propagation constant in MPB

Dear all,

 I am now simulating a 2D (square lattice) photonic crystal. I set k-points to be Gamma-X-M. instead of the relationship between frequency and parallel k-vectors, I am more concerned about the propagation constant(generally named as beta, actually is z component of wave vector in this situation I think). So how could I convert frequency into kz? Could it be deduced by the formula shown as follows:
beta^2=(2pi*f*n)^2-(2pi*kx)^2-(2pi*ky)^2
( suppose a=1 and f, kx, ky are all got from MPB)

Thanks for your attention!

Best wishes
Yiling

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