We apply the moment method to nonlinear pulse propagation in silicon waveguides in the presence of two-photon absorption (TPA), free-carrier dispersion, and free-carrier absorption (FCA). The evolution equations for pulse energy, temporal position, duration, frequency shift, and chirp are obtained. We derive analytic expressions for the free-carrier induced blueshift and acceleration and show that they depend only on the pulse peak power. Importantly, these effects are independent of the temporal duration. The moment equations are then numerically solved to provide fast estimates of pulse evolution trends in silicon photonic waveguides. We find that group-velocity and free-carrier dispersion dominate the pulse dynamics in photonic crystal waveguides. In contrast, two-photon and FCA dominate the temporal dynamics in silicon nanowires. To the best of our knowledge, this is the first time the moment method is used to provide a concise picture of free-carrier effects in silicon photonics. The treatment and conclusions apply to any semiconductor waveguide exhibiting TPA.
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