Ultrafast opto-protonics in a hydrogen-bonded π-molecular ferroelectric crystal

We investigated the ultrafast photo-response of a ferroelectric co-crystal of Hdppz–Hca composed of protonated 2,3-di (2-pyridinyl)pyrazine (Hdppz) and deprotonated chloranilic acid (Hca). Whereas the intermolecular proton transfer triggers the ferroelectricity in Hdppz–Hca, the majority of the large spontaneous polarization has a quantum mechanical origin from the highly polarizable π-electron system. In this study, we prepared a carrier-envelope phase-stable mid-infrared pulse tuned to the proton vibration of this system and investigated the time dependence of the subsequent change in the second harmonic generation (SHG) yield. By exciting the proton vibration, the relative change in SHG yield increased by about 100%, and the enhancement was only observed within the duration of the applied electric field. The huge enhancement and ultrafast response of the SHG, which is not seen in usual ferroelectrics, is attributed to the fact that the photoexcitation dynamically changes the stable position of protons and π-electrons, resulting in an ultrafast increase in the value of χ(2) of Hdppz–Hca. The phenomena observed here indicate a new property of this system as a quantum material with nonlinearity and can be regarded as opto-protonics in proton-mediated ferroelectrics.