We investigated the angular radiation patterns, a key characteristic of an emitting system, from individual silver nanowires decorated with rare earth ion-doped nanocrystals. Back focal plane radiation patterns of the nanocrystal photoluminescence after local two-photon excitation can be described by two emission channels: Excitation of propagating surface plasmons in the nanowire followed by leakage radiation and direct dipolar emission observed also in the absence of the nanowire. Theoretical modeling reproduces the observed radiation patterns which strongly depend on the position of excitation along the nanowire. Our analysis allows to estimate the branching ratio into both emission channels and to determine the diameter dependent surface plasmon quasimomentum, important parameters of emitter-plasmon structures. KeywordsPlasmonics; metallic nanowires; rare earth ions; back focal plane imaging; up-conversion Propagating surface plasmon polaritons (SPPs) are electromagnetic waves bound to a metaldielectric interface. They offer the distinguished possibility to concentrate light to subwavelength scales and transport this energy over a length several magnitudes larger. [1][2][3][4] These properties provide the basis for plasmonics, a very active research area aiming at optical device miniaturization and the integration of optics and electronics on a single chip. 5,6 Metallic nanowires (NWs) have drawn particular attention as plasmonic building blocks due to their successful implementation as waveguides, 7-9 routers and logic gates. 10-12 SPPs on metallic NWs have been investigated by direct visualization, 13 using a scanning aperture probe, 14 by electrical detection 15 as well as by calculations. 16 A key step in plasmonic applications of NWs is the coupling of the initial energy source to the NW and the contributing energy relaxation pathways. 17,18 Importantly, subwave-length light confinement by the SPPs can be used to enhance the interaction between objects and light. 19 This coupling and the excitation and propagation of SPPs have been experimentally visualized by leakage radiation microscopy 20-22 combined with imaging of the back focal plane (BFP) for a variety of plasmonic structures and devices. [23][24][25] In this manuscript we studied the coupling of the emission from rare earth doped nanocrystals to SPP modes in silver NWs on glass. We use the ability of these nanocrystals to exhibit stable, non-bleaching upconverted photoluminescence (PL) on the anti-Stokes side * To whom correspondence should be addressed achim.hartschuh@cup.uni-muenchen.de. Europe PMC Funders Group Europe PMC Funders Author ManuscriptsEurope PMC Funders Author Manuscripts of the laser energy 26 to avoid temporal intensity fluctuations and to exclude any background contribution from laser scattering, metal luminescence or the sample substrate. Importantly, since two-photon excitation requires high excitation densities it will only be efficient at the position of the laser focus. Two-photon excitation of NCs located outsid...
Ensemble and single-molecule spectroscopy demonstrates that both emission and absorption of peridinin−chlorophyll−protein photosynthetic antennae can be largely enhanced through plasmonic interactions. We find up to 18-fold increase of the chlorophyll fluorescence for complexes placed near a silver metal layer. This enhancement, which leaves no measurable effects on the protein structure, is observed when exciting either chlorophyll or carotenoid and is attributed predominantly to an increase of the excitation rate in the antenna. The enhancement mechanism comes from plasmon-induced amplification of electromagnetic fields inside the complex. This result is an important step toward applying plasmonic nanostructures for controlling the optical response of complex biomolecules and improving the design and functioning of artificial light-harvesting systems.Strong enhancement of electromagnetic fields generated through plasmon resonances in metal films and particles has recently stimulated a considerable interest in diverse research fields such as optical spectroscopy, cell imaging, quantum information processing, nanophotonics, and biosensors. [1][2][3][4][5] This versatility results from a dramatic influence that plasmons impose on the absorption and emission properties of nearby located dipoles, for example, semiconductor nanocrystals and nanowires 6-12 or dye molecules. [13][14][15][16][17][18] Optical response of an emitter coupled to a plasmonic structure depends upon spatial arrangement as well as spectral characteristics of a studied system. Remarkable progress has been made in on-demand design of metal nanostructures, which is essential for tuning the resonance frequency and thus the coupling strength. 13,14,19 Complementary efforts focused on developing advanced experiments to study dipoles placed in the vicinity of a metal nanoparticle have shed light on the interplay between radiative and nonradiative processes in these systems. 16,18 This very relation determines whether the fluorescence is enhanced [9][10][11]16 or quenched due to the dominating role of nonradiative energy transfer from the dipole to the metal. 15,18 Metal-enhanced fluorescence (MEF) has been observed for many hybrid systems that include nanocrystals on corrugated metal surfaces, 10,11 dye molecules coupled to metal nanoparticles, 18 and nanocrystal-nanoparticle bioconjugates. 8In all these cases, very stable and highly fluorescing emitters have been selected. It would be, however, highly desirable to apply MEF to weakly fluorescing systems such as DNA, 20 carbon nanotubes 21 or, yet experimentally unexplored in this context, light-harvesting complexes. These latter proteinpigment systems, which contain chlorophyll (Chl) and carotenoid molecules embedded in a protein matrix, participate in the photosynthesis process by collecting sunlight energy and transferring it to reactions centers. The presence of fluorescing Chls and the protein, separated by a few nanometers, renders light-harvesting complexes ideal for studying the prote...
The influence of plasmon excitations in spherical gold nanoparticles on the optical properties of a light-harvesting complex 2 (LH2) from the purple bacteria Rhodopseudomonas palustris has been studied. Systematic analysis is facilitated by controlling the thickness of a silica layer between Au nanoparticles and LH2 complexes. Fluorescence of LH2 complexes features substantial increase when these complexes are separated by 12 nm from the gold nanoparticles. At shorter distances, non-radiative quenching leads to a decrease of fluorescence emission. The enhancement of fluorescence originates predominantly from an increase of absorption of pigments comprising the LH2 complex.
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