A pulse of wave is a superposition of steady-state waves. Group velocity, phase velocity and dispersion relation have one-to-one correspondence. I agree that t

A pulse of wave is a superposition of steady-state waves. Group velocity, phase velocity and dispersion relation have one-to-one correspondence. I agree that the phase-kick classical model is a very simplified material model, which probably breaks down in certain ways. But it does yield a dispersion relation, therefore both phase and group velocities.