In metallic nanoparticles (NPs), collective oscillation of conduction electrons can be excited upon light illumination. This is known as the excitation of localized surface plasmons (LSPs), giving rise to the concentration of incident light into deep subwavelength volumes. As the result, the electromagnetic (EM) fields can be significantly enhanced, enabling a number of significant applications, e.g. fluorescence enhancement and control, surface enhanced Raman scattering, optical sensing, and photo-thermal conversion. If placed in the proximity to a metallic film, the optical properties of NPs can be greatly altered and can be highly dependent on the distance between NPs and the film.
Here we discuss two types of film-coupled NPs resonators and their applications. The first one comprises a gold nanocube and a gold film with an extremely small gap spacing (<3 nm). When the nano-gap is doped with dye molecules, the gap resonances can strongly couple with the dye molecules, inducing not only spectral but also profound spatial modification to the gap modes. Specifically within the spectral span of a single gap mode in a nanotube-film cavity with a 3-nm wide gap, the introduction of narrowband J-aggregate dye molecules enables an anti-crossing behavior in the spectral response, splitting the single mode into two distinct spatial modes that are easily identified by their far-field scattering profiles. This provides us significant insights into the nature of coherent light-matter interaction at the nanometre scale.
The second structure acquires a similar configuration to the first one, comprising gold NPs separated from a gold mirror with a polymer spacer. An increase in relative humidity (RH) causes the spacer to expand, which induces a significant reduction of nanoparticle scattering intensity, as the scattering is highly dependent on the cavity-nanoparticle coupling that closely relates to the nanoparticle-mirror distance. This lithography-free structure enables a remarkable averaging sensitivity at 0.12 dB/% RH and 0.25 dB/% RH over RH range (45-75%), possessing an estimated resolution better than 0.5% RH with full reversibility and almost zero-hysteresis, exhibiting notable gas sensing potentials.