Appl. Phys. Lett. 104, 241101 (2014)

Photonic crystal cavities can confine light of specific frequencies to small volumes of a few hundred nm3. In such artificial structures it is possible to achieve very strong coupling between light and matter, and for this reason, photonic crystal cavities could be essential elements in photonic circuits. It is, however, important to optimize their quality factor, which is a measure of how precisely defined the frequency of the confined light is. Antonio Badolato and colleagues at the University of Rochester, the University of Pavia and the Ecole Polytechnique Federale de Lausanne have now found a way to design photonic crystal cavities with optimized quality factors.

A photonic crystal is a semiconductor slab with a periodic array of holes. A photonic cavity is created by fabricating photonic crystals with one or more holes missing from the array, thus leaving a small area where light can be trapped. The quality factor can be improved by varying the size and position of the holes surrounding the cavity. Efforts in the past have been based primarily on educated guesses on how these geometric parameters should be varied. Badolato and colleagues have instead used a procedure that works by finding the structures with the highest quality factors from a population of candidates by successive optimization, a process resembling genetic evolution. The designs were used to fabricate silicon photonic crystal cavities that exhibited quality factors that were an order of magnitude higher than those previously reported.