  The unique material property of silicon carbide (SiC) and the recent
demonstration of low-loss SiC-on-insulator integrated photonics platform have
attracted considerable research interests for chip-scale photonic and quantum
applications. Here, we carry out a thorough investigation of the Kerr
nonlinearity among 4H-SiC wafers from several major wafer manufacturers, and
reveal for the first time that their Kerr nonlinear refractive index can be
significantly different. By eliminating various measurement errors in the
four-wave mixing experiment and improving the theoretical modeling for
high-index-contrast waveguides, the best Kerr nonlinear refractive index of
4H-SiC wafers is estimated to be approximately four times of that of
stoichiometric silicon nitride in the telecommunication band. In addition,
experimental evidence is developed that the Kerr nonlinearity in 4H-SiC wafers
can be stronger along the c-axis than that in the orthogonal direction, a
feature that was never reported before.
