Fig. 1: Experimental setup and bright–dark near-field contrast in gold disk on (100) sapphire.

a Schematics for the s-SNOM nano-spectroscopy setup and sample crystal orientation. Inset at bottom shows the photo of the sapphire wafer. The c-axis is identified by corner-cuts. The scale bar is 10 mm. b AFM topography image of the disk antenna on (100) sapphire. c The corresponding white light near-field image. k_in-plane denotes the in-plane component of the incident light wavevector. Scale bars are 1 µm in both b and c. d Hyperspectral image normalized to a separate gold reference \(\frac{{S}_{2}(\omega ,x)}{{S}_{{\rm{ref}}}(\omega )}\) obtained by scanning the tip across the disk (indicated by the white dashed arrow in c) while collecting a spectrum at each pixel. Here \({\theta }\approx {9}{{0}}^{\circ }.\) A gaussian filter of small width is imposed on the hyperspectral image for reducing noise and better display without creating distortion.