  In this work we use multiple scattering in conjunction with a genetic
algorithm to reliably determine the optimized photonic-crystal-based structure
able to perform a specific optical task. The genetic algorithm operates on a
population of candidate structures to produce new candidates with better
performance in an iterative process. The potential of this approach is
illustrated by designing a spot size converter that has a very low F-number
(F=0.47) and a conversion ratio of 11:1. Also, we have designed a coupler
device that introduces the light from the optical fiber into a
photonic-crystal-based waveguide with a coupling efficiency over 87% for a
wavelength that can be tuned to 1.5 microns.
