Boris D. Fainberg

Light-induced current in molecular tunneling junctions excited with intense shaped pulses

A theory for light-induced current by strong optical pulses in molecular tunneling junctions is described. We consider a molecular bridge represented by its highest occupied and lowest unoccupied levels. We take into account two types of couplings between the molecule and the metal leads: electron transfer that gives rise to net current in the biased junction and energy transfer between the molecule and electron-hole excitations in the leads. Using a Markovian approximation, we derive a closed system of equations for the expectation values of the relevant variables: populations and molecular polarization that are binary, and exciton populations that are tetradic in the annihilation and creation operators for electrons in the molecular states. We have proposed an optical control method using chirped pulses for enhancing charge transfer in unbiased junctions where the bridging molecule is characterized by a strong charge-transfer transition. An approximate analytical solution of the resulting dynamical equations is supported by a full numerical solution. When energy transfer between the molecule and electron-hole excitations in the leads is absent, the optical control problem for inducing charge transfer with a linearly chirped pulse can be reduced to the Landau-Zener transition to a decaying level. When the chirp is fast with respect to the rate of the electron transfer, the Landau theory is recovered. The proposed control mechanism is potentially useful for developing optoelectronic single-electron devices with optical gating based on molecular nanojunctions.

Light-induced current in molecular junctions: Local field and non-Markov effects

We consider a two-level system coupled to contacts as a model for charge pump under external laser pulse. The model represents a charge-transfer molecule in a junction, and is a generalization of previously published results [B. D. Fainberg, M. Jouravlev, and A. Nitzan. Phys. Rev. B 76, 245329 (2007)]. Effects of local field for realistic junction geometry and non-Markov response of the molecule are taken into account within finite-difference time-domain (FDTD) and on-the-contour equation-of-motion (EOM) formulations, respectively. Our numerical simulations are compared to previously published results.

Resonance heterodyne optical Kerr spectroscopy of solvation dynamics in water and D2O

Abstract We have used the technique of time resolved resonance heterodyne optical Kerr effect to measure the ultrafast solvation dynamics of rhodamine 800 (R800) and 3,3′-diethylthia-tricarbocyanine bromide (DTTCB) in water and D2O. We find a significant isotope effect in the picosecond range for R800 but we do not reveal it for DTTCB. We explain the R800 results by a specific solvation due to formation (breaking) of an intermolecular solute-solvent hydrogen bond. Another important part of the experimental data is a bimodal solvation correlation function with an ultrafast femtosecond component

Compensation of Coulomb Blocking and Energy Transfer in the Current Voltage Characteristic of Molecular Conduction Junctions

We have studied the influence of both exciton effects and Coulomb repulsion on current in molecular nanojunctions. We show that dipolar energy-transfer interactions between the sites in the wire can at high voltage compensate Coulomb blocking for particular relationships between their values. Tuning this relationship may be achieved by using the effect of plasmonic nanostructure on dipolar energy-transfer interactions.

Coherent charge transport through molecular wires: Exciton blocking and current from electronic excitations in the wire

We consider exciton effects on current in molecular nanojunctions, using a model comprising a two twolevel sites bridge connecting free-electron reservoirs. Expanding the density operator in the many-electron eigenstates of the uncoupled sites, we obtain a 16 16 density matrix in the bridge subspace whose dynamics is governed by Liouville equation that takes into account interactions on the bridge as well as electron injection and damping to and from the leads. Our consideration can be considerably simplified by using the pseudospin description based on the symmetry properties of Lie group SU2. We study the influence of the bias voltage, the Coulomb repulsion, and the energy-transfer interactions on the steady-state current and, in particular, focus on the effect of the excitonic interaction between bridge sites. Our calculations show that in case of noninteracting electrons this interaction leads to reduction in the current at high voltage for a homodimer bridge. In other words, we predict the effect of “exciton” blocking. The effect of exciton blocking is modified for a heterodimer bridge and disappears for strong Coulomb repulsion at sites. In the latter case the exciton type interactions can open new channels for electronic conduction. In particular, in the case of strong Coulomb repulsion, conduction exists even when the electronic connectivity does not exist.

