Self-trapping of a wannier exciton in the optical phonon field

Stability for self-trapping of a Wannier exciton in the polarization field of optical phonons is studied by means of variational calculations based on the adiabatic approximation. The phase diagram of free and self-trapped states is presented for 1s, 2s and 2p excitons. It is shown that optical phonons can be responsible for self-trapping of 2s (2p) exciton as a trigger of self-trapping.     

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.     

Experimental evidence for exciton scaling effects in self-assembled molecular wires

Resonant Rayleigh scattering from self-assembled one-dimensional molecular J-aggregate wires reveals a distinct dependence of the exciton energy on the width of lateral extension. For the J aggregates used in this study, strong in-line dipole coupling leads to a delocalization of the exciton wave function over several molecular units. Polarization dependent measurements of resonantly scattered light from the wires show that the exciton dipole moment is oriented perpendicular to the long axis. The experimental observations can be described by applying a quantization condition to the center of mass motion of the J-band exciton in the wires.     

Specific features of amplification of almost ultimately short pulses in media with a permanent dipole moment

Amplification of electromagnetic field pulses approximately one cycle in duration in multilevel media with an effective permanent dipole moment is studied theoretically without application of the approximation of slowly varying envelopes for the case of unidirectional propagation of the radiation. For three-level and quasi-two-level media, it is shown that the set of Maxwell-Schrödinger equations can be reduced to an integrable set of equations for an effective two-level medium. The presence of an (effective) permanent dipole moment of the medium was found to lead to new specific features of the amplification and control of the dynamics of field pulses.     

Anomalous Stark shifts in single vertically coupled pairs of InGaAs quantum dots

Vertically coupled Stranski Krastanow quantum dots (QDs) are predicted to exhibit strong tunnelling interactions that lead to the formation of hybridised states. We report the results of investigations into single pairs of coupled QDs in the presence of an electric field that is able to bring individual carrier levels into resonance and to investigate the Stark shift properties of the excitons present. Pronounced changes in the Stark shift behaviour of exciton features are identified and attributed to the significant redistribution of the carrier wavefunctions as resonance between two QDs is achieved. At low electric fields coherent tunnelling between the two QD ground states is identified from the change in sign of the permanent dipole moment and dramatic increase of the electron polarisability, and at higher electric fields a distortion of the Stark shift is attributed to a coherent tunnelling effect between the ground state of the upper QD and the excited state of the lower QD.     

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.     

Solitary Frenkel Excitons in One-Dimensional Molecular Aggregates with Static Dipole Moment

It is shown theoretically that solitary Frenkel excitons in one-dimensional molecular aggregates can exist and propagate when the Frenkel exciton is accompanied by a static dipole moment along the aggregate axis.     

Real-time spectroscopy of Frenkel exciton system

We performed selective excitations of the first- and second-excited Frenkel exciton states (Q- and B-excitons) with use of sub-40-fs and sub-5-fs visible laser pulses. The transient absorption signals exhibit oscillatory features associated with molecular vibrations. From an analysis of the spectral profiles of the phase and amplitude of the oscillation, the experimental result is well explained by the modulated transition dipole moment, which is caused by the dynamic intensity borrowing from the intense B-transition to the weak Q-transition.     

Exciton Dynamics in a Helical Molecular Aggregate

The formation of excitons in a helical molecular nanochain is considered. The model equations are derived with allowance for a spatial dispersion of the polarization of optical transitions treated in the approximation of nearest-neighbor interaction and constant dipole moment of molecules. Models of the medium are formulated, which determine the formation of polarization domains. Some examples are considered, which show that a helical medium admits the formation of stable localized structures (solitons) due to the curvature of the molecular chain. These features of the nanosystem can produce a critical influence on the luminescent properties of the chiral medium.     

Exciton delocalization and superradiance in tetracene thin films and nanoaggregates

The structure and dynamics of luminescent excitons in tetracene thin films and nanoaggregates are investigated using time-resolved spectroscopy and theoretical calculations. The orientation of tetracene's transition dipole moment along its short molecular axis leads to properties qualitatively different from those observed in aggregates of phenylene-vinylene and thiophene oligomers, despite similar crystal structures. The spectral shape, temperature dependence, and radiative lifetime are consistent with a short-lived superradiant exciton delocalized over approximately 10 tetracene molecules.     

