Dirac-like spectrum for surface waves in an ultrasonic crystal

Absent of dissipation, wave energy transport occurs with the group velocity, and we exploit this principle to resolve the Dirac cone at the Brillouin-zone edge for ultrasonic surface waves in a hexagonal lattice of resonant cavities. Wave propagation measured along varying real-space lattice angles resolves dispersion along the Dirac cone in reciprocal space where the isofrequency contour gradient is oriented along the angle for which energy propagation is measured. Through the Dirac point, we measure group and phase velocities, and the Dirac-point frequency, which agree with finite-element predictions. The results afford the study of Dirac cone evolution in development of alternative acoustic systems, and demonstrate how a one-dimensional line-scan measurement can resolve targeted reciprocal space dispersion about a specific high-symmetry point for a two-dimensional crystal.