Two-photon resonant third-harmonic generation in La2CuO4

Combining linear absorption and nonlinear third harmonic generation (THG) e xperiments, we investigate details of the electronic structure of the highl y correlated electronic system in La2CuO4. We demonstrate strong THG mainly due to the charge transfer excitation from O (2p(sigma)) to Cu (3d(x2-y2)) . The THG spectrum shows pronounced features due to three-photon and two-ph oton resonance enhancement as well as quantum interference effects. We obta in excellent agreement with a THG spectrum calculated in terms of the excit onic cluster model and can identify both odd and even symmetry excitation m odes.