Measurement of a Photoinduced Transition from a Nonordered Phase to a Transient Ordered Phase in the Organic Quantum-Paraelectric Compound dimethyltetrathiafulvalene-dibromodichloro-p-benzoquinone Using Femtosecond Laser Irradiation

We report a new photoinduced transition from a nonordered phase to a transient ordered phase with symmetry breaking in an organic charge-transfer compound, dimethyltetrathiafulvalene (DMTTF)-dibromodichloro-$p$-benzoquinone ($2,6Q{\mathrm{Br}}_2{\mathrm{Cl}}_2$), which is a neutral compound located near the neutral-ionic phase boundary and shows quantum paraelectricity at low temperatures. By an irradiation of a femtosecond laser pulse, an ionic domain consisting of $\sim 40$ molecules is introduced into the neutral lattice per photon, giving rise to coherent molecular oscillations with fractional charge modulations over $\sim 400$ molecules. This response is due to the recovery of ferroelectric nature from the quantum paraelectricity by a photoinjection of an ionic domain with a large dipole moment.