Nonlinear optical properties of perovskite YMnO3 studied by real-space recursion method

New types of second-order nonlinear optical properties of hexagonal YMnO3 h ave been studied, caused by the coexistence of the noncentrosymmetric ferro electric ordering of charges and the centrosymmetric spin ordering below it s Neel temperature T-N. Using time-dependent perturbation theory and the Gr een's function method, we have derived an expression for the nonlinear opti cal susceptibility in terms of the real-space Green's function. Then, based upon the multiband Hubbard Hamiltonian and the unrestricted Hartree-Fock a pproximation, the real-space recursion method has been used to calculate th e electric structure of YMnO3 below T-N, and its nonlinear optical suscepti bility. The calculated results are very consistent with the experimental ob servation, and clearly show different contributions to the nonlinear optica l response Oem the Mn e(g) and Op bands in YMnO3.