Fig. 5: HPhP probing near the hBN edge.
a Optical microscope image of the hBN edge in the Fourier crystal, with a coordinate system used for the near-field analysis: the edge is parallel to the x-axis, and the y-axis is at 32° angle to the lattice vector G. The hBN edge launches the polariton mode in y-direction (yellow wavy arrow) which produces interference fringes in the near-field images, while the s-SNOM nanotip excites the Bloch modes in the Fourier crystal (red wavy arrow). b Near-field images of the hBN edge shown in (a), measured at the excitation frequencies around the expected upper limit of the M0 bandgap in the direction of G. Dashed lines indicate the maxima of the underlying gold surface in contact with hBN. c Corresponding Fourier spectra of the near-field images in (b). Yellow arrows mark the spectral signature of the near-field interference due to the edge-launched HPhP; the white dashed circle indicates k = G. d Numerically calculated band structure of the HPhP propagating in y-direction (yellow wave in a). Yellow circles indicate the experimentally measured momentum of the edge-launched mode: 1.0, 1.25, and 1.65 μm–1 at 1420, 1440, and 1460 cm–1, respectively. Red tick marks indicate the corresponding experimental frequencies and the red dashed lines show the expected M0 bandgap in the direction of G from 1410 to 1470 cm–1. Red dot-dashed lines show the analytically calculated M0 dispersion in G-direction (also shown in Fig. 4b).
