Fig. 1: Slowing down polaritonic modes by mirroring.
a Schematic diagram of our sample. A silicon substrate is coated with a 100-nm-thick e-beam evaporated gold coating. A 121-nm-thick h-BN flake is deposited on the gold film. b Optical image of the h-BN crystal flake, which was obtained using a phase-contrast microscope. The yellow arrow points to the edge that is imaged in this work. c AFM profile of the flake at the edge. d Polaritonic mode effective index. e Polaritonic mode dispersion. f Field configuration for the first mode in each of the RS bands. The main effect of the gold substrate (a perfect conductor at these wavelengths) is to reduce the polariton group and phase velocities by inhibiting less-confined modes. For example, in the RS2 band at 1500 cm−1, the effective refractive index of the first guided mode in our configuration is neff = 12.9 while the effective group index is n g = 191. The same mode on a SiO2 substrate would have neff = 4.02 and n g = 101
