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

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.     

Optical bistability in thin molecular films

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.     

Methods for calculation of ir intensities in polyatomic molecules: A comparative study

Various methods for calculating intensities in ir spectra of polyatomic molecules are reviewed against the current theory of small vibrations of polyatomic molecules. The valence optical theory based on the representation of the molecular dipole moment as the sum of dipole moments of bonds has been shown to be erroneous. Of the two calculation procedures which use no assumptions on the molecular dipole moment, the Mayants-Averbukh method is shown to have certain advantages over the polar tensor method.     

Regular and chaotic dynamics of the dipole moment of square dipole arrays

Dipole lattices, which represent square dipole arrays, are investigated. Various types of equilibrium configurations of arrays are obtained, and conditions are shown under which these configurations are established. On the basis of parametric bifurcation diagrams, the main types of regular and chaotic oscillation regimes of the total dipole moment of a system are considered and their dependence on the amplitude, frequency, and polarization of an alternating field, as well as on the initial equilibrium configuration of arrays, is analyzed. Scenarios of the onset of chaotic regimes are demonstrated, including those that occur via the establishment and variation of quasiperiodic oscillations of the dipole moment of a system. The dynamic bistability state is revealed in which a stochastic resonance—an increase in the response of a system to a harmonic signal in the presence of noise—can be implemented.     

Optical bistability in nonlinear subwavelength metal hole array involving Kerr materials

We investigate an optical bistability phenomenon in a structure consisting of a silver film with a hole array filling with Kerr medium with single wavelength. We explore the mechanism of optical bistability in our structures and find that the excitation condition of surface plasmon at this moment is related with that at last moment. The contrast of optical bistability can reach to the maximum at the wavelength of the transmission peak. Our structure can realize the function of the beam splitter.     

利用带模型探讨染料环形腔系统的光学双稳特性

利用染料分子带模型，探讨了染料环形腔系统的光学双稳特性，得到光学双稳可在一个比较大的频率范围内实现；其双稳区域是染料分子带结构参数（带宽及偶极分布的Ｌｏｒｅｎｔｚｉａｎ宽度等）的函数． 关键词：     

Optically and thermally driven bistability of FH(OH−) centres in KBr: Structure of the bistable configurations

Abstract

Pairs of F centres and OH^? molecules (F_H(OH^?) centres) exhibit a very interesting optical and thermal bistability in KBr. The analysis of our Electron Nuclear Double Resonance (ENDOR) investigation yields structure models. In both bistable configurations the OH^? molecules reside on a next nearest anion site (4^th shell) relative to the F centre. The difference lies in the orientation of the OH^? dipole with respect to the defect pair axis. The thermal bistability will be explained tentatively as being entropy-driven.     

Dynamics of a magnetic moment of finite dipolar lattices in the AC field

Dynamic regimes of a magnetic moment of 2 × 2, 3 × 3, and 4 × 4 square dipole lattices in the linearly and circularly polarized ac magnetic field and the perpendicular static field have been investigated. The possibility of regular and chaotic precession dynamics of the magnetic moment of the lattices has been demonstrated. A shift of the main magnetic resonance owing to the dipole–dipole interaction and additional resonances have been revealed. Quasiperiodic regimes and bistability states with a metastable chaotic attractor have been discovered.     

Molecules with a permanent dipole moment in a periodic medium with quadratic nonlinearity

Nonlinear optical effects in a one-dimensional periodic medium containing spatially oriented molecules of J aggregates of a dye and silver nanoparticles are studied. It is shown that, in such composite medium, the presence of a permanent dipole moment of an exciton transition of molecules of J aggregates results in the appearance of new nonlinear optical properties.     

