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1. Evolution of near- and far-field optical properties of Au bipyramids upon epitaxial deposition of Ag

   https://doi.org/10.1039/D0NR00330A  

   Xi, Min; Reinhard, Björn M. (March 2020, Nanoscale)

   Bimetallic plasmonic nanostructures provide composition and spatial distribution of the individual components in the nanostructure in addition to overall size and morphology as degrees of freedom for tuning near- and far-field optical responses. AgAuAg nanorods (NRs) generated through epitaxial deposition of Ag on the tips of Au bipyramids (BPs) are an important bimetallic model system whose longitudinal dipolar plasmon mode first shows a spectral blue-shift upon initial deposition of Ag on the Au BP tips followed by a red-shift after additional deposition of Ag. Here, we quantify the relative contributions from morphological and compositional effects to the far-field spectral shift of the longitudinal and vertical dipolar plasmon modes during the initial deposition of Ag and compare the near-field in Ag and AgAuAg NRs with lengths between L = 130 nm–280 nm under whitelight illumination through electromagnetic simulations. Subsequently, we experimentally characterize the near-field around AgAuAg NRs with lengths between L = 88.1–749.0 nm at a constant excitation wavelength of 1064 nm on a silicon (Si) support through scattering type near-field scanning microscopy (sNSOM). We detect Fabry–Perot resonance-like higher order multipolar plasmon resonances whose order and near-field pattern depends on the length and composition of the NRs as well as the refractive index of the ambient medium. We find that under oblique illumination higher order multipolar modes with an even symmetry dominate on the high refractive index Si substrate due to strong electromagnetic interactions between the NR and the substrate. 

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2. Plasmon-Coupled Gold Nanoparticles in Stretched Shape-Memory Polymers for Mechanical/Thermal Sensing

   https://doi.org/10.1021/acsanm.1c00309  

   Yadav, Prachi R.; Rizvi, Mehedi H.; Kuttich, Björn; Mishra, Sumeet R.; Chapman, Brian S.; Lynch, Brian B.; Kraus, Tobias; Oldenburg, Amy L.; Tracy, Joseph B. (March 2021, ACS Applied Nano Materials)

   null (Ed.)

   The organization of plasmonic nanoparticles (NPs) determines the strength and polarization dependence of coupling of their surface plasmons. In this study, plasmon coupling of spherical Au NPs with an average diameter of 15 nm was investigated in shape-memory polymer films before and after mechanical stretching and then after thermally driving shape recovery. Clusters of Au NPs form when preparing the films that exhibit strong plasmon coupling. During stretching, a significant polarization-dependent response develops, where the optical extinction maximum corresponding to the surface plasmon resonance is redshifted by 19 nm and blueshifted by 7 nm for polarization parallel and perpendicular to the stretching direction, respectively. This result can be explained by non-uniform stretching on the nanoscale, where plasmon coupling increases parallel to the shear direction as Au NPs are pulled into each other during stretching. The polarization dependence vanishes after shape recovery, and structural characterization confirms the return of isotropy consistent with complete nanoscale recovery of the initial arrangement of Au NPs. Simulations of the polarized optical responses of Au NP dimers at different interparticle spacings establish a plasmon ruler for estimating the average interparticle spacings within the experimental samples. An investigation of the temperature-dependent recovery behavior demonstrates an application of these materials as optical thermal history sensors. 

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3. Plasmon-Induced Electron Transfer between Gold Nanorods and a Carbon Thin Film

   https://doi.org/10.1021/acs.jpcc.3c07754  

   Vo, Tamie; Chang, Wei-Shun (January 2024, The Journal of Physical Chemistry C)

   Plasmonic nanostructures have been demonstrated as emergent photocatalysts because of their efficient photon absorption and their ability to produce hot carriers. However, the plasmon-generated hot carriers decay through ultrafast relaxation pathways, resulting in a short lifetime that impedes the exploitation of hot carriers for chemical reactions. Charge separation at the heterojunction of the hybrid nanostructures can counteract the ultrafast decay to extend the carrier lifetime. Here, we fabricate hybrid nanostructures composed of gold nanorods and a carbon thin film and demonstrate efficient charge transfer between these two materials. Using single-particle dark-field scattering spectroscopy, we observe a broadening of the longitudinal plasmon for gold nanorods on a carbon film compared to those on a glass substrate. We attribute this plasmon damping to the electron transfer from gold nanorods to the carbon film and exclude the contribution from plasmon-induced resonance energy transfer. The electron transfer efficiencies are calculated as 52.8 ± 4.8 and 57.4 ± 4.0% for carbon films with thicknesses of 10 and 25 nm, respectively. This work demonstrates efficient charge separation at the gold–carbon film interface, which can extend the lifetime of hot carriers to promote plasmonic photocatalysts. 

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4. Efficient TE-polarized mode coupling between a plasmonic tunnel junction and a photonic waveguide

   https://doi.org/10.1364/OE.543072  

   Upcraft, Daniel; Vaz, Daniel; Youngblood, Nathan; Oh, Sang-Hyun (December 2024, Optics Express)

   Nanoscale plasmonic gaps are useful structures both electrically, for creating quantum tunnel junctions, and optically, for confining light. Inelastic tunneling of electrons in a tunnel junction is an attractive source of light due to the ultrafast response rate granted by the tunneling time of electrons in the system as well as the compact dimensions. A main hurdle for these light emitting tunnel junctions, however, is their low external efficiency given by both low electron-to-plasmon conversion as well as low plasmon-to-photon conversion. Inversely, coupling light into a nanogap for high confinement and field enhancement can be difficult due to the size mismatches involved. We show a 3 nm gap metal-insulator-metal plasmonic tunnel junction evanescently coupled to the fundamental TE mode of a standard silicon waveguide in a tapered directional coupler configuration with a transmission efficiency of 54.8% atλ =1.55μm and a 3-dB coupling bandwidth of 705 nm. In the inverse configuration, we show an electric field enhancement of |E|/|E₀| ≈120 within a plasmonic tunnel junction in the technologically important optical telecommunications band. 

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5. Understanding plasmon coupling in nanoparticle dimers using molecular orbitals and configuration interaction

   https://doi.org/10.1039/C9CP03890F  

   Alkan, Fahri; Aikens, Christine M. (October 2019, Physical Chemistry Chemical Physics)

   null (Ed.)

   We perform a theoretical investigation of the electronic structure and optical properties of atomic nanowire and nanorod dimers using DFT and TDDFT. In both systems at separation distances larger than 0.75 nm, optical spectra show a single feature that resembles the bonding dipole plasmon (BDP) mode. A configuration interaction (CI) analysis shows that the BDP mode arises from constructive coupling of transitions, whereas the destructive coupling does not produce significant oscillator strength for such separation distances. At shorter separation distances, both constructive and destructive coupling produce oscillator strength due to wave-function overlap, which results in multiple features in the calculated spectra. Our analysis shows that a charge-transfer plasmon (CTP) mode arises from destructive coupling of transitions, whereas the BDP results from constructive coupling of the same transitions at shorter separation distances. Furthermore, the coupling elements between these transitions are shown to depend heavily on the amount of exact Hartree–Fock exchange (HFX) in the functional, which affects the splitting of CTP and BDP modes. With 50% HFX or more, the CTP and BDP modes mainly merge into a single feature in the spectra. These findings suggest that the effects of exact exchange must be assessed during the prediction of CTP modes in plasmonic systems. 

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