THEORETICAL STUDY OF EXCITON–EXCITON CORRELATION EFFECT ON EXCITON MIGRATION IN MOLECULAR AGGREGATE

Two-exciton migration dynamics of a molecular aggregate is performed using the density matrix approach. Exciton–exciton correlation is shown to cause an oscillatory behavior in the one-exciton population dynamics. Such a feature is found to be well explained by the bypass transition from the ground to a one-exciton forbidden state via a two-exciton state.

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.

Excited-state dynamics in light-harvesting complex of Rhodobacter sphaeroides

A four-level model, involving the ground, one-exciton and two-exciton states as well as a high-lying molecular term through which the two-exciton state annihilates, is used for analyzing the bistable optical response of a thin film built up of oriented molecular aggregates. We focus on the effects of inhomogeneous broadening of the exciton optical transitions and exciton-exciton annihilation on bistability of the thin film response. It turns out that the inhomogeneous broadening, preventing generally the occurrence of bistability, may be suppressed considerably due to a fast exciton-exciton annihilation.

Comparison of density matrix and spectral simulation approaches for the calculation of isotopic selectivities

The question of how to treat free-carrier screening in the description of highly nonequilibrium carrier plasmas is still widely disputed. To further inquire into this problem we compare different techniques to derive generalized Boltzmann Bloch equations (BBEs) for optically excited semiconductor systems which account for both the coherent and incoherent dynamics. Mostly such BBEs have been obtained either within the framework of non-equilibrium Green’s functions (NEGFs) or by using the density matrix approach (DMA). A loose end of the latter approach has been the lack of a consistent way to treat free-carrier screening, and the conventional procedure has been to replace the bare Coulomb potential in the basic Hamiltonian by an appropriately screened one. We show that from the equation of motion (EoM) for the two-particle NEG F one can derive a simple prescription of how to treat free-carrier screening within the DMA. As an example we obtain the semiconductor Bloch equations for a homogeneous bulk semiconductor and we show that the ad-hoc implementation of screening within the DMA introduces errors.

The engineering and manipulation of delocalized molecular exciton states is a key component for artificial biomimetic light harvesting complexes as well as alternative circuitry platforms based on exciton propagation. Here we examine the consequences of strong electronic coupling in cyanine homodimers on DNA duplex scaffolds. The most closely spaced dyes, attached to positions directly across the double-helix from one another, exhibit pronounced Davydov splitting due to strong electronic coupling. We demonstrate that the DNA scaffold is sufficiently robust to support observation of the transition from the lowest energy (J-like) one-exciton state to the nonlocal two-exciton state, where each cyanine dye is in the excited state. This transition proceeds via sequential photon absorption and persists for the lifetime of the exciton, establishing this as a controlled method for creating two-exciton states. Our observations suggest that DNA-organized dye networks have potential as platforms for molecular logic gates and entangled photon emission based on delocalized two-exciton states.

One of the computational models proposed to study emerging phenomena in decentralized systems is the swarmalator model [Nat. Commun. 8, 1504 (2017)2041-172310.1038/s41467-017-01190-3], where only long-range interactions are considered. But in living systems many of the collective behaviors observed arise from the combined effect of long- and short-range interactions. In this work we present an extension to the swarmalator model which includes a Gaussian short-range repulsive term of the type 1/σe^{-|x|^{2}/σ} along with the results of numerical simulations. Using several order parameters we can distinguish between static and dynamic aggregations and between synchronous and asynchronous states. We have found six long-term collective states, some of them not previously reported and the most remarkable one showing oscillatory collective behavior. The results obtained show a multiplicity of complex behaviors that extend the applicability of the swarmalator model.

A hybrid approach consisting of two neural networks is used to model the oscillatory dynamical behavior of the Kuramoto‐Sivashinsky (KS) equation at a bifurcation parameter α = 84.25. This oscillatory behavior results from a fixed point that occurs at α = 72 having a shape of two‐humped curve that becomes unstable and undergoes a Hopf bifurcation at α = 83.75. First, Karhunen‐Loève (KL) decomposition was used to extract five coherent structures of the oscillatory behavior capturing almost 100% of the energy. Based on the five coherent structures, a system offive ordinary differential equations (ODEs) whose dynamics is similar to the original dynamics of the KS equation was derived via KL Galerkin projection. Then, an autoassociative neural network was utilized on the amplitudes of the ODEs system with the task of reducing the dimension of the dynamical behavior to its intrinsic dimension, and a feedforward neural network was usedto model the dynamics at a future time. We show that by combining KL decomposition and neural networks, a reduced dynamical model of the KS equation is obtained.

