Evolution of Collective, Radiative Plasmon Coupling in Confined, Soft Colloidal Films

Abstract Plasmon resonance coupling is a strongly distance‐dependent phenomenon that also critically depends on the spatial arrangement of its plasmonic constituents, for example, plasmonic nanoparticles in periodic or random assemblies. Here, we report electromagnetic coupling in ordered monolayers of gold nanoparticles. The interparticle distances range from two to almost six times the particle diameter. This is achieved by continuously monitoring the optical response of a soft colloidal monolayer film that is confined at the air/water interface and compressed through the barriers of a Langmuir trough. The colloidal building blocks contain monodisperse, spherical gold nanoparticles that are homogeneously encapsulated in soft, deformable microgel shells. The soft shell enables continuous tuning of the interparticle distance. This work directly compares the optical response measured in situ at the fluid interface to the response of monolayers that are transferred to glass substrates. Supported by COMSOL simulations and calculations using the coupled dipole approximation this work observes plasmonic surface lattice resonances for large spacings at the fluid interface. Reducing the interparticle distance leads to increased damping and broadening of the coupled resonances. These results demonstrate the feasibility of engineering 2D plasmonic structures with a wide range of optical properties via mechanical manipulation of soft colloidal films.