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Optoelectronics Instrumentation and Data Processing

Dipole-Dipole Interaction in a Helical Molecular Aggregate

Alexander A. Zabolotskii

Published Jul 1, 2018• 1 citation
Publisher

Abstract

The formation and dynamics of excitations in an atomic or molecular nano chain of a helical shape are studied. The dipole-dipole interaction between the circular polarizations of the optical transitions of neighboring molecules is taken into account. The derived Bloch equations are solved in a continuum approximation using the Frenet–Serret coordinate system. Solutions are obtained that describe the formation of stable localized structures (solitons) due to the curvature of the chain, which can have a critical influence on the luminescent properties of the chiral medium.

DOI
10.3103/s8756699018040155

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The formation and dynamics of excitations in an atomic or molecular nano chain of a helical shape are studied. The dipole-dipole interaction between the circular polarizations of the optical transitions of neighboring molecules is taken into account. The derived Bloch equations are solved in a continuum approximation using the Frenet–Serret coordinate system. Solutions are obtained that describe the formation of stable localized structures (solitons) due to the curvature of the chain, which can have a critical influence on the luminescent properties of the chiral medium.
Abstract The dynamics of a field in a thin waveguide surrounded by helically arranged two-level atoms is studied. The interaction of the induced polarizations of atoms with the field propagating inside the waveguide is described by a system of reduced Maxwell–Bloch equations (RMBEs) in the approximation of unidirectional propagation of the field. Nonlocal dipole–dipole interaction (DDI) of the polarizations of atoms in the helix is described in the approximation of the interaction of nearest neighbors in a curvilinear medium. Soliton solutions of an integrable reduction of the system of equations describing the asymmetric propagation of field pulses in the waveguide in the forward and backward directions are found by the method of Riemann problem with zeros. It is shown that, depending on the chirality sign or the propagation direction, a field pulse in the waveguide can have either the shape of a sharp peak or a nearly rectangular shape. Solutions describing the evolution of field pulses on a nonzero pedestal show that the shape and amplitude of the field pulses can be controlled by the external pumping parameters.
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.
We consider crystal formation of particles with dipole-dipole interactions that are confined to move in a one-dimensional helical geometry with their dipole moments oriented along the symmetry axis of the confining helix. The stable classical lowest energy configurations are found to be chain structures for a large range of pitch-to-radius ratios for relatively low density of dipoles and a moderate total number of particles. The classical normal mode spectra support the chain interpretation both through structure and the distinct degeneracies depending discretely on the number of dipoles per revolution. A larger total number of dipoles leads to a clusterization where the dipolar chains move closer to each other. This implies a change in the local density and the emergence of two length scales, one for the cluster size and one for the inter-cluster distance along the helix. Starting from three dipoles per revolution, this implies a breaking of the initial periodicity to form a cluster of two chains close together and a third chain removed from the cluster. This is driven by the competition between in-chain and out-of-chain interactions, or alternatively the side-by-side repulsion and the head-to-tail attraction in the system. The speed of sound propagates along the chains. It is independent of the number of chains although depending on geometry.
The evolution of polarization of a light pulse in a system consisting of two-level atoms located on symmetric helices twisted into a bundle has been considered. The interaction of induced dipoles is taken into account by additional third order differential terms in Maxwell’s equations in the nearest neighbor approximation. An integrable generalization of the reduced Maxwell-Bloch equations has been derived under the conditions of almost unidirectional propagation. The analysis of the solutions obtained has shown that the evolution of field pulses critically depends on the direction of their propagation or on the chirality of the medium.
Abstract The possibility of mirrorless optical bistability in molecular aggregates with dipole–dipole interaction was studied theoretically. Dimers and ordered layers of the Langmuir–Blodgett type or J‐aggregates consisting of identical polyatomic fluorescent molecules were chosen as molecular aggregate models. The investigation was carried out on the basis of the Bogolubov equation chain for the density matrix of molecular aggregates and its components. It is shown that the fluorescence intensity and absorption ability are described by bistable and hysteresislike behaviour when the parameters of the molecular aggregate and pumping radiation are chosen suitably. An analysis of the bistability mechanism in such aggregates is given. Possibilities of controlling one light beam by another and switching polychromatic light flow by using a bistable device based on molecular aggregates are discussed.
A theory of resonant interaction with radiation of J aggregates based on chains of two-level molecules coupled by the retarded dipole‒dipole interaction has been developed. The effect of pair correlations between the chain molecules on the bistable response of this system under the influence of external resonant radiation has been investigated within the homogeneous chain model. Traditionally, these systems have been described using single-particle density matrices corresponding to each molecule. In this description, twoparticle interactions are represented in the factorized form and do not include correlations between the interacting molecules. In this study, the correlation corrections have been estimated taking into account the influence of only the nearest neighbors, while their values have found to be of the same order of magnitude as the factorized two-particle expectation values for which these corrections have been calculated. As a result, the dipole‒dipole interaction of a particular molecule with the nearest neighbors is so strong that the description of this interaction in the factorized form becomes inappropriate and can be used only for a qualitative analysis of the response of the chain. In order to obtain correct quantitative characteristics, it is necessary to abandon the factorization of the two-particle expectation values, at least for nearest neighbors.