Fig. 2: First experimental demonstration of natural planar hyperbolic phonon and plasmon polaritons.

a The first 2D hyperbolic polariton demonstration was the phonon polariton in α-MoO₃, for which the wavefronts are shown here. b 3D and 2D isofrequency surfaces of hyperbolic phonon polaritons in vdW α-MoO₃. k_x, k_y, and k_z represent wave vectors along the three principal axes. c Absolute values of the Fourier transform near-field images of hyperbolic phonon polaritons recorded at 944 cm⁻¹ in vdW α-MoO₃ flake, where k₀ denotes the wave vector of free-space. d A demonstration of plasmonic 2D hyperbolic polariton. Isofrequency contours of plasmon at different frequencies are indicative of the topological transition in WTe₂ thin films. k_a and k_b denote wave vectors along a and b axes. Panel c reproduced with permission from AAAS³⁵ under a CC BY-NC 4.0 license; Panel d adapted with permission from ref. ²³.