Existence of a photonic pseudogap for visible light in synthetic opals

Synthetic opals, composed of submicron silica spheres close packed in a three-dimensional fee lattice, are shown to display photonic stop bands at optical frequencies. We have investigated the light attenuation within the stop band as a function of refractive index contrast. Based on measurements of the Bragg attenuation length and on theoretical considerations, we show that a prominent depletion of the photonic density of states (pseudogap) can be achieved in opals by adjusting the volume packing fraction of the spheres and increasing the refractive index of the pores. To approach the pseudogap criterion the pores of opal were impregnated with CdS nanocrystals. We find a dramatic decrease of the attenuation length in opal-CdS, which indicates the strong perturbation of photonic states.