6.1. Introduction

Since Yablonovitch s proposal regarding photonic band gap crystals [YAB 94], numerous studies have been performed throughout the world to realize and use such nanostructures in the optical domain [JOA 95a; JOA 95b; LOU 03]. These studies, whilst fascinating at a fundamental level, may also lead to applications concerning, for example, lazers without thresholds, guided propagation with very small curvature radii, or optically active elements for optical signal processing, all of which open the way to the optical information processing microcircuits of the future. To realize these optical information processing functions, the implementation of nonlinear optical interaction processes will be required, in which third order nonlinear effects will play an essential role.

Here we hit an important scientific stumbling block: how to obtain sufficient optical nonlinearities to realize, with sufficiently small lazer energies, efficient control devices on optical microcircuits. As is well known, the use of one or many photon resonant, and possibly non-resonant, interactions in semiconductors produces high nonlinear effects on relatively small interaction lengths. However, for potential future information processing optical circuits, the interaction length will have to be on the micrometer range, leading to the necessity to further reinforce the optical nonlinearity of these materials. The nanostructuring of the material in the form of photonic band gap crystals, otherwise necessary for other linear elements of circuits, may give us an elegant and efficient solution. This will particularly be the case for photonic crystals used...