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Optics and Spectroscopy

Exciton self-trapping in molecular media with an elastic dipole moment

Alexander A. Zabolotskii

Published Nov 1, 2005• 0 citations
Publisher

Abstract

Exciton self-trapping in a molecular medium is considered within a self-consistent model taking into account the change in the dipole moment as a result of a displacement of molecules and the resonance interaction with an electromagnetic field. New mechanisms and specific features of the formation of localized structures in quasi-one-dimensional molecular structures are investigated. It is shown that the dependence of the dipole moment on molecular vibrations leads to new conditions for exciton self-trapping and intrinsic optical bistability, as well as for the formation of electromagnetically induced transparency. The theory proposed is used to explain the specific features of exciton self-trapping in a conjugated polymer and J aggregates of dyes under the action of an external laser field.

DOI
10.1134/1.2135844

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Exciton self-trapping in a molecular medium is considered within a self-consistent model taking into account the change in the dipole moment as a result of a displacement of molecules and the resonance interaction with an electromagnetic field. New mechanisms and specific features of the formation of localized structures in quasi-one-dimensional molecular structures are investigated. It is shown that the dependence of the dipole moment on molecular vibrations leads to new conditions for exciton self-trapping and intrinsic optical bistability, as well as for the formation of electromagnetically induced transparency. The theory proposed is used to explain the specific features of exciton self-trapping in a conjugated polymer and J aggregates of dyes under the action of an external laser field.
Within the framework of a developed model of a medium consisting of molecular J aggregates and placed in an external resonant light field, it is shown how exciton self-localization and the dependence of the dipole moment components on molecule deformation affect optical bistability. Deformation of the molecules substantially facilitates the observation of a bistable dependence of the state of the medium and of the transmitted field amplitude on the incident field amplitude. The evolution of the system to a quasi-stationary, nonground state after the passage of an electromagnetic field pulse under the conditions of self-localization of excitons is studied.
Nonlinear optical properties of dye J-aggregates embedded in a one-dimensional periodic medium are analyzed theoretically and numerically. The effect of strain-induced exciton self-localization in a chain of monomers on nonlinear optical properties of the system is investigated for weakly and highly ordered J-aggregates. It is found that exciton self-localization in a periodic medium enhances light absorption and effective optical susceptibility. Optical bistability thresholds are examined for weakly and strongly excited dye J-aggregate thin films. It is shown that the threshold intensity of incident electromagnetic field is lower by more than an order of magnitude as compared to that for a homogeneous medium.
The resonance interaction of a three-component electromagnetic field with a planar polymer film containing J aggregates of dyes is studied. The field consists of two counterpropagating waves traveling along the J aggregate film and a wave directed along a normal to the plane of the film. It is shown that, compared to the case of a one-component field, the polariton-phonon interaction in such a system leads to qualitatively different conditions of the occurrence of localized exciton states and bistability.
Abstract A model which allows to treat self‐consistently many‐body exciton effects, including biexciton formation, is proposed for the theoretical description of an excitonic polarization induced by an intense electromagnetic field. In the presence of a pump field the normal spectrum of electromagnetic waves in the vicinity of the nonlinear biexciton resonance is shown to be composed of four polariton branches, where each is the superposition of two photons and two excitons. In the self‐consistent field approximation, the structure of the interconnected, strongly nonlinear exciton and biexciton resonances is studied, including optical Stark shift, multistable behaviour, etc. For a certain range of the exciton interaction parameters at sufficiently strong pump wave the system of excitons and excitonic molecules transforms abruptly to a dense coherent electron–hole “liquid”. Appearance and temporary evolution of an exciton–biexciton system after switching on a pump is also considered.
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 address a two-level system in an environment interacting with an electromagnetic field in the dipole approximation. The resonant optical bistability induced by local-field effects is studied by considering the relationship between the population difference and the excitation field. The diversity of various systems is included by accounting for system self-action via the surface part of Green's dyadic in the general form. The bistability condition and the exact solution of the steady-state optical Bloch equations at the absolute bistability threshold are derived analytically.