Photonic stopband and reflectance asymmetry from thickness gradient in opals

The influence of thickness gradient and structural order on the spectral response of opal photonic crystals (PhCs) grown by evaporation-induced self-assembly (EISA) are presented. SEM imaging and angle resolved optical transmission spectroscopy are used to investigate the evolution of the PBG along a thickness gradient for opals grown from five different colloidal sphere concentrations at two different evaporation rates. The degradation of structural order along the thickness gradient is demonstrated, the occurrence of which attenuates the PBG with the thinning of the opal film and results in asymmetrical angle-resolved transmission spectra. The asymmetry in transmission intensity becomes more pronounced for opals grown from lower volume fractions, where secondary Bragg reflections also appear at low incident angles.