Nonlinear optical response of a new Si-Ge waveguides with mid-infrared using femtosecond optical pulses

We describe the nonlinear optical response of low-loss Si_0.6Ge_0.4/Si waveguides in the mid-IR between 3.3 μm and 4 μm using femtosecond long pulses. Furthermore, we estimate the three-photon and four-photon absorption coefficients together with the nonlinear refractive index of Si_0.6Ge_0.4/Si waveguides, which are found to be comparable to previous work on the same material but with different waveguide length and geometries, and with longer optical pulses. Therefore, this work corroborates the previous measurements and thus provides some guidelines for future nonlinear device work. Using the estimated coefficients, the nonlinear FOM of Si_0.6Ge_0.4/Si waveguides in the mid-infrared(mid-IR) is evaluated. An optimum FOM is observed around 4 μm due to the decrease of the three-photon absorption coefficient, together with a small four-photon absorption coefficient. We find that this optimum of the FOM also depends on the coupled peak intensity since the three and four-photon absorption scales differently with this parameter. Finally, we compare the impact of free carrier absorption at mid-infrared wavelengths versus near-infrared (near-IR) wavelengths for these femtosecond long pulses, and highlights the need to accurately account for free carrier effects even when using ultrashort optical pulses in the mid-IR. This work provides useful groundwork for the design of photonic devices such as supercontinuum sources based on this material platform.