(a) Transmission spectra for three metasurfaces increasingly close to the singular limit. COMSOL simulations (red dots) and analytic calculations performed with TO (blue lines) show perfect agreement. The top inset shows the spatial conductivity profile δ(y ^′)/δ ₀ = σ(y ^′)/σ _g along the metasurface. (b) In-plane component of the electric field normalized to the incident amplitude for the n = 1, 3 and 5 modes at f ≈ 7.4, 12.4 and 15.9 THz, respectively. The vertical extent of all plots h = 200 nm, highlighting the increase in GSP confinement with mode order.