Photo-induced phase transition (PIPT): How it is advancing through collaborations with materials and measurement sciences.

This study reviews developments in experimental studies of photo-induced cooperative phenomena (photo-induced phase transition, PIPT), which involve ultrafast dynamical changes in electronic, spin, and lattice structures. The growth in PIPT research has been driven by deep collaboration among quantum beam, materials, and theoretical sciences. In the early stages of PIPT research, it was thought that the dynamic PIPT process is dominated by energy relaxation from the photoexcited state, a process classified as classical and non-coherent. Using ultrashort-pulse quantum beam technologies, a new ordered phase, the "hidden state", unique to the photoexcited state, has been observed. In addition, modern laser technology has enabled control over the initial processes of PIPT, especially the formation of hidden states, through coherent coupling between a photon's electromagnetic field and the materials' electronic states. This review demonstrates that the PIPT control method has been extended from classical (non-coherent) to coherent processes (a class of quantum processes) through experimental results in collaboration with theoretical science.