Our group has made several recent advances using single-particle nonlinear optical (NLO) microscopy techniques to examine nanostructure-specific plasmon-mediated interactions with electromagnetic energy. These efforts are directed toward the larger goal of optimizing the structure of photonic nanoparticle assemblies for the use and control of energy at the nanoscale. By combining statistical localization methods with second harmonic generation (SHG) imaging, nonlinear signal hot spots can be located within a plasmonic network with nanometer spatial accuracy. This experimental capability was applied to study electromagnetic lensing effects in plasmonic nanoparticle assemblies, with the efficiency of cascaded energy transfer through the network being determined by the resonantly excited plasmon mode. The polarization and time dependencies of the NLO signals were examined using a sequence of phase-locked broad bandwidth femtosecond laser pulses. The methodologies for generating, characterizing, and incorporating these laser pulses into an optical microscope are described. Attosecond control over the temporal separation between two phase-locked laser pulses projected onto orthogonal planes allowed for the generation of a multitude of excitation polarization states; interpulse time delays gave rise to phase shifts ranging from 33 mrad to 2π radians. Left and right circularly polarized light was generated by inducing an interpulse time delay of positive or negative 667 attoseconds (800 nm carrier wave), respectively. These pulse sequences were applied to quantify NLO circular dichroism (CD) signals and to study chiro-optical plasmon amplification in nanostructures predicted to be CD active based on interparticle mode interferences. In order to investigate the time dependence of the NLO responses, plasmon-mediated SHG and two-photon photoluminescence (TPPL) signals were acquired using spectral interferometric detection by projecting both pulse replicas onto a common plane. The SHG signals resulting from plasmon-resonant and nonresonant excitation were analyzed to quantify electronic relaxation rates in metal nanoparticle assemblies. Analysis of TPPL signals revealed structure-specific emission phase shifts reflecting mode-dependent electronic relaxation dynamics. In summary, these results highlight the advantages of NLO imaging with phase-locked femtosecond laser pulses, yielding methodologies for single-structure examination with high spatial accuracy and temporal resolution. NLO signals are inherently plasmon-mode specific, making these methods highly attractive for probing the interplay between nanoscale structure and photonic properties.
All Science Journal Classification (ASJC) codes
- Electronic, Optical and Magnetic Materials
- Physical and Theoretical Chemistry
- Surfaces, Coatings and Films