Fig. 1. Concept of image polaritons.

a Schematic illustrations of image polaritons. Charge distributions for b symmetric mode and c anti-symmetric mode. In the presence of a mirror, image polaritons with out-of-phase charge oscillations are launched and confine radiation to the gap between the hBN layer and the mirror. d The normalized in-plane momentum and e figure of merit as a function of the gap size for the symmetric and anti-symmetric polariton modes, and graphene plasmons at five different frequencies of 1522, 1545, 1569, 1592, 1616, and 1639 cm⁻¹. The arrows represent the direction of increasing frequency. For graphene, the doping level and damping rate are 0.4 eV and 0.00378 fs⁻¹, respectively.