Exciton condensation in quantum wells: Temperature effects

Our previous theory of the formation of exciton condensed phases in a two-dimensional system is applied to the analysis of the temperature dependence of a number of parameters of the system. The pumping intensity-temperature phase diagram is constructed, some of its features characteristic of indirect excitons having a dipole moment are established, the dependences of the intensity of the radiation band corresponding to the condensed phase on the pumping intensity and temperature are calculated, and the results are compared with experimental data.     

Broadening of triplet-triplet absorption band in molecular structures with different spatial dimensionalities

Within the framework of a dipole approximation, the form of the light absorption band for exciton transitions between triplet zones in one-, two-, and three-dimensional periodic molecular structures is calculated. Allowance is made for exciton attenuation as a frequency-independent parameter. The analytical expressions obtained make it possible to analyze the band for molecular structures of different spatial dimensionalities as a function of the difference in bandwidths, attenuation, and lattice temperature. Institute of the Surface Chemistry, National Academy of Sciences of Ukraine, 31, Nauka Ave., Kiev 252022, Ukraine. Translated from Zhurnal Prikladnoi Spektroskopii, Vol. 64, No. 5, pp. 586–590, September–October, 1997.     

Semiempirical law for the dipole moments of low-molecular-weight gelators

Some small chiral organic molecules are capable of forming gel in low-concentration solutions. This phenomenon is of great theoretical interest, but its molecular details are still unclear. High chemical diversity of such gelators impedes the investigation of their common structural properties important for gel formation. Here, we calculated the total dipole moment for a set of gelators with sufficiently different chemical structures by means of molecular dynamics simulations. It was found that all considered molecules have a dipole moment of ～3 D or more. This means that the energy of the dipole-dipole interaction between two point dipoles at a distance of 6–8 Å is ～kT. This distance roughly corresponds to molecules in close contact. This makes it possible to conclude that the dipole moment can orient gelator molecules before aggregation, thereby playing a key role in the process of gelation. This orientation effect determines the anisotropy of aggregates and the gelation of the solution.     

Localized self-trapping in two-dimensional molecular lattice with interaction between Wannier-Mott excitons and phonon lattice

We investigate the interactions of lattice phonons with Wannier-Mott exciton, the exciton that has a large radius in two-dimensional molecular lattice, by the method of continuum limit approximation, and obtain that the self-trapping can also appear in two-dimensional molecular lattice with a harmonic and nonlinear potential. The exciton effect on molecular lattice does not distort the molecular lattice but only makes it localized and the localization can also react, again through phonon coupling, to trap the energy and prevents its dispersion.     

Mechanistic model for the dielectric spectrum of a simple dielectric material

ABSTRACT

In this paper, we perform molecular dynamics simulations of a dielectric fluidic material composed of permanent molecular dipoles. The dielectric spectrum features two peaks at lower frequencies than the system phonon frequency. The primary peak is observed at all temperatures studied and shifts toward lower frequencies as the temperature decreases. During this shift, the secondary peak emerges with a higher peak frequency than the primary peak. The secondary peak amplitude increases as the temperature decreases. Both peaks are dependent on the wavevector; in the small wavevector regime, the primary peak is shifted to higher frequencies as the wavevector squared and the secondary peak amplitude increases as the wavevector increases, but shows no shift in frequency. From the polarisation balance equation, we propose a model for the dielectric spectrum. This captures the spectrum features, and we conjecture that the primary peak is due to dipole moment correlations (Debye-type) and the secondary peak is due to the correlation between the dipole moment and a microscopic local field.     

Jahn-Teller effect at exciton self-trapping

In the adiabatic approximation two limiting cases of exciton self-trapping have been considered for a degenerate atomic term creating the bands of Frenkel excitons, which interact with all crystal deformations admitted by symmetry: a case of a large radius excitation on the self-trapping barrier and a small-radius case. In the first case wave functions and deformations have been found which correspond to the barrier, in the second case the adiabatic surface of self-trapping excitons have been obtained. It is shown that the interaction with non-totally symmetric deformations leads to a lowering of the crystal point symmetry in a state corresponding to the lowest self-trapping barrier, that is the manifestation of the Jahn-Teller effect applied to a self-trapping exciton.     

