Fig. 4: Universal even energy-momentum (ω-k) dispersions in isotope-homogenous hyperbolic systems.
From: Van der Waals isotope heterostructures for engineering phonon polariton dispersions

a–c The ω-k dispersions of naturally abundant hBN, h11BN, and h10BN, respectively. Thickness: 50 nm. Adjacent polariton branches are separated evenly by identical momentum ∆k (red arrows) in all panels. White dashed lines indicate the transverse optical (TO) phonon frequencies of naturally abundant hBN, h10BN, and h11BN, respectively. Various polariton branches are marked by N = 1, 0, and −1, etc.