Radiation Channels Close to a Plasmonic Nanowire Visualized by Back Focal Plane Imaging

Abstract

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...

Delocalization of Nonlinear Optical Responses in Plasmonic Nanoantennas

“…Excitation at the left extremity of the antennas systematically produces fringe patterns with an intensity increasing towards positive k x /k em . Such fringe pattern was already observed in the linear regime for nanowires decorated with emitters [40]. SHG Fourier images show a strict dependence on the rod length.…”

“…When the laser is focused on the left extremity, the TPL Fourier distribution [ Fig. 1(d)] features a pattern with two maxima aligned along the antennas x axis coinciding with the excitation polarization direction at variance with a single x-oriented dipole [40]. Vastly different is the SHG angular image shown in Fig.…”

Silver nanowires as receiving-radiating nanoantennas in plasmon-enhanced up-conversion processes

“…33 We conclude that the polarization contrast of the NaYF 4 :Er 3+ /Yb 3+ nanocrystal luminescence is controlled by the nanowire. We note that for uncoupled nanocrystals we find no polarization-sensitive response, which we interpret is due to the absence of a preferred orientation of the transition dipole moment in these nanostructures.…”

“…33,38 This technique allows to determine both the orientation of the dipolar emitters and the direction of plasmon propagation by analysing the angular distribution of emitted light in Fourier space. This facet of plasmon-mediated emission in coupled NCs-NW hybrids can be directly probed using back focal plane microscopy.…”

Comparative analysis of imaging configurations and objectives for Fourier microscopy

“…1, there are four commonly used configurations for BFP imaging. The first two simply place an achromatic doublet (Thorlabs AC254-050-A and AC254-100-A, respectively) either before [3][4][5] or after [7,27] the microscope's image plane as shown in Fig. 1(a) and Fig.…”