  This work deals with the inverse design in the field of photonic crystal
based devices. Here an inverse method containing a fast and accurate simulation
method integrated with a competent optimization method is presented. Two
designs yielded from this conjunction of multiple scattering theory with a
genetic algorithm is analyzed. The potential of this approach is illustrated by
designing a lens that has a very low F-number (F=0.47) and a conversion ratio
of 11:1. We have also designed a coupler device that introduces the light from
an optical fiber into a PC based wave-guide with a predicted coupling
efficiency that exceeds 87%.
