Fig. 6
Comparison between PEPPs and SPPs in parametric plots. a Normalized propagation length (Lm/λ) versus normalized mode area (Am/λ2), revealing the trade-off between confinement and loss. Inset: since the confinement/propagation length increases toward the left/up direction, the trajectory located at the upper-left area indicates the best trade-off relation. b Normalized momentum (Re(k)/k0, k0: wavevectors of free-space photons) versus normalized mode area (Am/λ2), revealing the trade-off between confinement and momentum mismatch to free-space photons (black dashed line). Inset: since the confinement/momentum mismatch increases toward the left/up direction, trajectories located at the bottom-left area indicate the best trade-off relation. Blue lines with triangles: exciton-like PEPPs for maximum confinement. Green and red lines with triangles: SPP-like PEPPs for maximum FOMs. Gray lines with triangles: SPPs