Koshihara Okimoto Lab.

Photoinduced phase transition in TTF-CA showing neutral-ionic (N-I) transition

It is well known that N-I phase transition can occur driven by the inherent long-range Coulomb interaction between donor (D; TTF) and acceptor (A; CA) molecules. In the case of TTF-CA, the transition temperature (TNI) is 81 K and the ionicity (r) is about 0.3 in the N-phase while about 0.7, relatively higher, in the ionic phase. In the ionic state, in addition, the dimerization of D and A molecules also occurs which causes ferroelectricity.  We have so far revealed that the photoinduced N-I and I-N transitions can be realized by irradiation of femtosecond laser pulses based on the change of the optical constant of the material. The photoinduced phase transition can be regarded as an ultimate model of photoreaction especially in the life-science and has been of current interest. Furthermore, we observed a nonlinear optical phenomenon (i.e, generation of the second halmonics) in accordance with the photoinduced N-I transition. The result indicates that the photoirradiation can break the inversion symmetry in the ionic phase, that is to say, the lattice structure shows ultrafast variation on the time scale of a few- 10 picosecond. To see the lattice change more directly, however, the dynamical structural analysis is necessary using ultrafast short X-ray pulses.

Based upon such wisdom, we proposed a research using femtosecond laser light and the synchrotron orbital radiation (SOR) pulse in the collaboration with Rennes 1st university and some European group and performed the study with revising the equipments of the ultrafast X-ray sources in ESRF (European Synchrotron Radiation Facility). As a result, we could observe the lattice change such as the occurance of the dimerization and breaking of the inversion symmetry and directly confirmed the resultant photoinduced ferroelectricity for the first time. Besides, the observation also enabled us to see that the lattice change modulation occurred driven by the slight and cooperative gliding of the molecules. Those observations were noticed as a first report that described the dynamical cooperative change in the materials with strong electron-lattice interaction on the microscopic (atomic) scale, and clearly shows the significance of the novel observation method using the synchronization of the SOR and femtosecond pulses.

Averaged lattice change in accordance with the photoinduced N-I transition. The left and right figures denote the diffraction intensities before (left) and 2 ns after (right) the photoirradiation. The photoexcitation causes the diffraction at (0 3 0), indicating the lattice dimerization, a direct evidence of the photoinduced N-I transition in TTF-CA.

"Laser-Induced Ferroelectric Structural Order in an 0rganic Charge-Transfer Crystal", Science, vol.300, p.612~615, 2003 E. Collet,et al.