This work reviews the experimental studies on photo-induced cooperative phenomena (photo-induced phase transitions, PIPT), which are related to ultrafast structural and electronic-state dynamics. The review includes a brief history of this research field. The current growth of PIPT research is facilitated by deep collaboration between quantum beam science and materials science. Meanwhile, owing to developments in ultrafast quantum beam technology, the concept of PIPT itself has largely expanded. In the initial stage of PIPT research, the dynamical PIPT process was thought to be governed by energy relaxation from the photoexcited state, which is a classical and incoherent process. Therefore, the main research target was realizing the quasi-stable state on the energy surface of the ground state. Using ultrashort pulsed light and quantum beam technologies (including X-ray and electron technologies), one can now directly probe the ultrafast and coherent electronic dynamics of PIPT materials coupled with changes in their structural properties. This new technology has realized a new ordered phase called the hidden state, which is unique to the photoexcited state. Modern laser technology has also enabled a controlled PIPT process utilizing coherent coupling between the light field and electronic states in materials. Exploiting these leaps in experimental techniques, we have expanded the way of controlling PIPT dynamics from classical (incoherent) to coherent/quantum processes. This review discusses the recent developments in PIPT tuning via quantum (coherent) control methods based on charge–lattice (orbital)–spin coupled materials.
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