Transmission phase of finite 1D photonic crystal

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 ≦ pi/2, I get negative phase angles in my plot. And since effective index neff(ω) = c/L/ω*Φ(ω) where L is the photonic crystal length, I get neff < 0 which should be positive. I have done calculations by assuming 0 ≦ Φ ≦ 2*pi but the results do not seem to be correct.

Appreciate much if someone can point me to the correct way of calculating Φ(ω).

Thanks!

I've managed to find an answer. Thanks.