The transmittance plots show the excitation of GSPs and SOPs for the three increasingly singular graphene metasurfaces covered by a similarly modulated SiO₂ superstrate. The presence of peaks due to SOPs in SiO₂ could pose an experimental challenge to a clear measurement of the close-to-singular plasmonic spectrum. The maximum thickness of the SiO₂ grating is 300 nm, whereas the minima are 50, 8 and 3 nm respectively (top to bottom). The bottom inset shows the 4th order GSP mode at f ≈ 11.3 THz, showing a maximum (in-plane) field enhancement of ≈70, and a minimum vertical decay length of ≈9 nm. The maximum total field enhancement |E|/|E ₀| ≈ 80.