2D Nonlinear Photonic Crystals Nanocavities in Chalcogenide for All-optical Processing

Abstract

We demonstrate that high-Q nanocavity mode in 2D photonic crystal made in highly nonlinear chalcogenide glass can be excited using evanescent coupling from a tapered optical flbre. This scheme provides a promising platform to realize low power integrated all- optical switching and logic functions. The key to all-optical processing is the ability to enhance nonlinear efiects. One approach is based on developing optical conflgurations in which nonlinear efiects in a weakly nonlinear material are enhanced by reducing the mode size and increasing the intensity of the light wave. 2D photonic crystal (PhC) membranes are expected to play a key role in this context. It has been shown that ultra small and high quality factor Q can be created by introducing a carefully designed defect in the PhC (1). Following those advances, there have been predictions of the possibility of light switching light using nonlinear compact PhC microcavities operating at power levels of only a few mW (2,3). Chalcogenide glasses are attractive materials for all-optical signal processing. These glasses are composed of heavy elements including the chalcogens: S, Se and Te. The refractive index of chalcogenide is high, typically between 2.4 and 3.0 allowing 2D PhC slab to be created. Absorption losses are low over a wide wavelength range (near- to mid-infrared). Chalcogenide glasses possess a relatively large third-order optical nonlinearity (100{1000 times that of silica), and low two- photon absorption. In addition to reducing the switching power requirements, the pure Kerr-like nonlinearities ofier the potential for near instantaneous response times (< 100fs) and are only limited by the resonator Q-factor. Using chalcogenide glasses to fabricate a 2D PhC nanocavity which conflnes light at the wave- length scale and thus enhances the nonlinear light-material interaction provides the essential in- gredients for all-optical ultra-fast switching at low powers. It is well known that such cavities can exhibit optical bistability at incident powers that scale as the inverse square of the cavity's quality factor: 1/Q 2 . It is also well known that the minimum fractional nonlinear change in the refractive index, -n/n, needed to operate the device has to be greater than the inverse of the cavity's quality factor 1/Q. A common characteristic with glasses is that an upper bound exists on the nonlinear index change, -n, of a few 10 i4 . This implies that a minimum Q-factor exists of about 5,000 in order to observe a nonlinear switching. In addition, a strong resonance depth is required to obtain high contrast between the two switching states. Using PhC nanocavities ofier the advantage to strongly localize the mode in a small volume while limiting radiation losses through careful design (Fig. 1).