(color online) Snapshots of wave packets crossing the interface, red dashed lines denote the interface and the arrows indicate the travelling direction. (a) LA, frequency 16.8 THz. (b) TA, frequency 12.0 THz. (c) ZA, frequency 2.5 THz. (d) ZA, frequency 9.0 THz. Reprinted with permission from Ref. [50], copyright (2014) by the American Institute of Physics.