Propagation Lengths and Group Velocities of Plasmons in Chemically Synthesized Gold and Silver Nanowires

Abstract

Recent advances in chemical synthesis have made it possible to produce gold and silver nanowires that are free of large-scale crystalline defects and surface roughness. Surface plasmons can propagate along the wires, allowing them to serve as optical waveguides with cross sections much smaller than the optical wavelength. Gold nanowires provide improved chemical stability as compared to silver nanowires, but at the cost of higher losses for the propagating plasmons. In order to characterize this trade-off, we measured the propagation length and group velocity of plasmons in both gold and silver nanowires. Propagation lengths are measured by fluorescence imaging of the plasmonic near fields. Group velocities are deduced from the spacing of fringes in the spectrum of coherent light transmitted by the wires. In contrast to previous work, we interpret these fringes as arising from a far-field interference effect. The measured propagation characteristics agree with numerical simulations, indicating that propagation in these wires is dominated by the material properties of the metals, with additional losses due to scattering from roughness or grain boundaries providing at most a minor contribution. The propagation lengths and group velocities can also be described by a simple analytical model that considers only the lowest-order waveguide mode in a solid metal cylinder, showing that this single mode dominates in real nanowires. Comparison between experiments and theory indicates that widely used tabulated values for dielectric functions provide a good description of plasmons in gold nanowires but significantly overestimate plasmon losses in silver nanowires.

Dye-Assisted Gain of Strongly Confined Surface Plasmon Polaritons in Silver Nanowires

“…17 Complete loss compensation and net output amplification of SPPs in strip waveguides of gold have been realized via an optical gain medium by De Leon and Berini, 19 and more recently by Kena-Cohen et al 20 However, to date there exists no demonstration of optical gain for subwavelength-confined SPPs like those in metallic NWs, despite the fact that plasmon waveguiding in 1D silver and gold NWs is well documented. [8][9][10][11][12][13][26][27][28] Chemically synthesized metal NWs with lateral dimensions of ~100 nm have emerged as an important class of plasmonic waveguides [8][9][10][11]27,28 supporting strongly confined SPPs. SPP propagation lengths in NWs are significantly enhanced compared to polycrystalline waveguides prepared lithographically because of reduced defect scattering and leakage radiation.…”

“…12,29 Despite reduced radiative losses, SPPs in chemically prepared NWs still suffer from absorptive losses (on the order of -0.4 dB µm -1 at 800 nm) because of strong confinement. 27 These losses represent an impediment toward the realization of integrated plasmonic circuitry. In this study, we demonstrate for the first time SPP gain in chemically prepared Ag NWs that support SPPs with a mode area of only λ 2 /40.…”

Propagation length enhancement of surface plasmon polaritons in gold nano-/micro-waveguides by the interference with photonic modes in the surrounding active dielectrics

“…In previous studies, the characterization of the LR-SPP propagation was accomplished through light scattered from the surface of the waveguide [17,18,[24][25][26][27][28][29][30][31][32], where Lp was indirectly estimated in silver nanowires [25][26][27], silver and gold planar films [18,19], and gold stripes of different lengths [31,32] by fitting the scattered light measured along the whole waveguides through exponential decays. Other more sophisticated experimental techniques have been proposed in literature, such as decoupling the LR-SPP by either prisms [16] or diffraction gratings [15,22] or studying the evanescent field along the surface of the waveguide by near-field microscopes [19,20,33].…”

“…It is worth mentioning that the analysis of the propagation length of the LR-SPP at visible wavelengths has been mainly carried out in silver nanostructures [24][25][26][27][28][29][30], because of the smaller losses of silver in this wavelength range. The propagation of LR-SPPs along gold waveguides is commonly studied at infrared wavelengths [31,32], and only a few works are found visible [22].…”

“…However, given the high attenuation introduced by metals, the estimation of Lp is usually carried out by indirectly measuring the scattering [17,18,[24][25][26][27][28][29][30][31][32] or by decoupling the SPP through additional optical components such as prisms [16], diffraction gratings [15,22], or more complicated systems as near-field scanning optical microscopes [19,20,33].…”