  We investigate the nonlinear optical susceptibilities of Mott insulators with
the dynamical mean field approximation. The two-photon absorption (TPA) and the
third-harmonic generation (THG) spectra are calculated, and the classification
by the types of coupling to external fields shows different behavior from
conventional semiconductors. The direct transition terms are predominant both
in the TPA and THG spectra, and the importance of taking all types of
interaction with the external field into account is illustrated in connection
with the THG spectrum and dcKerr effect. The dependence of the TPA and THG
spectra on the Coulomb interaction indicate a scaling relation. We apply this
relation to the quantitative evaluation and obtain results comparable to those
of experiments.
