Exciton Wave Packet Research Articles

Frenkel exciton model of ultrafast excited state dynamics in AT DNA double helices

Recent ultrafast experiments have implicated intrachain base-stacking rather than base-pairing as the crucial factor in determining the fate and transport of photoexcited species in DNA chains. An important issue that has emerged concerns whether or not a Frenkel excitons is sufficient one needs charge-transfer states to fully account for the dynamics. Here we present an SU(2) ⊗ SU(2) lattice model which incorporates both intrachain and interchain electronic interactions to study the quantum mechanical evolution of an initial excitonic state placed on either the adenosine or thymidine side of a model B DNA poly(dA).poly(dT) duplex. Our calculations indicate that over several hundred femtoseconds, the adenosine exciton remains a cohesive excitonic wave packet on the adenosine side of the chain where as the thymidine exciton rapidly decomposes into mobile electron/hole pairs along the thymidine side of the chain. In both cases, the very little transfer to the other chain is seen over the time-scale of our calculations. We attribute the difference in these dynamics to the roughly 4:1 ratio of hole versus electron mobility along the thymidine chain. We also show that this difference is robust even when structural fluctuations are introduced in the form of static off-diagonal disorder.

Excitonic wave packet dynamics in semiconductor photonic-crystal structures

Significant aspects of the light--matter interaction can be strongly modified in suitably designed systems consisting of semiconductor nanostructures and dielectric photonic crystals. To analyze such effects, a microscopic theory is presented, which is capable of describing the optoelectronic properties of such hybrid systems via a self-consistent solution of the dynamics of the optical field and the photoexcitations of the material. The theory is applied to investigate the local excitonic resonances, which arise as a consequence of the modified Coulomb interaction in the vicinity of a structured dielectric medium. The excitation of a coherent superposition of the spatially inhomogeneous optical transitions induces an intricate wave packet dynamics. In the presence of dephasing and relaxation processes, the coherent oscillations are damped and the photoexcited carriers relax into spatially inhomogeneous quasi-equilibrium distributions.

Coherent control of Bloch oscillations by means of optical pulse shaping

We excite excitonic wave packets in biased semiconductor superlattices with spectrally shaped ultrashort optical pulses. We tailor the shape and phase of the pulse spectrum in order to control the coherent dynamics of the excitonic wave packets formed from a superposition of three excitonic states. Via careful shaping, we are able to excite either wave packets that exhibit standard Bloch oscillations (BO's) or breathing-mode (BM) motion. These two types of motion are characterized by the presence (BO) or absence (BM) of an internal intraband polarization caused by the electron-hole separation within the excitonic wave packet. The wave packet evolution is monitored using spectrally resolved four-wave mixing. This ability to control the BO's provides a way to control the emitted THz radiation.

Controlling excitonic wavepacket motion in the PS1 core-antenna system

Sub-picosecond laser pulse driven localization of electronic excitation energy is suggested for a biological chromophore complex. Based on an exciton model of the photosynthetic core antenna PS1 of Synechococcus elongatus the shape of the respective laser pulse is calculated using optimal control theory combined with a density matrix theory accounting for energy relaxation and dephasing caused by the protein environment. As a target observable we choose the population oscillation after photo-excitation between the two Chlorophylls forming the special pair. The temperature dependence of the related control yield is studied as well as its dependence on the pulses duration.

Terahertz generation by difference-frequency mixing of exciton Wannier–Stark ladder states in biased semiconductor superlattices

We report the generation of terahertz (THz) radiation in a biased semiconductor superlattice by difference-frequency mixing of interband transitions. The Wannier–Stark spectrum of the superlattice was selectively excited by two spectrally narrow laser lines. The emitted THz radiation was measured as a function of the energetic splitting and spectral position of the bichromatic excitation. The generation of tunable THz radiation is verified. The results clearly show enhanced THz emission when either continuum or 1s exciton wave packets are excited. Good agreement is obtained between the experiment and the results of an exciton model of the nonlinear coherent dynamics.

