Reflection spectra of synthetic opal at liquid-nitrogen temperature

Reflection spectra of the (111) surface of synthetic opal crystals with sphere diameters of 210, 250, and 300 nm have been measured at room and liquid-nitrogen temperatures. At cryogenic temperatures, we observed a narrowing and shift of photonic band gaps due to liquid nitrogen infiltration into the opal pores. We have calculated ω(k) dispersion relations and determined the reflectivity of the opal surface. The results have been compared to the measured reflection spectra. We have detected photon conversion from the edges of the band gap to its center.