Essential parameter in the formation of photonic band gaps

It is generally believed that the spatial periodic variation of a dielectric constant epsilon or refractive index root mu epsilon can give rise to photonic band gaps. However, through microwave experiments of microstrip lines with a periodic array of holes, we demonstrate that the wave impedance, which depends on root mu/epsilon, is an essential parameter, giving photonic band gaps rather than the refractive index. We explain physically the effect of the magnetic permeability mu on photonic band gaps, already reported.