Optical response of artifical opals oriented along the ΓX direction

The optical response of artificial opals in the surroundings of the (100) crystallographic direction has been measured by means of microreflectance and transmittance spectroscopies. The results indicate that for such sample orientation, the optical properties are determined entirely by low dispersion bands responsible for out-of-plane diffraction. This is corroborated by phase-sensitive spectroscopy which shows strong anomalies in the measured phase above the onset of diffraction. Such anomalies translate into group velocity values of ±c/ 20, evidencing slow and superluminal light propagation. These findings could be relevant in developing future routes for enhanced light-matter interactio