Nonlinear resonance reflection from and transmission through a dense glassy system built up of oriented linear Frenkel chains: Two-level model

A theoretical study of the resonance optical response of assemblies of oriented short (as compared to an optical wavelength) linear Frenkel chains is carried out. Despite the fact that the energy spectrum of a single chain is composed of the bands of Frenkel exciton states, a two-level model is used to describe the optical response of a single linear chain. We account for only the (on-resonance) optical transition between the ground state and the state of the one-exciton band bottom as having the dominating oscillator strength as compared to the other states of the one-exciton manifold. The (off-resonance) process of creation of two excitons per chain is neglected because it requires a higher excitation frequency due to the quasi-fermionic nature of one-dimensional Frenkel excitons. A distribution of linear chains over length resulting in fluctuations of all exciton optical parameters, such as the transition frequency and dipole moment as well as the radiative rate, are taken explicitly into account. We show that both transmittivity and reflectivity of the film may behave in a bistable fashion, originating from saturation of the nonlinear refraction index, and analyze how the effects found depend on the film thickness and on the inhomogeneous width of the exciton optical transition. Estimates of the driving parameters show that films of oriented J-aggregates of polymethine dyes at low temperatures seem to be suitable species for the experimental verification of the behavior found. (C) 2001 American Institute of Physics.