Light localization induced enhancement of optical nonlinearities for optical switching

In recent years, a significant amount of attention has been drawn towards all-optical semiconductor switching. To that respect, exploitation of light localization, in slow light photonic crystal waveguides (PCWs) or in microcavities, is of considerable interest because it enables enhancement of optical nonlinearities. Recently, we observed experimental and theoretical enhancement of optical Kerr effect, two-photon absorption (TPA) and free-carrier index changes (FCI) in GaAs PCWs, which demonstrated the important role of group velocity. This paper presents an original approach to map and design light localizing semiconductor structures for optical switching applications. In any nonlinear process, each nonlinear coefficient of the bulk material must be enhanced by a power of the local field factor f, which is simply related to the square root of the slow down factor for PCWs, or of the Q-factor for microcavities. So, a p-th order nonlinear susceptibility of the bulk material should be multiplied by f^p+1 in the nonlinear propagation equation. The desired nonlinearity for switching applications is the Kerr effect through which a relative nonlinear phase-shift of pi must be achieved. However, semiconductor materials present n-photon absorption (n-PA). n-PA transfers power from light to free- carriers that produce FCI, which adds an additional phase-shift to the optical beam and thus competes with the Kerr effect, though some argue that FCI switching can be used when carrier lifetime is short.