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Open Access Publications from the University of California

All-optical nonlinear switching in optical micro- resonators on a silicon chip

  • Author(s): Ikeda, Kazuhiro
  • et al.

Silicon-based optical interconnects compatible with the standard complementary metal-oxide semiconductor (CMOS) technology are desirable for the realization of on-chip optical interconnections, which have the advantages of high refractive index resulting in nanometer scale waveguides, and of highly cost-effective integrations. Numerous researches thus have been recently conducted in this field, so called "Silicon Photonics". This dissertation addresses all-optical nonlinear switching processes, specifically, the critical issue of long free- carrier lifetime in optical micro-resonators where small nonlinearities of the material can be enhanced due to the accumulation of the optical amplitude and phase. The phenomenon observed previously was that the all-optical nonlinear switching in silicon micro-resonators was induced through free carrier refraction via two-photon absorption (TPA) and the lifetime of the photo-excited free-carriers had been the limit for the response time of the switching or modulating operations. For faster switching, bound electronic nonlinearity, i.e. Kerr nonlinearity is preferred. First, we quantitatively confirm that silicon cannot be considered as the materials for the resonator-enhanced Kerr nonlinear switch at 1.55mm of wavelength, due to the large free-carrier nonlinearity excited by TPA. Therefore, the material of micro- resonators is reconsidered in two directions to solve the issue of the long-lived free carriers; (i) amorphous silicon as a material with faster carrier recombination, (ii) silicon nitride (SiN) as a material with a smaller TPA coefficient. We present the first measurements of optical nonlinearity due to free carrier effects in amorphous silicon films using z-scan technique, demonstrating enhanced nonlinearity due to existence of midgap localized states. We also introduce a new composite waveguide structure consisting of amorphous and crystalline silicon whose measured free-carrier lifetime is shorter than that in pure crystalline silicon waveguides due to existence of the midgap localized states in amorphous silicon. We then present experimental evaluations of loss and nonlinear optical response in waveguides and micro-resonators, both implemented with a SiN/silicon dioxide (SiO2) material platform. The fabricated ring resonators are used to observe the all- optical switching and to measure both the thermal and ultrafast Kerr nonlinearities. The nonlinear refractive index of SiN is measured for the first time

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