Intrinsic optical bistablility of an ultrathin film consisting of orientedlinear aggregates

A theoretical study of transmittivity of an ultrathin film (of thickness sm aller than an optical wavelength) consisting of oriented linear aggregates is carried out. We show that within a certain domain of the incident field amplitude, optical transitions of a single disordered aggregate, the eigens tates of which are represented by the Frenkel exciton states, can be treate d in the framework of a two-level approximation. This allows us to claim th at an ensemble of linear aggregates is analogous to that comprised of inhom ogeneously broadened two-level systems. Based on this analogy, we show that the transmittivity of the film can manifest a bistable behavior originatin g from the coupling of the aggregates via the emission (far-zone) field. Th is mechanism of bistability differs from that occurring in a single perfect aggregate. In the latter case, bistability results from the population-dep endent resonance frequency shift of an individual aggregate, originating fr om the coupling of the molecules via the dipole-dipole (near-zone) field. E stimates of the driving parameters show that real candidates, to realize th e type of behavior we found, are such well-studied objects as J-aggregates of polymethine dyes or thin films of thiophene oligomers. (C) 2000 American Institute of Physics. [S0021-9606(00)70427-2].