Thermal properties of a photon gas in photonic crystals

We investigated the thermal properties of a photon gas in photonic crystals. Since the density of photon states in photonic crystals is modified, thermal radiation of photonic crystals is suppressed inside forbidden gaps and the heat capacity of photon gas is more enhanced than $T^3$ as temperature increases. The heat capacities of phonons in periodic elastic composites and magnons in periodic ferromagnetic composites are also enhanced at sufficiently low temperature.