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Nanoscale imaging of plasmonic hot spots and dark modes with the photothermal-induced resonance technique.

The collective oscillation of conduction electrons, responsible for the localized surface plasmon resonances, enables engineering nanomaterials by tuning their optical response from the visible to terahertz as a function of nanostructure size, shape, and environment. While theoretical calculations helped tremendously in understanding plasmonic nanomaterials and optimizing their light matter interaction, only a few experimental techniques are available to study these materials with high spatial resolution. In this work, the photothermal-induced resonance (PTIR) technique is applied for the first time to image the dark plasmonic resonance of gold asymmetric split ring resonators (A-SRRs) in the mid-infrared (IR) spectral region with nanoscale resolution. Additionally, the chemically specific PTIR signal is used to map the local absorption enhancement of poly(methyl methacrylate) coated on A-SRRs, revealing hot spots with local enhancement factors up to ≈30 at 100 nm lateral resolution. We argue that PTIR nanoscale characterization will facilitate the engineering and application of plasmonic nanomaterials for mid-IR applications.