Ultrafast Laser Pulse Control of Exciton Dynamics: A Computational Study on the FMO Complex

Femtosecond laser-pulse-induced spatial localization of excitation energy in the FMO complex is suggested theoretically. Based on the Frenkel exciton model for the bacteriochlorophyll (BChl) Q y -excitations, the laser-pulse-driven exciton wave packet motion into a single, spatially localized Q y state is described in detail. First, the manner in which excitation energy relaxation and dephasing, as well as the length and overall intensity of the control pulse, determine the control yield is discussed. In a next step, two-exciton states decaying via exciton-exciton annihilation are included, and a preliminary attempt is undertaken to account for structural and energetic disorder. Although the latter effects may decrease the efficiency of excitation energy localization drastically, it is demonstrated that, even under realistic conditions, tailored laser pulses may deposit energy in a single BCh1 of the FMO complex.

Coherent Excitonic Wavepackets in Two-Dimensional SquareDot Arrays Driven by an In-Plane Uniform Electric Field

We investigate the evolution behaviour ofelectron–hole pair wavepacket in optically excited squarequantum-dot arrays driven by in-plane (x-y plane) uniform electricfield E (viz, E = Exex+Eyey, ex,ey are unit vectors along x and y directions respectively), in thetime domain. It is found that if the ratio of the x-componentelectric field Ex to the y-component electric fieldEy is a rational p/q (p and q being coprime integernumbers), the wavepackets undergo a time-periodic breathing mode,with the period 2πp/ωBx, where ωBx = eExa/ℏ,with a being the lattice constant of square dot arrays, ℏ beingPlanck's constant, e being the electron charge. This finding providesa time-domain demonstration of the recent spectral result [Phys. Rev.Lett. 86 (2001) 3116].

Nonlinear optical response of highly energetic excitons in GaAs: Microscopic electrodynamics at semiconductor interfaces

A spectroscopic investigation of bound electron-hole pairs in GaAs propagating with large center-of-mass momentum is presented. The approach is based on transmission experiments exploiting the coupling of excitonic polarization to the electromagnetic field of femtosecond laser pulses. The dispersion relations of the coherent excitations are determined up to excess energies of 300 meV above the band edge. A surprisingly low Frohlich coupling of light-hole excitons is observed in excellent agreement with theoretical simulations. The influence of different phonon scattering processes on the excitonic damping is discussed. The interaction dynamics of excitonic wave packets with nonthermal carrier distributions is analyzed with femtosecond time resolution. A theoretical model based on the exciton-polariton concept including additional boundary conditions reproduces our experimental results. The observations in extremely thin samples show deviations from our phenomenological model. This finding is important for a detailed understanding of the microscopic polarizability near semiconductor surfaces.

PROPAGATION PROPERTIES OF HIGH-DENSITY EXCITON POLARITONS

We investigate the time evolution of the exciton wave packet by an interacting exciton polariton model. The analysis is based on the equation-of-motion for the expectation value of exciton polariton operator with respect to the macroscopic quantum state. We find that the nonlinear interaction between exciton polariton leads to the extraordinary fast propagation of high-density exciton polaritons. This result helps the proper understanding of the experiments on the unusually fast propagation of exciton polaritons in strongly excited BiI 3.

Exciton Delocalization in the B850 Light-Harvesting Complex: Comparison of Different Measures

A much debated topic in recent studies of ultrafast exciton dynamics in molecular aggregates has been the precise definition of the exciton coherence domain in the B850 aggregate of photosynthetic purple bacteria. In this work we compare several measures of exciton delocalization and their variation with temperature, inhomogeneous broadening, and time. We find that the exciton wave packet prepared by a 40 fs pulse is initially delocalized over a substantial portion of the ring and contracts to a significantly smaller size on the time scale of a few hundreds of femtoseconds. Steady-state absorption and time-dependent pump−probe spectra of the B850 aggregate are simulated and compare well with previously reported experimental data. The calculated thermalized exciton wave packet size is in good agreement with previous experimental and theoretical estimates.

Exciton (De)Localization in the LH2 Antenna of Rhodobacter sphaeroides As Revealed by Relative Difference Absorption Measurements of the LH2 Antenna and the B820 Subunit

We report the results of relative difference absorption measurements for the LH2 antenna of Rhodobacter sphaeroides and the B820 subunit of Rhodospirillum rubrumat room temperature. It is shown that significant differences between shapes and amplitudes of photoinduced, absorption changes reflect a different degree of exciton delocalization in the intact antenna compared with the dimeric subunit. Using the exciton model in the presence of static disorder, we have obtained a consistent and quantitative fit of the amplitudes and shapes of the pump-probe spectra of the LH2 antenna and the B820 subunit. We estimate that the nearest-neighbors interaction energy in the antenna is about 400 cm -1 and the diagonal disorder is about 450 cm -1 . For these values the coherence length (FWHM) of the steady-state exciton wavepacket corresponds to 5 BChl molecules at room temperature. The amplitude of the difference absorption reflects a cooperative behavior within at least 12 BChls of the B850 antenna. The calculations suggest that the dimeric subunit is characterized by a decrease of the interaction energy to 300 cm -1 together with an increase of the disorder value to about 600 cm -1 .

