We are interested in solids, which are composed of many microscopic components, e.g., atoms and molecules. Such materials exhibit dramatic and interesting phenomena, such as phase transitions and nonlinear responses, due to interactions among their constituents. There remain many not yet understood phenomena that will require highly-sophisticated theoretical treatments beyond one-body approximations. Our group is trying to explain these exoctic phenomena using analytical and computational techniques. Specifically, we study dynamical/nonlinear responses of condensed matter, in particular, cooperative phenomena related to optically-excited states of quantum many-body systems. In other words, we explore coupled systems, where the fermionic (electronic) fields and the bosonic (photonic, phononic, excitonic, biexcitonic) fields are interacting with each other. One of our main subjects is the "electron-hole (e-h) systems" in photoexcited semiconductors, where the Coulomb interaction leads to the formation of various phases such as the exciton gas, e-h plasma, e-h liquids, exciton Bose-Einstein condensation, and e-h BCS condensate. We perform systematic and quantitative surveys of such phases focusing on the following: (i) The phase diagram of e-h ensembles including exciton condensation and Mott transition. (ii) Evaluation of dynamical and coherence properties of light emitted from such e-h ensembles. (iii) Theoretical modeling of the dynamics of quantum phase transition of e-h ensembles.

Schematic illustration of a photoinduced

neutral-ionic transition.