Fig. 3

Dispersion curves of the bilayer. a, b Experimental (image) and theoretical (blue dashed and green dotted-dashed lines) dispersion curves for a 0 ≤ f ≤ 10 GHz and 0 ≤ k ≤ 4.5 μm⁻¹ b at frequencies near the first ZGV mode for 1.5 ≤ f ≤ 1.9 GHz and −2.5 ≤ k ≤ 2.5 μm⁻¹. The solid navy blue section of the dispersion curves corresponds to the portions where the phase and group velocities are anti-parallel. a The box corresponds to the enlarged region in b for k > 0. a, b Arrows indicate ZGV points. Plots are independently normalized