Periodic arrays of resonant nanoparticles supporting localized surface plasmon polaritons (LSPPs) at optical frequencies allow the control of electromagnetic fields over large areas and define extended open cavities. Enhanced far-field coupling of LSPPs across the array by orders of diffraction enables the precise control of the distribution of the field enhancement, the losses of the system, and the strength of interaction with the surrounding. These degrees of freedom can lead to strong light-matter coupling, and the formation and condensation of exciton-polaritons, which produces a non-linear emission with similar characteristics than lasing. We show that in contrast to lasers, where high efficiency emitters are a requisite for low threshold emission, the reduction of the quantum efficiency of emitters strongly coupled to periodic arrays of nanoparticles leads to a remarkable reduction of the threshold for non-linear emission.
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