Nanoscale structure-specific plasmonics

Abstract: Recent breakthroughs in two areas, resulting from my current postdoctoral research, will be presented: 1)controlled interparticle plasmon resonances in hollow gold nanospheres (HGNs) and 2) structure-specificdetermination of nanoscale optical properties using nonlinear single-particle spectroscopy. The broaderimpacts of this research include the development of a predictive understanding of the structure specificityof nanoscale optical properties (surface plasmon resonance frequencies and electromagnetic fields) andthe development of new experimental techniques that are capable of providing previously inaccessiblechemical information. In the first part of my presentation, structure-specific plasmon resonances of HGN dimers will bediscussed. In the case of HGNs, an unexpected, but systematic, blue shift of the spectral position of thesurface plasmon resonance was observed upon nanoparticle dimerization, an effect not observed for solidnanospheres. Finite-Difference Time-Domain calculations and high-resolution TEM demonstrate that thisblue shift results from interparticle plasmon coupling. I will show that the collective plasmonic propertiesof coupled HGNs can be systematically controlled by tailoring the interplay between the structure of theisolated HGN building blocks and the resultant interfacial structure of the dimer. In the second part of the talk, I will describe the development of a novel single-particle nonlinear optical(NLO) microscope, which enables the study of excitation-induced electromagnetic surface fields inplasmonic nanostructures. Specifically, interfacial electromagnetic fields of single gold nanospheredimers were probed using second harmonic generation (SHG) spectroscopy. SHG is a second-order NLOprocess, which is sensitive to local electromagnetic fields. The interfacial fields (located within the inter-particle gap) were found to be asymmetric and structure dependent. Unambiguous circular dichroism(CD) was observed in the SH signal obtained from gold nanosphere dimers that were formed by a bottom-up assembly, indicating that the interfacial plasmon field was chiral. The origin of this effect was probedby continuous-polarization-variation- SHG measurements. Detailed multipolar analysis of the polarizationline shapes of the SH intensities revealed that the CD resulted from strong magnetic-dipole contributionsto the nanostructure’s interparticle plasmon coupling. This is the first-ever observation of magnetic-dipolecontributions to the plasmon coupling of colloidal nanostructures in the optical frequency domain.Detection of these magnetic-dipole contributions was made possible only by the development and use ofadvanced single-particle spectroscopy techniques; these data would not have been resolvable usingensemble averaging methods. I will conclude with a brief outline of my future research plans.