Photoinduced ultrafast polarization reversal and terahertz emission in an organic ferroelectric crystal

Abstract Croconic acid represents a prototype organic dielectric, whose polarization exhibits a rapid response to external stimulus owing to its proton-transfer type ferroelectricity. Photoexcitation has shown a great potential to achieve efficient and fast polarization reversal of croconic acid crystal, but the microscopic dynamics of photoinduced ultrafast polarization reversal remain elusive due to the delicate interactions and ultrafast timescale. Here, we unravel the microscopic mechanism of photoinduced complete polarization reversal in croconic acid crystal via first-principles coupled electron-nuclear dynamics simulations. Upon photoexcitation, the croconic acid crystal undergoes the ultrafast depolarization within 150 femtoseconds. The depolarization is primarily governed by proton dynamics, with electronic contribution following those of protons. The depolarization and proton transfer take place in a half-transfer mode, driven by the different charge redistribution of two groups of hydrogen atoms. Ab-initio molecular dynamics simulations confirm the complete reversal of polarization and transfer of all protons during the picosecond relaxation process. Moreover, the sudden polarization switch and subsequent oscillations lead to efficient terahertz emission, consistent well with experiments.