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Structural Examined Through the Combined Lenses of Ultrafast and Magnetooptical Spectroscopy



By: Ken Knappenberger
Associate Professor of Chemistry and Biochemistry
From: Florida State University
When: Wednesday, March 1, 2017
4:00 PM - 5:00 PM
Where: Dell Butcher Hall
Abstract: Plasmonic nanoparticle assemblies offer unique opportunities for controlling energy at the nanoscale. Here, we describe experimental outcomes in three key areas critical to understanding nanoscale-structure-specific light-matter interactions: 1) selective amplification of specific light polarization states; 2) structure-dependent plasmon coherence times; and 3) plasmon-mode-specific spatial localization of electromagnetic energy to nanoscale volumes. Interferometric single-particle second harmonic generation (SHG) and two-photon photoluminescence (TPPL) imaging techniques developed in our lab provide high spatial accuracy and precision along with femtosecond time resolution for examining nanoparticle assemblies. Femtosecond time-resolution is achieved by employing a sequence of phase-locked laser pulses to examine the nanostructures. These imaging methods have been employed to quantify plasmon coherence times for assemblies of nanospheres and nanorods. Determination of plasmon coherence times provides a quantitative measurement of mode-specific quality factors, which are important for assessing the efficiency of nanostructures for using electromagnetic energy. Based on our coherence data, one-dimensional nanorods are promising building blocks of nanoparticle networks for using electromagnetic energy at the nanoscale. The effectiveness of the nanorods results from the inherent length-to-diameter aspect-ratiodependent tunability of the longitudinal plasmon resonance (LSPR) frequency. The LSPR of high aspect ratio nanorods can be energetically decoupled from interband relaxation channels, which are a major source plasmon decoherence, thus explaining the experimental results. Indeed, preliminary interferometric nonlinear optical studies of nanorod trimers indicate the plasmon coherence time can be increased by approximately 100% for nanoparticle networks of specific symmetries.