Fig. 2: Experimental near-field mapping of ℓ = 2 SPhP vortex.

a, In sSNOM, the pure SPhP field interferes with the bare material response. b, False-colour SEM image of a VG with L = 1 excited by LCP light. Scale bar, 2.5 μm. The dashed lines indicate the region where experimental data are shown. c, Experimental near-field amplitude |sO2| and phase Φ(sO2) maps (second order demodulation) from the VG in b at ω = 913 cm⁻¹. d, Simulated corresponding vortex profile, where a constant complex background is added to the SPhP E_z field. e, Removal of the material contrast through FFT filtering. Only the region between the dashed lines is retained. The rest of the map is shown multiplied by 0.03. The bright spot at the centre represents the position-independent material contrast. Scale bar, 10k₀. f, Experimental maps obtained by the inverse FFT of the filtered data shown in f. g, Simulated corresponding SPhP E_z field.