  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.
