Multi-photon absorption and third-order nonlinearity in silicon at mid-infrared wavelengths.

Silicon based nonlinear photonics has been extensively researched at telecom wavelengths in recent years. However, studies of Kerr nonlinearity in silicon at mid-infrared wavelengths still remain limited. Here, we report the wavelength dependency of third-order nonlinearity in the spectral range from 1.6 μm to 6 μm, as well as multi-photon absorption coefficients in the same range. The third-order nonlinear coefficient n2 was measured with a peak value of 1.65 × 10(−13) cm2/W at a wavelength of 2.1 μm followed by the decay of nonlinear refractive index n2 up to 2.6 μm. Our latest measurements extend the wavelength towards 6 μm, which show a sharp decrement of n2 beyond 2.1 μm and steadily retains above 3 μm. In addition, the analysis of three-photon absorption and four-photon absorption processes are simultaneously performed over the wavelength range from 2.3 μm to 4.4 μm. Furthermore, the effect of multi-photon absorption on nonlinear figure of merit in silicon is discussed in detail.