  One-dimensional Mott insulators exhibit giant nonlinear optical response.
Based on the dynamical density matrix renormalization group method,
photoexcited states and optical response in the insulators are studied as
functions of the on-site and the nearest neighbor Coulomb interactions, $U$ and
$V$, respectively. We find that the lowest optically-allowed and forbidden
excited states, which have odd and even parities, respectively, are degenerate
for $V/t\ltsim 2$ with $t$ being the hopping integral of an electron between
nearest neighbor sites. For $V/t\gtsim 2$, the bound states with even and odd
parities occur and are not degenerate. The nature of the degeneracy and its
effect on the optical response are examined.
