  We study the density of states (DOS) in band gaps of ideal and disordered
three-dimensional photonic crystals of finite size. The ideal crystal is a
diamond lattice of resonant point scatterers (atoms) whereas the disordered one
is obtained from it by displacing the scatterers by random distances in random
directions. We find that DOS inside a band gap of the ideal crystal decreases
as the inverse of the crystal size. Disorder narrows the band gap and DOS
exhibits enhanced fluctuations near the new band edges. However, the average
DOS still exhibits the same scaling with the crystal size within the remaining
band gap. A phenomenological explanation of this scaling suggests that it
should hold for one- and two-dimensional photonic crystals as well.