Effect on optical anisotropy of intramolecular lability in various molecular rotational configurations

It is shown that the effect of intramolecular lability in complex molecules on the degree of fluorescence polarization of rarified vapors is mainly mediated through perturbations of rotational motion in the region of separatrixes of A- and C-forms of rotation stereo dynamics. It is most pronounced for the orientation of optical transition dipole moment components along the axis of the molecule average moment of inertia (B) and manifests itself in all types of molecular tops. Such lability for dipole orientations along the A and C axes usually causes the degree of anisotropy to decrease by an order of magnitude and more. So it is advisable to use optical transitions with dipole moments oriented along the A or C axes for optical polarization analysis of molecules in the gaseous state, for example in analytical high-temperature polarized fluorescence.     

Equilibrium values and dynamics of the net magnetic moment of a system of magnetic dipoles

Equilibrium states of different systems formed by coupled spherical bodies with dipole magnetic moments have been investigated using a numerical analysis. The bistable states and the corresponding values of the net magnetic moment are determined for a number of planar and three-dimensional systems of dipoles, and the conditions providing the existence of orientational configurations of coupled dipoles involved in the bistability are analyzed. The disturbances of the magnetic moment due to the quasi-static passage of an additional dipole and the dynamic modes excited by a homogeneous alternating magnetic field and represented by periodic, quasi-periodic, and chaotic oscillations of the magnetic moment of the system are considered for several types of systems. The bifurcation diagrams of the dynamic modes are constructed, and the specific features typical of the systems under consideration are revealed.     

Theory of pressure-induced vibrational and rotational absorption of diatomic molecules colliding with atoms at high temperatures

A derivation is given for the integrated absorption coefficient of pressure-induced pure rotational and vibrational transitions in binary collisions of homonuclear diatomic molecules with neutral atoms. The previously neglected effects of excited vibrational states, mechanical anharmonicity, vibration-rotation interaction, transverse components of the dipole moment, and a higher order term in the expansion of the z component of the dipole moment are taken into account to obtain more accurate absorption coefficients at temperatures above 423 K. In the region of the fundamental wave number the excited vibrational states make more of a contribution to the absorption than their relative populations would indicate.     

Polar nano-rods under shear: From equilibrium to chaos

The orientational dynamics of rod-like particles with permanent (electric or magnetic) dipole moments in a plane Couette shear flow is investigated using mesoscopic relaxation equations combined with a generalized Landau free energy. The free energy contribution due to the coupling between average alignment and dipole orientation is derived on a microscopic basis. Numerical results of the resulting eight-dimensional dynamical system are presented for the case of longitudinal dipoles and thermodynamic conditions where the equilibrium state is a (polar or non-polar) nematic. Solution diagrams reveal presence of a large variety of periodic, transient chaotic, and chaotic dynamic states of the average alignment and dipole moment, respectively, appearing as a function of Deborah number and tumbling parameter. Compared to rods without dipoles we observe a significant preference of out-of-plane kayaking-tumbling states and, generally, a higher sensitivity to the initial conditions including bistability. We also demonstrate that the average (electric) dipole moment characterizing most of the observed states yields electrodynamic (magnetic) fields of measurable strength.     

Scaling and universality in the optics of disordered exciton chains

The joint probability distribution of exciton energies and transition dipole moments determines a variety of optical observables in disordered exciton systems. We demonstrate numerically that this distribution obeys a one-parameter scaling, originating from the fact that both the energy and the dipole moment are determined by the number of coherently bound molecules. A universal underlying distribution is found, which is identical for uncorrelated Gaussian disorder in the molecular transition energies or in the intermolecular transfer interactions. The universality breaks down for disorder in the transfer interactions resulting from variations in the molecular positions. We suggest the possibility to probe the joint distribution by means of single-molecule spectroscopy.     

