Elementary nonlinear processes in the formation of domains after photoexcitation

We theoretically study lattice relaxation dynamics of a photogenerated exci ton which finally results in a macroscopic domain in insulators. Especially , we focus on three elementary nonlinear processes of this domain formation ; aggregation, growth and proliferation. We consider a strongly coupled man y exciton-Einstein phonon system. We also assume the system to interact wit h a reservoir, in order to take into account the relaxation channels such a s vibrational relaxations, non-radiative and radiative decays of excitons. The excitons are assumed to be strongly repulsive on the same sites and att ractive on the nearest-neighboring sites. The third-order anharmonic coupli ng between excitons is also assumed. Our system is in the so-called multist able situation where various multi-exciton states are energetically close t o 1-exciton states. Within the Markov approximation for the reservoir, we i nvestigate full-quantummechanically the time evolution of the system. We ha ve shown that in our model, the aforementioned elementary processes success fully work against the relaxation channels. We conclude the anharmonicity a nd the multistability of excitons are indispensable for the domain formatio n. (C) 2000 Elsevier Science B.V. All rights reserved.