  We investigate the optical Kerr nonlinearity and multi-photon absorption
(MPA) properties of DSTMS excited by femtosecond pulses at a wavelengths of
1.43 {\mu}m, which is optimal for terahertz generation via difference frequency
mixing. The MPA and the optical Kerr coefficients of DSTMS at 1.43 {\mu}m are
strongly anisotropic indicating a dominating contribution from cascaded
2nd-order nonlinearity. These results suggest that the saturation of the THz
generation efficiency is mainly related to the MPA process and to a spectral
broadening caused by cascaded 2nd-order frequency mixing within DSTMS
