ABSTRACT We investigate exciton–polariton condensation in square arrays, composed of either dielectric silicon (Si) or plasmonic silver (Ag) nanodisks, covered with a dye‐doped layer. Both arrays support symmetry‐protected bound states in the continuum (BICs) at normal incidence, featuring electric quadrupolar and magnetic dipolar characters. Due to differences in mode coupling, these BICs are split by meV in the Si array, whereas they remain nearly degenerate in the Ag array. Simulations reveal that interference in the Ag array results in hybrid modes, and , which are polarized along orthogonal directions. Interestingly, this results in similar condensation thresholds in both Si and Ag arrays, regardless of the inherent nonradiative losses of Ag, and the confinement of the polariton condensates in the Ag array. Although condensation in the Si array occurs in the BIC, producing a characteristic donut‐shaped far‐field emission in k‐space, condensation in the Ag array populates the hybrid modes, leading to a double‐cross emission pattern extending over a broad range of wave vectors due to the quasi‐flat band nature of this mode. As a result, the Ag array also exhibits a strong confinement along the polarization axis in real space. However, for unpolarized emission, there is a similar spatial confinement in both Si and Ag arrays. This control over the confinement of condensates could also be exploited to control interactions. Our results highlight a novel mechanism for condensate confinement with potential applications in quantum computing and polaritonic circuitry.
Berghuis et al. (Wed,) studied this question.