Long-lived Photostriction in Croconic Acid Thin Film

Croconic Acid (CA) shows ferroelectricity due to proton displacement between molecules. Based on photovoltaic and converse piezoelectric effect the photostriction effect can appear in CA crystals. In order to study this photostriction effect, we carried out pump-probe experiments on CA thin film using time-resolved X-ray diffraction. The results of this experiment show a lattice expansion under laser exposure, but the anisotropy of this lattice expansion is different than the anisotropy for thermal expansion. In particular, the largest lattice expansion happens along polarization direction instead of weakly bonded direction. After the laser is turned off, the lattice distortion persists for approximately 20 minutes, indicating a metastable state. The appearance of this photostriction may be due to the reduction of depolarization field under the distribution of excited electrons. And the existence of a long-lived metastable state may be due to the polycrystalline thin film and single domain in small crystallites. This result helps us unveil the real relation between proton displacement and polarization switching, which should promote the development of proton-transfer ferroelectric organics.