42 research outputs found
Examining the nanoworld using a molecular spectroscopist's toolbox
I will describe recent advances in understanding the influence of nanoscale structure on plasmon-mediated electron dynamics. Steady-state extinction spectra of plasmonic nanoparticle networks are accurately described using hybridization models reminiscent of molecular orbitals. We have extended these molecular-based descriptions to account for nanoparticle electron dynamics by quantifying the coherence dephasing times of collective inter-particle plasmon modes of single nanostructures. In particular, we demonstrate that interference between plasmon modes of different angular momenta leads to increased coherence times. These observations are consistent with a model based on superpositions of molecular-like electronic states. These fundamental studies are important for understanding the structure-photonic-function relationship of plasmonic nanoparticles. This is because the spectroscopically determined coherence times reflect mode quality factors, which determine achievable amplification factors of optical signals. These new insights are made possible by recent advances in single-nanoparticle/molecule spectroscopy based on interferometric nonlinear optical detection. I will describe how the generation of sequences of phase-locked femtosecond laser pulses (33mrad phase stability) and their integration to an optical microscope were critical for this research
Communication: SHG-detected circular dichroism imaging using orthogonal phase-locked laser pulses
Magnetic Dipolar Interactions in Solid Gold Nanosphere Dimers
We report the first observation of a magnetic dipolar
contribution
to the nonlinear optical (NLO) response of colloidal metal nanostructures.
Second-order NLO responses from several individual solid gold nanosphere
(SGN) dimers, which we prepared by a bottom-up approach, were examined
using polarization-resolved second harmonic generation (SHG) spectroscopy
at the single-particle level. Unambiguous circular dichroism in the
SH signal was observed for most of the dimeric colloids, indicating
that the plasmon field located within the interparticle gap was chiral.
Detailed analysis of the polarization line shapes of the SH intensities
obtained by continuous polarization variation suggested that the effect
resulted from strong magnetic-dipole contributions to the nanostructure’s
optical properties
Excitation Wavelength Dependence of Fluorescence Intermittency in CdSe/ZnS Core/Shell Quantum Dots
Linking On-State Memory and Distributed Kinetics in Single Nanocrystal Blinking
Memory effects in single nanocrystal fluorescence blinking
are investigated as a function of the on-state kinetics for CdSe/ZnS
quantum dots and CdSe nanorods. The on-state duration probability
distributions for single nanocrystal blinking traces are characterized
by an inverse power law, which crosses over to exponential decay for
long on-state durations. The correlations of subsequent on-state durations
(<i>R</i><sub>log,on</sub>) are found to decrease for nanocrystals
that display earlier crossover times and smaller power law coefficients.
Specifically, <i>R</i><sub>log,on</sub> increases from 0.14
± 0.02 to a saturation value of 0.44 ± 0.01 for nanocrystals
with average crossover times of ∼100 ms to more than 5.0 s,
respectively. The results represent the first link between memory
effects and blinking kinetics and are interpreted in the framework
of two competing charge trapping mechanisms. A slow fluctuation-based
trapping mechanism leads to power-law-distributed on durations and
significant memory effects; however, the additional contribution of
an ionization-induced trapping pathway is found to induce crossover
to exponential decay and decreased memory. Monte Carlo simulations
of nanocrystal blinking based on the two trapping mechanisms reproduce
the experimental results, suggesting that the power law component
and the memory effects correlate with a fluctuation-based mechanism.
This effect is found to be universal, occurring for two nanocrystal
morphologies and in blinking data measured using a wide range of continuous
and pulsed excitation conditions
Optical Properties and Electronic Energy Relaxation of Metallic Au<sub>144</sub>(SR)<sub>60</sub> Nanoclusters
Electronic energy relaxation of Au<sub>144</sub>(SR)<sub>60</sub><sup>q</sup> ligand-protected nanoclusters,
where SR = SC<sub>6</sub>H<sub>13</sub> and <i>q</i> = −1,
0, +1, and +2,
was examined using femtosecond time-resolved transient absorption
spectroscopy. The observed differential transient spectra contained
three distinct components: (1) transient bleaches at 525 and 600 nm,
(2) broad visible excited-state absorption (ESA), and (3) stimulated
emission (SE) at 670 nm. The bleach recovery kinetics depended upon
the excitation pulse energy and were thus attributed to electron–phonon
coupling typical of metallic nanostructures. The prominent bleach
at 525 nm was assigned to a core-localized plasmon resonance (CLPR).
ESA decay kinetics were oxidation-state dependent and could be described
using a metal-sphere charging model. The dynamics, emission energy,
and intensity of the SE peak exhibited dielectric-dependent responses
indicative of Superatom charge transfer states. On the basis of these
data, the Au<sub>144</sub>(SR)<sub>60</sub> system is the smallest-known
nanocluster to exhibit quantifiable electron dynamics and optical
properties characteristic of metals