Light localization effect on the optical properties of opals doped with gold nanoparticles

Polystyrene artificial opals having both different microsphere diameters and gold nanoparticles (AuNP) doping levels have been prepared and their spectroscopic characterization has been performed. Reflectance spectra of doped opals show a red shift of the photonic band gap and a reduction of its width and intensity upon increasing AuNP doping level, but they do not show any direct evidence of AuNP plasmon resonance. Transmittance spectra of the same samples recorded on the same spot area show, instead, a doping level dependent line shape. For the highest doping level a complicated sigmoidal line shape due to the overlap of broad AuNP absorption and photonic crystal stop band is observed. This unusual effect is accounted for by considering the localization of standing electromagnetic waves within both the microspheres and the interstices where the AuNP are embedded.