Tuning-cooperative phenomena such as phase transition artificially obtained by external stimulation are currently a key subject for materials science, device application, and biological science. Especially, achieving the control of the phase transition by light excitation, which is named photo-induced phase transition (PIPT), is an important target for the wide field of optical science, because ultrafast and sensitive modifications of magnetic, dielectric, structural, and optical properties of materials by light are expected in virtue of cooperative interactions.
Our laboratory is the pioneer of PIPT research. In 1990, we published the first report on photo-induced neutral-ionic (N-I) phase transition in organic charge-transfer (CT) crystals. In the 90’s, we expanded the application of PIPT to various classes of materials such as photo-induced inverse Spin–Peierls transition in Mott insulator, bi-directional PIPT in polymer crystals with strong electron-lattice interaction, and photo-induced ferromagnetic order in Mn-doped III-V semiconductor. During the period 1995–2014, we reported that the combined photo-modification of three main physical degrees of freedom in solids (Spin–Charge–Structure, S–C–S) at ultra-high speed (within 100 femtoseconds [fs:10-15s]) beyond the vibrational speed of elementary excitations in solids plays an essential role in PIPT dynamics when fs spectroscopy and picosecond [ps:10-12s] X-Ray techniques are used. One typical example of the exotic nature of PIPT reflecting the S–C–S modification is represented by the photo-induced ferroelectric order in CT crystal of which experimental evidence was obtained by international collaboration with Professor H.Cailleau and Professor E.Collet Group. In addition, in manganite and organic CT crystals, we have suggested the emergence of a hidden state from quantum dynamics reflecting strong S–C–S coupling immediately after photoexcitation before spin, charge and lattice vibrate by their single periods. To obtain the structural identification of the hidden state in organic CT crystal, PI started an international collaboration with Professor D. Miller Group, who utilizes pulsed electron diffraction systems with widths of 200–250 fs. This collaboration reached fruitful results and has become an important concept and strong input for new project which has just started in Japan. However, regrettably, the formation process of the hidden state, i.e., initial quantum dynamics of PIPT including strongly combined and quantum fluctuations of S–C–S freedoms, remained a big question due to the limitation of time-resolution and the lack of versatility of the probing method of S–C–S (Electronic state) photo-modifications. Then our group is promoting continuously various projects for developing new PIPT materials based on S-C-S coupling combing with the development of ultrafast electronic-structural dynamical measurement systems.
For details: Grand-in-Aid, SPR