We develop an effective way to engineer a two-dimensional GaAs photonic crystal slab with its leaky eigenmodes at desired wavelengths by investigating its spectral dispersion, particularly in terms of transmission efficiency spectra at different launch angles of the light beam. Structural parameters for the photonic crystal slab with leaky eigenmode wavelengths at both 1492nm and 1519nm are obtained. This may lead to the enhanced luminescence from erbium-doped trinitride-template fullerenes (Er 3 N@C 80 ) on the surface of the photonic crystal slabs.