Large second-order optical nonlinearity in a ferroelectric molecular crystal of croconic acid with strong intermolecular hydrogen bonds

Linear and nonlinear optical responses in a molecular crystal, croconic acid, showing electronic-type ferroelectricity were studied by reflection and second harmonic generation spectroscopy. The second-order nonlinear susceptibility χ(2) was very large, exceeding 10−6 esu in the near-infrared region. The enhancement of χ(2) was attributed to the large dipole moment of the lowest π–π* transition and the large difference between the molecular dipole moments for the ground state and the photoexcited state. We deduced the molecular orbitals (MOs) and dipole moments responsible for the large χ(2) by comparing the experimental optical parameters and MO calculation results based upon density functional theory.