Photonic Stopband Tuning in Metallo-Dielectric Photonic Crystals

One of the most appealing aspects of photonic crystal structures is the photonic bandgap created in structures with sufficiently high dielectric contrasts between constituent materials. Periodic structures with a modest dielectric contrast between high and low index regions instead form a photonic stopband; the photonic stopband is linked to the principal diffraction resonance from the (111) crystal plane in the photonic crystal. Understanding how specific photonic crystal structures and their associated stopband positions can selectively interfere with incoming light is vital for implementing these structured dielectrics in a range of optical applications. Among the many methods existing to modify the signature optical response of photonic crystal materials, metallo-dielectric photonic crystals act to incorporate metal particles into the ordered arrangement of these structures. We examined the optical changes to the transmission spectrum resulting from copper, nickel and gold metal infiltration into polystyrene opals and TiO2 inverse opals. We report a consistent and interesting optical phenomena directly associated with the creation of metallo-dielectric photonic crystal structures. More pronounced and numerous diffraction resonances emerge in opal photonic crystals with a metal deposited across the top layer. Common to both opal and inverse opal structures, was a blue-shift in the position of the (111) photonic stopband which increased in magnitude with greater metal content in the structure. We investigate the origin of the photonic stopband blue-shift by variation of the metal content and the placement of metal in the photonic crystal structure. Our results suggest that metal introduced to structured dielectric media acts to tune the position of the photonic stopband by slight alterations to the effective dielectric constant or effective refractive index of the photonic crystal material.