Plasmonic Surface Lattice Resonances at the Strong Coupling Regime

Aaro Ilmari Väkeväinen COMP Centre of Excellence, Department of Applied Physics, Aalto University, Espoo, Finland Robert Jan Moerland COMP Centre of Excellence, Department of Applied Physics, Aalto University, Espoo, Finland Heikki Tapani Rekola COMP Centre of Excellence, Department of Applied Physics, Aalto University, Espoo, Finland Antti-Pekka Eskelinen COMP Centre of Excellence, Department of Applied Physics, Aalto University, Espoo, Finland Jani-Petri Martikainen COMP Centre of Excellence, Department of Applied Physics, Aalto University, Espoo, Finland Dong-Hee Kim COMP Centre of Excellence, Department of Applied Physics, Aalto University, Espoo, Finland Päivi Törmä COMP Centre of Excellence, Department of Applied Physics, Aalto University, Espoo, Finland

We show strong coupling in plasmonic nanoarrays involving three different types of resonances: surface lattice resonances (SLRs), localized surface plasmon resonances (LSPRs) and excitations of organic molecules, Rhodamine 6G in PMMA (R6G) [1]. The nanoarrays consist of cylindrical silver nanoparticles arranged in a regular square lattice. The transmission/extinction spectra of the arrays are measured, both without and with the R6G layer on top. We systematically vary the lattice spacing, h, in order to scan the first-order SLR through the LSPR frequency. The dispersions of the extinction spectra display anti-crossings and bending of the modes, typical for the strong coupling regime (see dispersion diagrams in figures (a) and (b), without and with R6G, respectively). Furthermore, the splittings of the extinction spectra depend on the molecule concentration. Here, k is defined as the reciprocal lattice wave vector: k = 2π / h.

Dispersion diagrams of the nanoarrays: (a) without and (b) with the R6G layer on top.

The experimental findings are analysed using finite-difference time-domain simulations, a coupled-dipole approximation, coupled-modes models, and Fano theory. The combination of the nanoparticle arrays with R6G molecules shows strong coupling between the molecules and the various modes of the arrays – also collective ones. The delocalized nature of the collective SLR modes suggests that in the strong coupling regime molecules near distant nanoparticles are coherently coupled. Our results pave the way for exploiting the combination of long lifetimes of the collective modes and the strong coupling to quantum emitters provided by plasmonic nanoparticles. This collective-mode plasmonic strong-coupling regime has potential for the creation of coherent light emission where collective effects are utilized.

[1] A.I. Väkeväinen, R.J. Moerland, H.T. Rekola, A.-P. Eskelinen, J.-P. Martikainen, D.-H. Kim, and P. Törmä, Nano Letters 14 (4), 1721–1727 (2014).

aaro.vakevainen@aalto.fi