Abstract
Metal–insulator–metal (MIM) nanogaps in the canonical nanoparticle-on-mirror geometry (NPoM) provide deep-subwavelength confinement of light with mode volumes smaller than V/Vλ < 10–6. However, access to these hotspots is limited by the impendence mismatch between the high in-plane k∥ of trapped light and free-space plane-waves, making the in- and out-coupling of light difficult. Here, by constructing a nanoparticle-on-foil (NPoF) system with thin metal films, we show the mixing of insulator–metal–insulator (IMI) modes and MIM gap modes results in MIMI modes. This mixing provides multichannel access to the plasmonic nanocavity through light incident from both sides of the metal film. The red-tuning and near-field strength of MIMI modes for thinner foils is measured experimentally with white-light scattering and surface-enhanced Raman scattering from individual NPoFs. We discuss further the utility of NPoF systems, since the geometry allows tightly confined light to be accessed simply through different ports.
Highlights
Metal−insulator−metal (MIM) nanogaps in the canonical nanoparticle-on-mirror geometry (NPoM) provide deep-subwavelength confinement of light with mode volumes smaller than V/Vλ < 10−6
In insulator− metal−insulator (IMI) systems, SPPs on either side of a metal film strongly hybridize when the metal thickness (t) is smaller than its skin depth (
A reliable, scalable, and robust construct for MIM geometries uses nanoparticle-on-mirror (NPoM) systems,[30,31] where a colloidally synthesized nanoparticle is placed on top of an atomically flat >100-nm-thick mirror, which is predeposited with a self-assembled monolayer of molecules
Summary
For the realization of single monolayers (SAMs), the Au-coated sample pieces are dipped in a 1 mM solution of biphenyl-4-thiol (BPT, Sigma-Aldrich, 97%). Nanoparticles of 80 nm in diameter (BBI Solutions) are deposited directly onto the BPT-treated Au surface. A spectrally filtered 632.8 nm diode laser (Matchbox, Integrated Optics) with 100 μm/μm[2] power on the sample and spectral line width of 0.1 pm is used as the excitation pump. For dark-field measurements, we used a halogen lamp to excite our samples. The reflected light is collected through the same objective and split to an imaging camera (Lumenera Infinity3−1) and a fiber-coupled spectrometer (Ocean Optics QEPRO) for dark-field spectroscopy. Underneath the BPT layer are different thicknesses of Au film placed above a thick SiO2 substrate. (2) Experimentally measured light scattering from air and glass slide from individual NPoF cavities in modified dual channel microscope.
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