Spectroscopic Investigation of Artificial Opals Infiltrated with a Heteroaromatic Quadrupolar Dye

We report on the optical properties of polystyrene opals infiltrated with solutions of ad hoc synthesized heteroaromatic quadrupolar dyes endowed with strong nonlinear optical properties (two-photon absorption). Transmittance spectroscopy of opals infiltrated with dye water solutions shows both a bathochromic shift and a reduced width of the photonic crystal stop band. Moreover, variable angle transmittance spectra highlight the high energy dispersion of the stop band upon increasing the incidence angle. Photoluminescence spectra recorded at different emission angles with respect to the normal of the sample for such infiltrated opals show that the emission spectrum is strongly modified at the stop band according to the opal dispersion properties. A careful comparison of the intensity and spectral properties of photoluminescence in opals possessing a properly tuned photonic stop band is reported and discussed. Two different effects are observed: when concentrated solutions are used, emitted light filtering due to the photonic stop band is observed. On the other hand, by properly reducing solution self-absorption, directional enhancement of emission in the high energy side of the stop band is observed. Physical mechanisms responsible for this phenomenon are discussed.