Simulation of nonlinear pattern formation dynamics in photoinduced structure change

We study the nonlinear dynamics of pattern formation triggered by injection of photoexcited states. In order to describe the nonadiabatic transition during the relaxation process, we employ a model of localized electrons coupled with a fully quantized phonon mode, and the time-dependent Schr\"odinger equation for the model is numerically solved. We found that the photoinduced nucleation process is switched on only when certain amount of excitation energy is supplied in a narrow part of the system, i.e., there exists a smallest cluster of excited molecules which makes the nucleation possible. As a result, the portion of the cooperatively converted molecules is nonlinearly dependent on the photoexcitation strength, which has been observed in various materials.