Surface Plasmon Photonics: From Optical Properties of Nanoparticles to Single Molecule Surface-enhanced Raman Scattering

Abstract: With the rapid development of nanoscience and nanotechnology, surface plasmon photonics using metal nanoparticles and nanostructures gain increasing interest, not only for fundamental scientific studies, but also for electronic, optical, and sensor applications. In the nanoscale, the physical and chemical properties of metallic particles, especially their optical properties, strongly depend on size and shape, as well as on surrounding media and structures. By modifying those features, one may design novel functional materials and devices. This thesis deals with the optical properties of silver and gold nanoparticles, focusing on applications and theory of surface-enhanced Raman scattering (SERS).

Localized surface plasmon resonances (SPR) determine the main optical properties of Ag and Au nanoparticles in the visible. Under certain conditions, the SPR can give rise to single molecule sensitivity in SERS. The experimental observation of dimer structures, where two Ag particles are bridged by a single hemoglobin molecule, probably reveal the simplest nanoparticle system that can amplify Raman scattering to the extent that vibrational spectra of single molecules can be recorded. Weaker, but more controllable, interparticle coupling effects are observed for nanofabricated Ag particles on Si. These observations may lead to important applications in the life sciences. Ag-particles are also investigated experimentally by scanning near-field microscopy. It is shown that this technique yields information on the optical phase-shift associated with SPR in individual nanoparticles.

The generalized Mie theory (GMT) has been used to analyze the mechanisms of single molecule SERS. It is shown that GMT can well explain the huge enhancement factor, as well as the polarization dependence, observed in this kind of experiments. This indicates that the electromagnetic enhancement mechanism is the main contributor to SERS. GMT is also used to theoretically quantify optical forces associated with Ag nanoparticles. The results indicate that molecules can be trapped and particle aggregates deformed by the optical forces induced at SPR excitation, even for moderate illumination intensities. The theory has also been used to investigate so-called SPR biosensors based on nanoparticles. It is shown that GMT accurately describes the experimentally observed variation in SPR with surrounding medium and particle size.