Breaking the symmetry of a photonic crystal allows external access to large near band-edge optical dispersion and exploitation of super-prism effects in a transmission mode device. Acceptable performance is obtained by spatially varying the scattering strength of the dielectric structure according to a modified exponential function. However, our analysis shows that useful devices operating with incident wavelength near 1550 nm will require fabrication tolerances on the scale of less than 10 nm. (C) 2003 Elsevier Science Ltd. All rights reserved.