Nonlinear optical response and spin-charge separation in one-dimensional Mott insulators

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.