Reflection spectra of uninfiltrated and Ba(NO3)2-infiltrated synthetic opal photonic crystals

The reflection spectra of uninfiltrated and Ba(NO₃)₂-infiltrated synthetic opal crystals have been measured using a halogen lamp with a quasi-continuous near-UV through IR spectrum as the light source. The spectra have been analyzed in a specular geometry at angles of incidence from 5° to 45°. The specular reflection spectra show a prominent maximum whose position depends on the angle of incidence. The infiltration of opal with barium nitrate shifts the maximum to longer wavelengths by 50 to 100 nm, depending on the angle of incidence. This effect has been interpreted in terms of the photonic band gap, which depends on the angle of incidence and the refractive index of the material filling the pores. The experimental data are in reasonable agreement with the peak-reflectance wavelengths calculated using well-known relations for Bragg diffraction.