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We show the existence of direct photonic transitions between modes of a silicon optical microcavity spaced apart in wavelength by over 8 nm. This is achieved by using ultrafast tuning of the refractive index of the cavity over a time interval that is comparable to the inverse of the frequency separation of modes. The demonstrated frequency mixing effect, i.e., the transitions between the modes, would enable on-chip silicon comb sources which can find wide applications in optical sensing, precise spectroscopy, and wavelength-division multiplexing for optical communications and interconnects.

We experimentally measure the optical nonlinearities in hydrogenated-amorphous silicon (a-Si:H) waveguides through the transmission of ultra-short pulses. The measured two-photon absorption coefficient beta is 4.1 cm/GW and we obtain a 3.5pi nonlinear phase shift at 4.1 W coupled input power corresponding to a nonlinear refractive index n(2) of 4.210(-13) cm(2)/W. The measured nonlinear coefficient gamma = 2003 (Wm)(-1) is at least 5 times the value in crystalline silicon. The measured free carrier absorption coefficient sigma = 1.910(-16) cm(2) agrees with the values predicted from the Drude-Lorenz model. It is seen that a-Si:H exhibits enhanced nonlinear properties at 1550 nm and is a promising platform for nonlinear silicon photonics.

We demonstrate that an integrated silicon microring resonator is capable of efficiently producing photon pairs that are completely unentangled; such pairs are a key component of heralded single photon sources. A dual-channel interferometric coupling scheme can be used to independently tune the quality factors associated with the pump and signal and idler modes, yielding a biphoton wavefunction with Schmidt number arbitrarily close to unity. This will permit the generation of heralded single photon states with unit purity.