Theory of photoinduced structure changes

Abstract

A unified theory is presented to understand the dynamical properties of photoinduced structure changes, investigating mainly switching phenomena. The way local microscopic structure changes induced by optical pumping lead cooperatively to global macroscopic changes is clarified. Both the optical and thermal transitions are discussed, stressing their respective roles and qualitative differences. The dynamics of the system is described by the kinetic Ising model with rather complicated transition probabilities. When the three-dimensional interaction is large enough and the energy difference between the two states is small, the nucleation process can be neglected. We apply the mean-field approximation to the kinetic Ising model, and the dynamics, including threshold behavior, the relaxation time, and the spontaneous emission are discussed for low temperatures compared with the lattice relaxation energy. A strong temperature dependence of the dynamics is predicted. As an application of our theory, the photoisomerization (A-B transition) of polydiacetylenes is discussed to point out their collective nature.