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

Abstract

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.

Ultrafast Photoinduced Electric-Polarization Switching in a Hydrogen-Bonded Ferroelectric Crystal

“…The results indicated that the ground state has the MO weighted on the side to which the protons are attached, whereas the final state of the lowest π-π* transition has the MO weighted on the other side [ Fig. 1(c)] [20]. This means that the direction of the dipole moment in the lowest photoexcited state is opposite to that in the ground state.…”

“…The photon energies of the incident and SH lights were 0.95 eV and 1.9 eV, respectively. The photon energy of the pump light was set at 3.2 eV and 2.6 eV, which correspond to the lowest π-π * transition [20] and the σ-π * transition [20,27], respectively. The pump and probe lights were obtained by two optical parametric amplifiers, which were excited by an output of the Ti:sapphire regenerative amplifier (repetition rate: 1 kHz, photon energy: 1.58 eV, and pulse width: 130 fs).…”

“…According to the THzradiation imaging experiment [28], the ferroelectric domain was uniform in the depth direction, i.e., the b-axis, and its size was larger than the spot diameters of the pump and probe pulses. Since the SH light was generated only from the back part of the crystal because of the short coherence length l C (=1.4 µm) [20], the pump light was incident from the back side. The time resolution was 0.37 ps.…”

Terahertz radiation by optical rectification in a hydrogen-bonded organic molecular ferroelectric crystal, 2-phenylmalondialdehyde

“…The light electric field E ω was set parallel to the c-axis, i.e., the direction of the spontaneous polarization. The increase in R and the sharp drop of T above 3 eV are attributed to the π-π * transitions [8] of the PhMDA molecule at approximately 3.9 eV. We performed the Kramers-Kronig transformation to extract the refractive index n spectrum, which is shown by the red line in Fig.…”

“…Thus, room-temperature hydrogen-bonded organic ferroelectrics are candidates for use in various electronic and photonic devices. Indeed, the second-order nonlinear susceptibility of croconic acid was reported to be fairly large [8], i.e., ∼ 2.5 × 10 −6 esu at 1000 nm, which is comparable to or higher than those of LiNbO 3 (1.2 × 10 −7 esu at 10640 nm) and dimethylamino-4-Nmethylstilbazolium tosylate (DAST) (4.8 × 10 −6 esu at 1318 nm). Furthermore, we have recently found that a terahertz electromagnetic wave is emitted from a single crystal of croconic acid at room temperature [9]; the magnitude of the terahertz wave was comparable to that emitted from a typical terahertz wave emitter ZnTe.…”