonstrated that nanoscale optical field structures in metal nanostructures can be directly visualized. For single noble metal nanoparticles, such as gold nanorods as typical examples, plasmon standing wave functions were visualized. Ultrafast imaging revealed that sub-picosecond relaxation was reflected on the plasmon wave function images through thermally induced dielectric function changes of the metal. In some cases optical field distribution features arising from the lightning rod effects were observed, depending on the resonance conditions of the incident wavelengths with the plasmon modes. We also found anomalous near-field transmission phenomenon for nanoapertures blocked by nanodisks near the plasmon resonance wavelengths, which arise from the efficient near-field to propagating-field conversion ability of the nanodisks. In assembled nanoparticles, enhanced optical fields at the gap sites between the particles were visualized, which elucidates experimentally the mechanism of surface-enhanced Raman scattering. The characteristic field distributions in many particle assemblies were also observed and analyzed. Through these studies, we established a valuable methodology to investigate optical and spectroscopic properties of metal nanostructures, and the information obtained is available only by optical measurements with high spatial resolution.
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