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The Journal of Chemical Physics

Intrinsic optical bistablility of an ultrathin film consisting of oriented linear aggregates

Viktor A. Malyshev, H. Glaeske, K.‐H. Feller

Published Jul 15, 2000• 25 citations
Publisher

Abstract

A theoretical study of transmittivity of an ultrathin film (of thickness smaller 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 eigenstates of which are represented by the Frenkel exciton states, can be treated in the framework of a two-level approximation. This allows us to claim that an ensemble of linear aggregates is analogous to that comprised of inhomogeneously broadened two-level systems. Based on this analogy, we show that the transmittivity of the film can manifest a bistable behavior originating from the coupling of the aggregates via the emission (far-zone) field. This mechanism of bistability differs from that occurring in a single perfect aggregate. In the latter case, bistability results from the population-dependent resonance frequency shift of an individual aggregate, originating from the coupling of the molecules via the dipole–dipole (near-zone) field. Estimates of the driving parameters show that real candidates, to realize the type of behavior we found, are such well-studied objects as J-aggregates of polymethine dyes or thin films of thiophene oligomers.

DOI
10.1063/1.481895

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Physical Review A•Feb 26, 2002•13 citations
A theoretical study of transmittivity of an ultrathin film (of thickness smaller 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 eigenstates of which are represented by the Frenkel exciton states, can be treated in the framework of a two-level approximation. This allows us to claim that an ensemble of linear aggregates is analogous to that comprised of inhomogeneously broadened two-level systems. Based on this analogy, we show that the transmittivity of the film can manifest a bistable behavior originating from the coupling of the aggregates via the emission (far-zone) field. This mechanism of bistability differs from that occurring in a single perfect aggregate. In the latter case, bistability results from the population-dependent resonance frequency shift of an individual aggregate, originating from the coupling of the molecules via the dipole–dipole (near-zone) field. Estimates of the driving parameters show that real candidates, to realize the type of behavior we found, are such well-studied objects as J-aggregates of polymethine dyes or thin films of thiophene oligomers.
Summary form only given. We provide a theoretical study of transmittivity of an ultrathin layer (of thickness smaller than an optical wavelength) consisting of oriented organic molecular aggregates (J-aggregates). In spite of the fact that those are complicated systems the excited states of which have the character of Frenkel excitons, an analysis of spectroscopic data of J-aggregates shows, that in a definite area of intensity of the incoming field such films can be modeled as an ensemble of inhomogeneously broadened two-level-systems. An appropriate renormalization of the spontaneous emission rate ensures, that mainly the fewest excitonic state couples to the radiation field. Because of a quite high concentration necessary for the aggregation process itself, in such systems a mechanism of optical bistability, that is due to coupling of the single aggregates via the far-zone field is shown to be possible.
A numerical study of reflectivity and transmittivity of an ensemble of short oriented linear Frenkel chains, which forms a thin film with a thickness of the order of an optical wavelength, is carried out. The eigenstates of a single chain are considered to be of a collective (excitonic) origin. A distribution of chains over lengths resulting in inhomogeneous broadening of the exciton optical transition is taken into account. We report a bistable behavior of both reflectivity and transmittivity of the film in a spectral domain close to the exciton resonance, caused by saturation of the nonlinear refraction index. Estimates of driving parameters show that thin films of oriented J-aggregates of polymethine dyes deposited on a dielectric substrate seem to be a suitable object for observation of the predicted behavior.
We perform a theoretical study of the nonlinear optical response of an ultrathin film consisting of oriented linear aggregates. A single aggregate is described by a Frenkel exciton Hamiltonian with uncorrelated on-site disorder. The exciton wave functions and energies are found exactly by numerically diagonalizing the Hamiltonian. The principal restriction we impose is that only the optical transitions between the ground state and optically dominant states of the one-exciton manifold are considered, whereas transitions to other states, including those of higher exciton manifolds, are neglected. The optical dynamics of the system is treated within the framework of truncated optical Maxwell-Bloch equations, in which the electric polarization is calculated by using a joint distribution of the transition frequency and the transition dipole moment of the optically dominant states. This function contains all the statistical information about these two quantities that govern the optical response and is obtained numerically by sampling many disorder realizations. We derive a steady-state equation that establishes a relationship between the output and input intensities of the electric field and show that within a certain range of the parameter space this equation exhibits a three-valued solution for the output field. A time-domain analysis is employed to investigate the stability of different branches of the three-valued solutions and to get insight into switching times. We discuss the possibility to experimentally verify the bistable behavior.
We generalize our recent work on the optical bistability of thin films of molecular aggregates [J. A. Klugkist et al., J. Chem. Phys. 127, 164705 (2007)] by accounting for the optical transitions from the one-exciton manifold to the two-exciton manifold as well as the exciton-exciton annihilation of the two-exciton states via a high-lying molecular vibronic term. We also include the relaxation from the vibronic level back to both the one-exciton manifold and the ground state. By selecting the dominant optical transitions between the ground state, the one-exciton manifold, and the two-exciton manifold, we reduce the problem to four levels, enabling us to describe the nonlinear optical response of the film. The one- and two-exciton states are obtained by diagonalizing a Frenkel Hamiltonian with an uncorrelated on-site (diagonal) disorder. The optical dynamics is described by means of the density matrix equations coupled to the electromagnetic field in the film. We show that the one- to two-exciton transitions followed by a fast exciton-exciton annihilation promote the occurrence of bistability and reduce the switching intensity. We provide estimates of pertinent parameters for actual materials and conclude that the effect can be realized.
We theoretically analyze the optical response of an ultrathin film with accounting for the effects of creating two exciton states as well as their annihilation in an individual aggregate. We show that these two processes do not violate mirrorless optical bistability of the film found in the framework of the ground-to-one exciton state transitions but, contrary to that, act towards improving the conditions for bistability to occur. In particular, the killing action of inhomogeneous broadening of the one-exciton absorption spectrum may be considerably reduced due to the exciton–exciton annihilation process giving hope for creating a unit for an all-optical device based on such systems.
We theoretically analyze the optical response from an ultrathin film built up of oriented molecular aggregates, the operating states of which are represented by Frenkel exciton states. A four-level model, involving transitions between the ground, one-exciton and two-exciton states, exciton-exciton annihilation from the two-exciton state as well as relaxation from the annihilation level back to the one-exciton and ground states, is used for describing the film optical response. It is proved that the exciton-exciton annihilation may act not as a destructive but, on the contrary, as a constructive factor tending towards the occurrence of bistability. In particular, the effect of inhomogeneous broadening of the exciton optical transition, preventing the bistable behavior, may be suppressed considerably due to a fast exciton-exciton annihilation.