Transient optical response of strongly correlated low-dimensional electron systems

We theoretically investigate transient optical response of strongly correlated low-dimensional electron systems by analyzing the free induction decay, the transient four wave mixing, and spin and charge correlation functions. We adopt the effective Hamiltonian of the Hubbard model for the photoexcited states in the strong correlation case, and these physical quantities are obtained by exactly calculating the time development of the photoexcited state. In the two-dimensional case, a well-separated two-step relaxation is observed. The faster relaxation originates from the transfer of photogenerated charges in the antiferromagnetic background, and the slower one originates from the spin structure rearrangement with the new charge distributions. The slower spin relaxation follows the faster charge relaxation as a result of the coupling between the spin and charge degrees of freedom. This shows that the coupling between the spin and charge degrees of freedom is weak. In the one-dimensional case, only the charge motion is observed, and the spin structure rearrangement does not occur. This is a result of the spin–charge separation. It is emphasized that optical methods provide us with a powerful tool to directly investigate the exotic properties of low-dimensional strongly correlated electron systems.