175 research outputs found
Localized heating in nanoscale Pt constrictions measured using blackbody radiation emission
Using thermal emission microscopy, we investigate heating in Pt nanowires
before and during electromigration. The wires are observed to reach
temperatures in excess of 1000 K. This is beyond the thermal decomposition
threshold for many organic molecules of interest for single molecule
measurements with electromigrated nanogaps. Blackbody spectra of the hot Pt
wires are measured and found to agree well with finite element modeling
simulations of the electrical and thermal transport.Comment: 4 pages, 3 figure
Coherent Fano resonances in a plasmonic nanocluster enhance optical four-wave mixing
Plasmonic nanoclusters, an ordered assembly of coupled metallic
nanoparticles, support unique spectral features known as Fano
resonances due to the coupling between their subradiant and
superradiant plasmon modes. Within the Fano resonance, absorption
is significantly enhanced, giving rise to highly localized, intense
near fields with the potential to enhance nonlinear optical
processes. Here, we report a structure supporting the coherent
oscillation of two distinct Fano resonances within an individual
plasmonic nanocluster. We show how this coherence enhances the
optical four-wave mixing process in comparison with other doubleresonant
plasmonic clusters that lack this property. A model that
explains the observed four-wave mixing features is proposed,
which is generally applicable to any third-order process in plasmonic
nanostructures. With a larger effective susceptibility χ (3) relative to
existing nonlinear optical materials, this coherent double-resonant
nanocluster offers a strategy for designing high-performance thirdorder
nonlinear optical media
Balancing near-field enhancement, absorption, and scattering for effective antenna-reactor plasmonic photocatalysis
Efficient photocatalysis requires multifunctional materials that absorb photons and generate energetic charge carriers at catalytic active sites to facilitate a desired chemical reaction. Antenna–reactor complexes are an emerging multifunctional photocatalytic structure where the strong, localized near field of the plasmonic metal nanoparticle (e.g., Ag) is coupled to the catalytic properties of the nonplasmonic metal nanoparticle (e.g., Pt) to enable chemical transformations. With an eye toward sustainable solar driven photocatalysis, we investigate how the structure of antenna–reactor complexes governs their photocatalytic activity in the light-limited regime, where all photons need to be effectively utilized. By synthesizing core@shell/satellite (Ag@SiO_2/Pt) antenna–reactor complexes with varying Ag nanoparticle diameters and performing photocatalytic CO oxidation, we observed plasmon-enhanced photocatalysis only for antenna–reactor complexes with antenna components of intermediate sizes (25 and 50 nm). Optimal photocatalytic performance was shown to be determined by a balance between maximized local field enhancements at the catalytically active Pt surface, minimized collective scattering of photons out of the catalyst bed by the complexes, and minimal light absorption in the Ag nanoparticle antenna. These results elucidate the critical aspects of local field enhancement, light scattering, and absorption in plasmonic photocatalyst design, especially under light-limited illumination conditions
Tunable optical tweezers for wavelength-dependent measurements
Optical trapping forces depend on the difference between the trap wavelength and the extinction
resonances of trapped particles. This leads to a wavelength-dependent trapping force, which should
allow for the optimization of optical tweezers systems, simply by choosing the best trapping wavelength
for a given application. Here we present an optical tweezer system with wavelength tunability,
for the study of resonance effects. With this system, the optical trap stiffness is measured for single
trapped particles that exhibit either single or multiple extinction resonances. We include discussions
of wavelength-dependent effects, such as changes in temperature, and how to measure them
Chiral Surface Plasmon Polaritons on Metallic Nanowires
Chiral surface plasmon polaritons (SPPs) can be generated by linearly
polarized light incident at the end of a nanowire, exciting a coherent
superposition of three specific nanowire waveguide modes. Images of chiral SPPs
on individual nanowires obtained from quantum dot fluorescence excited by the
SPP evanescent field reveal the chirality predicted in our theoretical model.
The handedness and spatial extent of the helical periods of the chiral SPPs
depend on the input polarization angle and nanowire diameter as well as the
dielectric environment. Chirality is preserved in the free-space output wave,
making a metallic nanowire a broad bandwidth subwavelength source of circular
polarized photons.Comment: 4 figure
Monitoring Chemical Reactions with Terahertz Rotational Spectroscopy
Rotational spectroscopy is introduced as a new in situ method for monitoring gas-phase reactants and products during chemical reactions. Exploiting its unambiguous molecular recognition specificity and extraordinary detection sensitivity, rotational spectroscopy at terahertz frequencies was used to monitor the decomposition of carbonyl sulfide (OCS) over an aluminum nanocrystal (AlNC) plasmonic photocatalyst. The intrinsic surface oxide on AlNCs is discovered to have a large number of strongly basic sites that are effective for mediating OCS decomposition. Spectroscopic monitoring revealed two different photothermal decomposition pathways for OCS, depending on the absence or presence of H_2O. The strength of rotational spectroscopy is witnessed through its ability to detect and distinguish isotopologues of the same mass from an unlabeled OCS precursor at concentrations of <1 nanomolar or partial pressures of <10 μTorr. These attributes recommend rotational spectroscopy as a compelling alternative for monitoring gas-phase chemical reactants and products in real time
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