Third-order nonlinearities in silicon at telecom wavelengths

The two-photon absorption coefficient and Kerr coefficient of bulk crystalline silicon are determined near the telecommunication wavelengths 1.3 mum and 1.55 mum using femtosecond pulses and a balanced Z-scan technique. The record sensitivity achieved by the combination of Z-scan and balanced detection, allowing the observation of phase shifts of the order of 1 mrad, enables the accurate measurement of third order nonlinear coefficients and the straightforward determination of the nonlinear figure of merit for all-optical switching under low excitation conditions, where thermal and free carrier contributions to the refractive index change are negligible.