Experimental observation of photonic and polaritonic gaps in a silica opal

Experimental observations of the simultaneous presence of a polaritonic and a photonic gap in a three-dimensional photonic crystal is reported, to the best of our knowledge, for the first time. The photonic crystal was made of monodispersed silica microspheres sedimented into a face-centered-cubic structure. Silica has a polaritonic gap for wavelengths between 8 and 9.35 microm. Four different sphere sizes were used, with diameters of d=0.49, 0.73, 0.99, and 1.57 microm. The photonic crystals were studied by normal incidence infrared reflectance measurements in the wavelength interval 0.8-12 microm. Four peaks with the a magnitude of approximately 0.6, originating from the periodicity of the crystal, were recorded in the interval between 1 and 4 microm. Another peak, the polaritonic reflectance peak (approximately 0.4), is observed for wavelengths around 9 microm for all four crystals.