Figure 2. Density of propagating photon modes ρ(ω) for vacuum and for PBG as a function of frequency ω. In vacuum, ρ(ω) varies continuously like ω2, whereas in a PBG, ρ(ω) exhibits a gap (or gaps) in which ρ(ω) is absolutely zero, ωv and ωc being the lower and upper band edge frequencies of the gap.

From

Frustrated Spontaneous Emission near the Edge of a Photonic Band Gap

Mesfin Woldeyohannes

International Journal of Physics. 2023, 11(1), 40-55 doi:10.12691/ijp-11-1-5