Solvation dynamics of LDS 750 in associative liquids by degenerate fout-wave mixing and time-resolved emission techniques

Abstract We developed theoretically and experimentally the principles of a spectroscopical method based on resonance transient population gratings for a quantitative description of solvation dynamics of large molecules in liquied solutions. The solvation dynamics of LDS 750 in methanol, 1,2-ethanediol, 1,3-propanediol and 1,4-butanediol have been measured over four time decades from 100 fs to 1000 ps. The solvation dynamics of LDS 750 in all solvents consists of ultrafast as well as slow components.

Absorption spectrum of intense chirped pulse by molecules in solution and the time evolution of vibrationally non-equilibrium populations

Abstract We have calculated the absorption spectrum of an intense chirped pulse exciting a solute molecule in a solvent. In general it depends on both the real and imaginary part of the susceptibility (a phase-dependent absorption in the non-stationary media). We have shown that the absorption spectrum directly reflects the time evolution of a vibrationally non-equilibrium population difference in the ground and excited electronic states at the configuration coordinate corresponding to instantaneous Franck–Condon transition, when measured using high-power and strongly chirped pulses. A method has been proposed for extracting this time evolution from the measured absorption spectrum.

Solvation dynamics of rhodamine 800 in water and D2O

Copyright (c) 1997 Elsevier Science B.V. All rights reserved.Time-resolved resonance heterodyne optical Kerr-effect spectroscopy is employed to measure the solvation dynamics of rhodamine 800 in water and D 2 O. Unlike other large dye molecules, rhodamine 800 solvation dynamics in water exhibits a long-time component which we attribute to hydrogen-bond formation (breaking). We also find a rather unusual large isotope effect which does not exist in other dyes. An important part of the experimental data is a bimodal solvation correlation function with an ultrafast femtosecond component <100 fs.

Theory of energy transfer interactions near sphere and nanoshell based plasmonic nanostructures

Theory of energy transfer interactions between a pair of two level molecules in the molecular nanojunction including surface plasmon (SP) dressed interaction of plasmonic nanostructure, replicating metallic leads is presented. Results on the modification of bare dipolar interaction, known to be responsible for molecular energy transfer processes, in the proximity of metallic nanosystem are presented. Specifically, the manuscript includes theoretical investigation of nanosphere (NSP) monomer, nanoshell (NSH) monomer, and coupled nanosphere pair (dimer) based nanosystems. Closed form analytical expressions for NSP and NSH structures tailored for molecular nanojunction geometry are derived in the theoretical framework of multipole spectral expansion (MSE) method, which is straightforwardly extendible to dimers and multimers. The role of size and dielectric environment on energy transfer is investigated and interpreted. Theory predicts that the monomer and dimer both enhance the dipolar interaction, yet, dimer geometry is favorable due to its spectral tuning potential originated from plasmon hybridization and true resemblance with typical molecular nanojunctions.

Time resolved spectroscopy of nonlinear solvation with pulses longer than electronic dephasing

Abstract In this study we show that transient four-photon spectroscopy with pulses longer than the electronic transition dephasing, can be used for nonlinear solvation study, i.e., when the linear response for the solvation dynamics breaks down. We derive new equations describing the time evolution of the moments of the nonlinear optical spectra and in particular, time resolved fluorescence in the case of nonlinear solvation. The formulae allow to use stochastic models and the information concerning time resolved Stokes shift is preserved. For broad and featureless electronic molecular spectra they provide simple analytical expressions. We demonstrate how solvation dynamics in the ground electronic state is obtained by time resolved hole-burning spectroscopy. We apply our results to the Debye model of rotational diffusion for the case of nonlinear solvation and obtain a long-time solution for the corresponding density of the conditional probability in a strong electrical field.