We show strong coupling involving three different types of resonances in plasmonic nanoarrays: surface lattice resonances (SLRs), localized surface plasmon resonances on single nanoparticles, and excitations of organic dye molecules [1]. The measured transmission spectra show splittings that depend on the molecule concentration. The results are analyzed using finite-difference time-domain simulations, a coupled-dipole approximation, coupled-modes models, and Fano theory. The delocalized nature of the collective SLR modes suggests that, in the strong coupling regime, molecules near distant nanoparticles are coherently coupled.
We study spatial coherence properties of a plasmonic nanoarray covered with a fluorescent organic molecule film by a double slit experiment [2]. The evolution of spatial coherence of this composite structure from the weak to the strong coupling regime is investigated by systematically varying the coupling strength between the localized molecular excitons and the collective, delocalized SLRs. We demonstrate the control of the exciton-SLR hybrid mode dispersion, spatial coherence length and the relative weights of the components in the hybrid, with high precision. A continuous evolution of coherence from the weak to the strong coupling regime is observed, with the strong coupling features clearly visible in the interference fringes. High degree of spatial coherence is maintained in the strong coupling regime, even when the mode is very exciton-like (80 %). This is in stark contrast to pure localized excitons which do not display long range coherence. We also present our new results on the theory of the photon BEC concept in plasmonic nanoarrays [3].
Finally, we briefly present the main topics of a new review article [4] on strong coupling phenomena in plasmonics. The review includes, for instance, a comparison of the classical, semiclassical and quantum theories of many-emitter surface plasmon coupling, and discussion on how these relate to the experiments performed so far in the field.
[1] A. I. Väkeväinen, R. J. Moerland, H. T. Rekola, A.-P. Eskelinen, J.-P. Martikainen, D.-H. Kim, and P. Törmä, Plasmonic surface lattice resonances at the strong coupling regime, Nano Letters 14, 1721 (2014)
[2] L. Shi, T. K. Hakala, H. T. Rekola, J.-P. Martikainen, R. J. Moerland, and P. Törmä, Spatial coherence properties of organic molecules coupled to plasmonic surface lattice resonances in the weak and strong coupling regime, Phys. Rev. Lett. 112, 153002 (2014)
[3] J.-P. Martikainen, M.O.J. Heikkinen, and P. Törmä, Phys. Rev. A in press (2014)
[4] P. Törmä and W. L. Barnes, Strong coupling between surface plasmon polaritons and emitters: a review, invited review article Rep. Prog. Phys., in press (2014)