Measurement of a Photoinduced Transition from a Nonordered Phase to a Transient Ordered Phase in the Organic Quantum-Paraelectric Compound dimethyltetrathiafulvalene-dibromodichloro-<mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline"><mml:mi>p</mml:mi></mml:math>-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 (2,6QBr(2)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 ~40 molecules is introduced into the neutral lattice per photon, giving rise to coherent molecular oscillations with fractional charge modulations over ~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.