High order nonlinearities and many photon effects in the exciton spectrum region of organic based crystals

The nonlinear effects of organic based crystals in which the Frenkel type o f excitons are realised were studied. It is shown by using of the nonequili brium Green function method that significant nonlinearities can arise in su ch type of crystals at moderate pump intensities owing to interaction of ex citons. Effective parameter responsible for nonlinear effects is "gN(P)", w here g is the constant of exciton-exciton interaction and N-P is the free p hoton density. The great crystal nonlinearities together with natural (nonl inear) boundary condition give rise to multistability effects in transmissi on and reflection light. The switching devices similar to optical key can b e realised on this base.