Department of Physics & Astronomy, The University of Utah, Salt Lake City, UT 84112, USA.
Sci Rep. 2013;3:2090. doi: 10.1038/srep02090.
The plasmonic resonances of nanostructured silver films produce exceptional surface enhancement, enabling reproducible single-molecule Raman scattering measurements. Supporting a broad range of plasmonic resonances, these disordered systems are difficult to investigate with conventional far-field spectroscopy. Here, we use nonlinear excitation spectroscopy and polarization anisotropy of single optical hot spots of supercontinuum generation to track the transformation of these plasmon modes as the mesoscopic structure is tuned from a film of discrete nanoparticles to a semicontinuous layer of aggregated particles. We demonstrate how hot spot formation from diffractively-coupled nanoparticles with broad spectral resonances transitions to that from spatially delocalized surface plasmon excitations, exhibiting multiple excitation resonances as narrow as 13 meV. Photon-localization microscopy reveals that the delocalized plasmons are capable of focusing multiple narrow radiation bands over a broadband range to the same spatial region within 6 nm, underscoring the existence of novel plasmonic nanoresonators embedded in highly disordered systems.
纳米结构银膜的等离子体共振产生了非凡的表面增强效应，使可重复的单分子拉曼散射测量成为可能。这些无序系统支持广泛的等离子体共振，因此难以用传统的远场光谱进行研究。在这里，我们使用非线性激发光谱和超连续产生中单光学热点的偏振各向异性来跟踪这些等离子体模式的转变，随着介观结构从离散纳米颗粒的薄膜调谐到聚集颗粒的半连续层。我们展示了具有宽光谱共振的衍射耦合纳米颗粒的热点形成如何转变为空间非局域表面等离子体激发，表现出多达 13 毫电子伏特的多个激发共振。光子局域化显微镜揭示了非局域等离子体能够将多个窄辐射带聚焦在同一空间区域内，带宽可达 6nm，这突显了在高度无序系统中存在新型等离子体纳米谐振器。
