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

Croconic acid crystals show proton displacive-type ferroelectricity with a large spontaneous polarization reaching $20\mu \mathrm{C}/{\mathrm{cm}}^2$, which originates from the strong coupling of proton and $\pi$-electron degrees of freedom. Such a coupling makes us expect a large polarization change by photoirradiations. Optical-pump second-harmonic-generation-probe experiments reveal that a photoexcited croconic-acid crystal loses the ferroelectricity substantially with a maximum quantum efficiency of more than 30 molecules per one absorbed photon. Based on density functional calculations, we theoretically discuss possible pathways toward the formation of a one-dimensional domain with polarization inversion and its recovery process to the ground state by referring to the dynamics of experimentally obtained polarization changes.

Figure 1

(a) Crystal structures of croconic acid with opposite directions of ferroelectric polarization $P$. (b) The optical absorption spectrum along the $c$ axis. Arrows show pump photon energies in the pump-probe experiments. (c) Molecular orbitals of an isolated croconic-acid molecule. Pump photon energies of 3.2 and 2.6 eV correspond to the lowest $\pi \text{−}{\pi }^{*}$ transition and the $\sigma \text{−}{\pi }^{*}$ transition, respectively.

Figure 2

(a, b) Time evolutions of $\mathrm{\Delta }{I}_{\mathrm{SHG}}/I_{\mathrm{SHG}}$ by the pump light with (a) 3.2 eV and $x_{\mathrm{ph}}=2.8\times {10}^{-4}\mathrm{ph}/\mathrm{molecule}$ and (b) 2.6 eV and $x_{\mathrm{ph}}=1.5\times {10}^{-4}\mathrm{ph}/\mathrm{molecule}$ (open circles). Solid lines show fitting curves. The experimental configuration is illustrated in the inset of Fig. 2. (c) Semilogarithmic plots of time evolutions shown in (a) and (b). (d) $x_{\mathrm{ph}}$ dependence of $\mathrm{\Delta }{I}_{\mathrm{SHG}}/I_{\mathrm{SHG}}$ at several typical delay times. (e),(f) Each component of fitting curves shown in (a) and (b) (see the text).

Figure 3

Adiabatic potential curves in the electronic ground state and schematic illustrations corresponding to the configurations. The carbon rings in the inner part of the domain and those within the DWs are shaded by pink and green colors, respectively.

Figure 4

(a) and (b) Adiabatic potential curves corresponding to the electronic ground and excited states, (c) optical conductivity spectrum, (d) schematics for the electron transitions, and (e) associated molecular orbitals. In (b), $\mathrm{\Delta }{b}_{1L}$ is fixed at 0.55 Å. The arrow drawn from the rightmost panel in (d) points the first excited state in (b).