  We experimentally show the spectrally averaged nonlinear refractive index and
absorption coefficient for liquid water, water vapor, {\alpha}-pinene, and Si
using a full-phase analysis in the terahertz regime through a standard
time-domain spectrometer. Our results confirm that the nonlinear index of
refraction of the liquid samples in this regime exceeds the near-infrared
optical nonlinear index by six orders of magnitude. In the case of liquid water
and water vapor at atmospheric pressure, we find a nonlinear index of
7.8x10^-10 cm^2/W and 6x10^-11 cm^2/W respectively, which are both much larger
than expected.