On the transferability of atomic contributions to the optical rotatory power of hydrogen peroxide,methyl hydroperoxide and dimethyl peroxide

The chirality of molecules expresses itself, for example, in the fact that a solution of a chiral molecule rotates the plane of linear polarised light. The underlying molecular property is the optical rotatory power (ORP) tensor, which according to time-dependent perturbation theory can be calculated as mixed linear response functions of the electric and magnetic dipole moment operators. Applying a canonical transformation of the Hamiltonian, which reformulates the magnetic dipole moment operator in terms of the operator for the torque acting on the electrons, the ORP of a molecule can be partitioned into atomic and group contributions. In the present work, we investigate the transferability of such individual contributions in a series of small, chiral molecules: hydrogen peroxide, methyl hydroperoxide and dimethyl peroxide. The isotropic atomic or group contributions have been evaluated for the hydrogen, oxygen and carbon atoms as well as for the methyl group at the level of time-dependent density functional theory with the B3LYP exchange-correlation functional employing a large Gaussian basis set. We find that the atomic or group contributions are not transferable among these three molecules.     

Nonadiabatic effects in the phonon spectrum of metal compounds

It is shown that nonadiabatic corrections to the dispersion law of optical phonons in the region of small wave vectors in the case of branches, for which the vibration with a zero wave vector is not accompanied by the appearance of a dipole moment in the ionic lattice, are significant for all possible directions of the wave vector. If a dipole moment arises, nonadiabatic corrections reach a noticeable value only for the wave-vector directions that are almost perpendicular to the direction of the dipole moment.     

Generation of terahertz radiation in the regime of resonant-nonresonant optical rectification

The generation of broadband terahertz radiation in quadratically nonlinear crystals containing additional resonant impurities having a permanent dipole moment has been theoretically analyzed. It has been shown that the contributions to the intensity of the generated radiation from resonance and nonresonance mechanisms of optical rectification are nonadditive. In semiconductor nanostructures and systems of organic molecules, the energy efficiency of generation due to the resonance mechanism can reach about 0.001, which is much higher than the corresponding efficiencies of the currently widespread tilted front technique. The spectra of the input optical pulses exhibit a noticeable red shift due to the resonance-nonresonance rectification.     

Effect of driving laser intensity fluctuations on optical bistability of a system with large permanent-dipole molecules

The statistical properties of an optical bistable system consisting of two-level atoms with large permanent dipole moments contained in a Fabry-Perot cavity are investigated assuming the external laser intensity fluctuations to be the main source of noise. The probability transformation rule is applied to obtain the probability distribution for the transmitted intensity whereas the probability distribution for the incident intensity is assumed to be that of the partially coherent laser light. It turns out that in the optical bistability region two values of fluctuation parameters are possible for a given value of input intensity, which produces interesting hysteresis effects.     

Electro-optics of molecules

The natural vibrations and electro-optics of diatomic molecules are considered in the framework of the perturbation theory. The energy of anharmonic vibrations and the matrix of a dipole moment function are determined with allowance made only for nonzero perturbation contributions. In particular, the case of strong electro-optical anharmonicity of diatomic molecules (i.e., the situation where the dipole moment function significantly deviates from linear behavior) is analyzed in the second order of the perturbation theory. Within the introduced formalism of polynomials of the quantum numbers, the perturbation theory is extended to polyatomic molecules and expressions for the matrix elements of the dipole moment are derived in explicit form with due regard for the vibrational quantum numbers.     

Systematics of vibrational intensities for “diatomic” molecules: Approximate formulas

Several approximations are discussed which enable one to estimate electric dipole moment transition matrix elements for diatomic molecules based on a very limited amount of experimental data. For strongly polar molecules, a simplistic point-charge dipole model in which there is no vibrational charge flux predicts a simple relation between the permanent moment, M₀, and its derivative with respect to internuclear separation, M₁. This is found to be approximately valid for a surprisingly large number of polar molecules. Next, approximate matrix elements for the fundamental and overtone bands are presented for a linear dipole moment function. Finally, systematic trends for molecules having similar electronic structures are investigated and combined with the approximations discussed above in order to estimate the transition moments for several molecules of astrophysical interest. An extension to the ν₃ fundamental and overtone bands of CH₄ is suggestive that some of these ideas may be applicable to certain polyatomic molecules as well.