Tuning cooperative phenomena such as phase transition artificially by external stimulation is a key subject for materials science, device application and even biological science today. Especially, achieving the control of the phase transition by light excitation which is named as photo-induced phase transition (PIPT) is becoming important target for wide field of optical science. Because ultra-fast conversion of magnetic, dielectric, structural and optical properties by weak light is expected for PIPT materials as a result of cooperative interactions.
In spite of various attractive natures of PIPT, the research of this field is facing a difficult and essential problem, i.e. can we realize and identify a new phase of solid based on novel lattice structure which is unique for the photo-excited condition so called as a ‘hidden phase’? This ‘hidden phase’ with electronic and structural order realized only by optical excitation is important merit of PIPT process for achieving ultrafast and sensitive phase control via pure photonic channel free from thermal effect. Here, we demonstrate that light excitation reveals a ‘hidden charge and orbital ordered phase (CO-OO)’ which can never be achieved under thermo-equilibrium condition, and it really becomes the origin of the change in optical property of various inorganic and organic crystals utilizing picosecond time-resolved X-ray and femtosecond time resolved electron diffraction techniques. As a realistic example, we demonstrate that photo-induced structural changes in Nd0.5Sr0.5MnO3 (NSMO) thin film attributed to the photo-induced formation of a new CO-OO based on the ps time-resolved X-ray diffraction (TR-XRD) combined with fs spectroscopic measurement. We also demonstarte that such a hidden state also plays a key role in the gigantic photo-response of organic correlated systems. In case of (EDO-TTF)2PF6 crystals, structural changes accompanied with the photo-induced conversion of CO patter can be probed by fs time-resolved electron diffraction and IR measurements. We will report the late results and discuss the role of these new technique in the development of a new class of photonic materials.
Shinya Koshihara is a Full Professor of Tokyo Institute of Technology, Department of Chemistry and Materials Science. He got master degree of Science in the field of Condensed Matter Physics and Ultrafast Spectroscopy from the Univ.Tokyo in 1985. He became the assistant professor of the Department of Physics, the Univ.Tokyo and got Ph.D. in 1991 in the field of optical properties of semiconductors. He started the research on the photo-induced cooperative phenomena (Photo-induced phase transition: PIPT) at this occasion. He moved to RIKEN as a post-doc researcher of Photo-dynamics Research Center (in Sendai), RIKEN in 1991. During 1993-2000, he worked as an Associate Professor of Tokyo Institute of Technology, Department of Applied Physics. He expended the work of PIPT into various class of materials during this period. He became the full Professor of Tokyo Institute of Technology, Department of Chemistry and Materials Science in 2000. At this occasion, he expanded his work of PIPT into the field of non-equilibrium structural dynamics and started the national project for the pulsed X-ray research based on the laser-synchrotron combined system. Now he also started the international collaborations with Professor D.Miller (CFEL Hamburg) group for the progress of the research in the field of the non-equilibrium cooperative phenomena in condensed matters utilizing femtosecond dynamical study.
Professional Activities :
- 1986-1991 Assistant Professor of the University of Tokyo, Department of Physics
- 1991-1993 Researcher of Photo-dynamics Research Center (in Sendai), RIKEN
- 1993-1998 Associate Professor of Tokyo Institute of Technology, Department of Applied Physics
- 1998-2000 Associate Professor of Tokyo Institute of Technology, Department of Physics
- 2000-Prsent Professor of Tokyo Institute of Technology, Department of Chemistry and Materials Science
- 2003-2004 Professor of KEK (High Energy Accelerator Research Organization)
- 2004-2009 Visiting Professor of KEK
- 2005-2009 Professor of Frontier Research Center, Tokyo Institute of Technology
- 1993-1997 Visiting Researcher of RIKEN, Photo-dynamics Research Center
- 1998-2003 Leader of the Project in Kanagawa Academy of Science and Technology (KAST)
- 2003-2009 Director of ERATO Project of JST (Japan Science and Technology Agency)
- 2009-2015 Leader of CREST Project of JST