Spectral investigations on 2,3-bis(chloromethyl)-1,4- anthraquinone: solvent effects and host–guest interactions

Solvent effects on 2,3-bis(chloromethyl)-1,4-anthraquinone (DCMAQ) and the molecular recognition of DCMAQ in calix[8]arene were investigated using optical absorption and fluorescence emission techniques. Optical absorption spectra show n→π^* band in 350–500 nm region. It also indicates that the dipole–dipole interaction and solvent reorganization energies are responsible for the observed features in different solvents. The observed quantum yield of DCMAQ in different solvents is due to the formation of intermolecular hydrogen bond and reorientation of solvent molecule in the excited state of DCMAQ. Excited state dipole moment of DCMAQ is calculated by solvatochromic data and it shows a higher excited state dipole moment than ground state dipole moment. Optical absorption and fluorescence studies of DCMAQ in calix[8]arene elucidate the evidence for the formation of complex between DCMAQ and calix[8]arene. The inclusion ratios and inclusion constant of the host–guest complexes are also determined.     

Polarized photoluminescence spectroscopy in WS2, WSe2 atomic layers and heterostructures by cylindrical vector beams

Due to the large exciton binding energy, two-dimensional (2D) transition metal dichalcogenides (TMDCs) provide an ideal platform for studying excitonic states and related photonics and optoelectronics. Polarization states lead to distinct light-matter interactions which are of great importance for device applications. In this work, we study polarized photoluminescence spectra from intralayer exciton and indirect exciton in WS₂ and WSe₂ atomic layers, and interlayer exciton in WS₂/WSe₂ heterostructures by radially and azimuthally polarized cylindrical vector laser beams. We demonstrated the same in-plane and out-of-plane polarization behavior from the intralayer and indirect exciton. Moreover, with these two laser modes, we obtained interlayer exciton in WS₂/WSe₂ heterostructures with stronger out-of-plane polarization, due to the formation of vertical electric dipole moment.     

Adiabatic surface of self-trapping excitons

The problem of the self-trapping of the Frenkel exciton interacting with phonons is considered in the adiabatic approximation. The self-trapping is assumed to take place at a single site. The exciton scattering by phonons at this site and the nearest neighbouring sites is taken into account. Analytical expressions for the adiabatic surface of self-trapping excitons, in particular, for the barrier height separating almost free and self-trapped states, are found. The condition for the applicability of the model to the describing of the self-trapping barrier is determined. The adiabatic surface is shown to have several minima separated by a barrier, i.e. the coexistence of several types of self-trapped excitons is possible.Dedicated to Professor Miroslav Trlifaj on the occasion of his sixtieth birthday.     

Identifying multi‐excitons in quantum dots: the subtle connection between electric dipole moments and emission linewidths

The emission linewidths of excitonic complexes confined in quantum dots (QDs) mirror their interaction with a defect‐induced, fluctuating charge environment, a phenomenon known as spectral diffusion. Interestingly, extended excitonic complexes that comprise several interacting excitons exhibit significantly smaller emission linewidths if compared to the optical fingerprint of their building block, a sole exciton. Hence, it is not the absolute, but the relative electric dipole moment that governs the directly accessible emission linewidths. Exemplarily we investigate this matter based on differing exciton and biexciton emission linewidths of single GaN QDs with varying emission energies, i.e. QD dimensions. Our results establish the full width at half maximum (FWHM) or any other linewidths criterion for the identification of excitonic complexes, a technique that can directly be applied to polar but even non‐polar QD materials. Additionally, we find an emission energy dependent trend for the FWHM ratios of the biexciton and the exciton (XX_FWHM/X_FWHM) in perfect agreement with their relative dipole moment ratios as derived from our 8‐band‐ k · p based treatment of the Coulomb and exchange interaction within these multi‐particle complexes. (© 2015 WILEY‐VCH Verlag GmbH &Co. KGaA, Weinheim)