The collective oscillation of conduction electrons, responsible for the LSPRs, enables engineering nanomaterials by tuning their optical response from the visible to THz 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 Photo Thermal 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 polymethylmethacrylate (PMMA) 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.
Citation: Nano Letters
Pub Type: Journals
nanoscale imaging, hot spots, metamaterial, dark mode, PTIR, AFM