We theoretically study the nonlinear optical response and photoexcited states of the Mott insulators. The nonlinear optical susceptibility ${\ensuremath{\chi}}^{(3)}$ is calculated by using the exact diagonalization technique on small clusters. From the systematic study of the dependence of ${\ensuremath{\chi}}^{(3)}$ on dimensionality, we find that the spin-charge separation plays a crucial role in enhancing ${\ensuremath{\chi}}^{(3)}$ in the one-dimensional (1D) Mott insulators. Based on this result, we propose a holon-doublon model, which describes the nonlinear response in the 1D Mott insulators. These findings show that the spin-charge separation will become a key concept of optoelectronic devices.
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 across the Mott gap, which have odd and even parities, respectively, are degenerate for $V∕t\ensuremath{\lesssim}2$ with
t being the hopping integral of an electron between nearest neighbor sites. For $V∕t\ensuremath{\gtrsim}2$, the bound states with odd and even parities occur and are not degenerate. The nature of the degeneracy and its effect on the optical response are examined.