Due to its horizontal polarisation, the incoming radiation can only weakly interact with the phonon band affecting the normal component of ϵ at ≈24 THz, so that the GSP spectrum is left intact by the presence of the hBN substrate even for significant thicknesses. In contrast, the in-plane phonon band is responsible for the strong transmittance dip at ≈41 THz. For our singular gratings, this lies way above the GSP frequency region. The field plot (bottom) shows the 4th order GSP mode for h _hBN = 20 nm at f ≈ 9.3 THz, exhibiting a maximum (in-plane) field enhancement , and a minimum vertical decay length is ≈5 nm. The maximum total field enhancement E|/|E ₀| ≈ 210.