Abstract
Nanoporous metallic networks are endowed with the distinctive optical properties of strong field enhancement and spatial localization, raising the necessity to map the optical eigenmodes with high spatial resolution. In this work, we used cathodoluminescence (CL) to map the local electric fields of a three-dimensional (3D) silver network made of nanosized ligaments and holes over a broad spectral range. A multitude of neighboring hotspots at different frequencies and intensities are observed at subwavelength distances over the network. In contrast to well-defined plasmonic structures, the hotspots do not necessarily correlate with the network morphology, emphasizing the complexity and energy dissipation through the network. In addition, we show that the inherent connectivity of the networked structure plays a key optical role because a ligament with a single connected linker shows localized modes whereas an octopus-like ligament with multiple connections permits energy propagation through the network.
Highlights
IntroductionNanoporous metallic networks constitute a new class of advanced materials comprising multimodal nanosized blobs connected to each other to form a disordered 3D structure
Nanoporous metallic networks constitute a new class of advanced materials comprising multimodal nanosized blobs connected to each other to form a disordered 3D structure.Their distinct structural properties, including high surface-tovolume ratio, ability to host guest materials, pure solid connectivity, and high surface curvature, endow them with optical properties that can be found neither in conventional plasmonic structures nor in bulk metals.[1−14]It has been previously shown by us[15−17] and others[18−33] that this class of connected metallic nanomaterials (two-dimensional (2D) networks as well) exhibit strong local fields over a broad optical range
The associated strong electric fields that exist in these hotspots lead to exceptional enhancement of the nonlinear optical properties of the network[13,16,20−23,33−38] and even to the occurrence of photochemical processes and reactions inside and on the network.[1,2,15,39−41] Both phenomena are outcomes of the high local density of optical states (LDOS) expected on the basis of the strong field enhancement and spatial localization,[18,31,38,42−44] raising the necessity to map the optical eigenmodes with high spatial resolution
Summary
Nanoporous metallic networks constitute a new class of advanced materials comprising multimodal nanosized blobs connected to each other to form a disordered 3D structure Their distinct structural properties, including high surface-tovolume ratio, ability to host guest materials, pure solid connectivity, and high surface curvature, endow them with optical properties that can be found neither in conventional (well-defined) plasmonic structures nor in bulk metals.[1−14]. It has been previously shown by us[15−17] and others[18−33] that this class of connected metallic nanomaterials (two-dimensional (2D) networks as well) exhibit strong local fields over a broad optical range. Diffraction-limited far-field optical measurements are not adapted for characterizing these local effects, as field fluctuations and spectral resonances are averaged and result in a broad spectral response.[4,15,45−47] Even near-field techniques, like near-field optical microscopy, are limited in their ability to retrieve spatial confinement in these highly topographic networks at a broad range of frequencies.[45,46,48]
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