We present the first study of nonlinear optical third harmonic generation in the strongly correlated charge-transfer insulator Sr2CuO2Cl2. For fundamental excitation in the near-infrared, the THG spectrum reveals a strongly resonant response for photon energies near 0.7 eV. Polarization analysis reveals this novel resonance to be only partially accounted for by three-photon excitation to the optical charge-transfer exciton, and indicates that an even-parity excitation at 2 eV, with a_1g symmetry, participates in the third harmonic susceptibility.
@article{arxiv.cond-mat/0105502,
title = {Parity forbidden excitations of Sr2CuO2Cl2 revealed by optical third-harmonic spectroscopy},
author = {A. B. Schumacher and J. S. Dodge and M. A. Carnahan and R. A. Kaindl and D. S. Chemla and L. L. Miller},
journal= {arXiv preprint arXiv:cond-mat/0105502},
year = {2009}
}