Department of Chemistry and The James Franck Institute, The University of Chicago, 929 East 57th Street, Chicago, Illinois 60637, USA.
ACS Nano. 2012 Jan 24;6(1):472-82. doi: 10.1021/nn203802e. Epub 2012 Jan 3.
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.
最近在化学合成方面的进展使得制造无大规模晶体缺陷和表面粗糙度的金银纳米线成为可能。表面等离激元可以沿着纳米线传播，使它们成为横截面远小于光波长的光学波导。与银纳米线相比，金纳米线具有更好的化学稳定性，但传播的等离激元的损耗也更高。为了描述这种权衡，我们测量了金和银纳米线中传播的等离激元的传播长度和群速度。通过等离子体近场的荧光成像来测量传播长度。群速度是从相干光在导线上传输的光谱中的条纹间距推断出来的。与之前的工作不同，我们将这些条纹解释为远场干涉效应的结果。测量的传播特性与数值模拟相符，表明这些导线上的传播主要由金属的材料特性决定，粗糙度或晶界的散射引起的额外损耗最多只能提供较小的贡献。传播长度和群速度也可以用一个简单的分析模型来描述，该模型仅考虑固体金属圆柱中的最低阶波导模式，表明在实际纳米线中，这种单模占主导地位。实验与理论的比较表明，广泛使用的介电函数表值可以很好地描述金纳米线中的等离激元，但严重高估了银纳米线中的等离激元损耗。