The relaxation mechanism and the strength of interaction in MBBA, EBBA, PAA and cholesteric liquid crystals have been theoretically studied using the density matrix approach to determine the physical basis. The exponent of the angular frequency (n) transforms the time independent transition rate to a time dependent transition rate and its value is around 0.54/spl plusmn/0.05. The ratio of the change in two time scales is related to a decrease in entropy and to the ordering of the molecular arrangement in the liquid crystals. The density matrix approach better explains the long time relaxation phenomenon and the singularity at n=0. This approach also gives a quantitative definition of the key parameter n. The time scale transformation under the density matrix approach is such that the causality principle of a Gaussian orthogonality ensemble random Hamiltonian is satisfied. An additional advantage is the use of 1-trick for cumulative perturbation to any desired long time relaxation. We found that there is no systematic variation of the key parameter versus temperature.

The optical Stark effect in a pump-probe setup is expected to show interesting additional features if the quantization of the pump field becomes important. A major effect is that the lineshape of a Stark-shifted resonance is strongly modified when squeezing the pump field. Furthermore, a probe gain is predicted here which does not appear in a semiclassical treatment. It appears for pump detunings considerably lower than the mean Rabi frequency. The nonclassical gain in the optical Stark effect is investigated on an ensemble of two-level systems (TLSs). A density matrix approach is presented which allows the accurate calculation of the probe absorption. The precise treatment of correlations between pairs of TLSs is crucial to explain details of the lineshape.

The effect of the excited two-exciton state on the transition from the ground state to the third molecular state is studied for a three-level molecular aggregate. Based on a Green function technique, the analytical expression is given for the line shape of pump–probe differential spectrum. A redshift peak of the transition from the ground state to the third state has been found because of introducing the coupling of the excited two-exciton states to the third state. Further, the dependence of the spectra on the aggregate length shows that the delocalization length of the exciton is decreased with an increase in the coupling strength. This result indicates that the coupling induces the exciton localization, leading to the reduction of the effective molecular number in the molecular aggregates.

We have performed femtosecond pump-probe spectroscopy measurements in 1,1’-diethyl- 3,3’-bis(4-sulfobutyl)-5,5’,6,6’-tetrachlorobenz imidazolocarbocyanine (also known as TDBC) J aggregates adsorbed onto silver colloidal surfaces. We show that the dependence on probe power and wavelength of the induced emission band dynamics, intensity, and position can only be explained by assuming stimulated emission from the one-exciton state. The stimulated emission originates from the amplification of the one-exciton state emission by an induced transition from the two-exciton state to the one-exciton state. One of the key causes of the stimulated emission is the formation of coherently coupled TDBC molecules on colloidal silver surfaces.

Oscillatory behavior is absolutely necessary for the normal functioning of various organisms and their performance. Therefore, it is necessary to protect the oscillatory behavior in an aging network which consists of oscillatory and nonoscillatory nodes. In this work, we investigate numerically and theoretically the effect of time delay on oscillatory behaviors in a network which includes active and inactive Stuart–Landau oscillators. Interestingly, we find a chimera oscillatory state where a part of oscillators is a steady state while other oscillators preserve oscillatory motion; such dynamical behaviors are considered generally to be impossible for globally coupled systems when the coupling strength is sufficiently large. Furthermore, our results reveal that time delay can effectively inhibit aging transition and recover the oscillatory behavior from the aging network.

The conceptual simplicity gained by the use of density matrices to describe the ground state of an electron gas moving in the field of a uniform background of positive charge (Sommerfeld model of a metal) is stressed. The diagonal element of the second-order density matrix (pair function) for a low-density electron gas has been discussed, previously byMarch andYoung [1], and it is shown, here that, to a similar degree of approximation, the first-order matrix and hence the momentum distribution may be obtained. In the high-density limit, where perturbation theory is valid,Daniel andVosko [2] have recently discussed the way in which the momentum distribution develops from the Fermi froms as the density is lowered (or the interaction switched on). A calculation which yields the pair function to the same degree of approximation is reported here, the results being obtained using Green’s functions, which are closely related to density matrices. In the light of the information thus available from the high and low density limits, the range of usefulness of the concept of the Fermi surface in an interacting electron gas is briefly discussed.