Colloidal Photonic Crystals Doped with Gold Nanoparticles: Spectroscopy and Optical Switching Properties

Polystyrene artificial opals with few gold nanoparticles (AuNp) embedded in the interstices (doping) are grown by using the meniscus technique starting from a mixed suspension of microspheres and AuNp. Samples having different sphere diameters and nanoparticle loads have been prepared. Their reflectance spectra clearly show a dramatic bathochromic shift of the photonic stop band (up to 1400 cm^–1) and a reduction of its full width half maximum, due to an increase of the effective refractive index of about 8 % with respect to bare opals, which is accounted for by analytical theoretical models. Reflectance spectra do not show any direct evidence of AuNp absorption even at the higher AuNp doping level. Nanosecond transient absorption measurements on these systems indicate that a variation of transmission (optical switching) of about 150 % is observed for AuNp doped opals upon photoexcitation with 9 ns laser pulses at 532 nm. No switching is instead observed for bare opals.