I use transfer matrix method. I try to plot the phase Φ(ω) versus frequency ω graph in the vicinity of a band gap. The transmission coefficient t(ω) is
t(ω) = |t(ω)|*exp(i*Φ(ω))
from which I get the transmission phase
Φ(ω) = atan(t_imaginary(ω)/t_real(ω)).
Since I assume that -pi/2 ≦ Φ ≦...