Two-photon resonant third-harmonic generation in La₂CuO₄

The strong third harmonic generation (THG) in La₂CO₄ was demonstrated and the merit of THG experiments for the investigation of energy states and crystal symmetry of insulating transition-metal oxides was emphasized. Results show an excellent agreement between experimental data and calculations based on the excitonic cluster model, verifying that this model provides an adequate description of the electronic structure in the region of the charge transfer gap. The application of this experimental method to other undoped and doped transition-metal oxides provide further insight into the electronic structure of these important materials.