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Si-based ultrafast photodetection provides the basis for a practical implementation of all-silicon integrated photodetectors. This system uses a Si-based microcavity and waveguide photonic crystal to receive an input optical signal. Optical energy is then absorbed by a nonlinear multi-photon absorption process such as two-photon absorption (TPA). This nonlinear process is enhanced by optical microresonators, which enable light confinement for times, orders of magnitude longer than the characteristic period of light, increasing bit-rates to up to 10Gbits/s. The silicon based structure also features electrodes that are responsive to the nonlinear multi-photon absorption process in the microcavity, and are responsible for producing an electronic signal indicative of the optical signal. Since both the microcavity and waveguide are photonic crystals made of silicon, the entire system can be manufactured with established CMOS techniques. Enhanced TPA in these silicon microcavities can yield a 15% increase in efficiency in photodetectors, paving the way for a new class of integrated photonic microdevices.