Near-field dynamics of excitonic wave packets in semiconductor quantum wells

A model study of the near-field optical excitation and spatiotemporal dynamics of excitons in semiconductor quantum wells is presented. It is shown that the spatially and temporally localized excitation results in the interference of radiative and nonradiative excitonic contributions, which form a propagating wave packet. The influence of excitation conditions, boundaries, and surface roughness on the wave packet are discussed. The experimentally measurable optical far-field emission of the wave packet is calculated.

Disordered Exciton Model for the Core Light-Harvesting Antenna of Rhodopseudomonas viridis

In this work we explain the spectral heterogeneity of the absorption band (Monshouwer et al., 1995. Biochim. Biophys. Acta. 1229:373–380), as well as the spectral evolution of pump-probe spectra for membranes of Rhodopseudomonas ( Rps.) viridis. We propose an exciton model for the LH1 antenna of Rps. viridis and assume that LH1 consists of 24–32 strongly coupled BChl b molecules that form a ring-like structure with a 12- or 16-fold symmetry. The orientations and pigment-pigment distances of the BChls were taken to be the same as for the LH2 complexes of BChl a–containing bacteria. The model gave an excellent fit to the experimental results. The amount of energetic disorder necessary to explain the results could be precisely estimated and gave a value of 440–545 cm −1 (full width at half-maximum) at low temperature and 550–620 cm −1 at room temperature. Within the context of the model we calculated the coherence length of the steady-state exciton wavepacket to correspond to a delocalization over 5–10 BChl molecules at low temperature and over 4–6 molecules at room temperature. Possible origins of the fast electronic dephasing and the observed long-lived vibrational coherence are discussed.

Exciton Delocalization in the B808–866 Antenna of the Green Bacterium Chloroflexus aurantiacus as Revealed by Ultrafast Pump-Probe Spectroscopy

A model of pigment organization in the B808–866 bacteriochlorophyll a antenna of the green photosynthetic bacterium Chloroflexus aurantiacus based on femtosecond pump-probe studies is proposed. The building block of the antenna was assumed to be structurally similar to that of the B800–850 light-harvesting 2 (LH2) antenna of purple bacteria and to have the form of two concentric rings of N strongly coupled BChl866 pigments and of N/2 weakly coupled BChl808 monomers, where N=24 or 32. We have shown that the Qy transition dipoles of BChl808 and BChl866 molecules form the angles 43°±3° and 8°±4°, respectively, with the plane of the corresponding rings. Using the exciton model, we have obtained a quantitative fit of the pump-probe spectra of the B866 and B808 bands. The anomalously high bleaching value of the B866 band with respect to the B808 monomeric band provided the direct evidence for a high degree of exciton delocalization in the BChl866 ring antenna. The coherence length of the steady-state exciton wave packet corresponds to five or six BChl866 molecules at room temperature.

Excitonic wave packets observed in space- and time-resolved pump and probe experiments

We investigate the coherent time regime in space- and time-resolved pump and probe experiments on a symmetrically strained InGaAs/GaAsP multiple quantum well. The spatial width of the transmission profiles shows a modulation which reflects the dynamics of an excitonic wave packet. This spatio–temporal beating originates from the combined action of various signal components which are detected simultaneously because of the collinearity of our experiment. The experimental results are in qualitative agreement with calculations obtained with a simplified model on the Hartree–Fock level.

Nonlinear dynamics of excitons in asymmetric coupled quantum wells

In this work, we have numerically integrated in space and time the effective-mass Schrödinger equation for an excitonic wave packet in a coupled quantum well system. Considering an electronic many-body interaction and an external electric field, we have obtained a nonlinear dynamical evolution for the excitonic wave function. At large electronic sheet densities, it is found that the charge dynamically trapped in both wells produces a reaction field which modifies the system resonant condition.

Inflating antenna: Dynamics of exciton wave packets

A theory of the free-induction signal from biexcitons and bound excitons is presented. Simultaneous existence of the exciton continuum and a bound state is shown to result in a new type of the time dependence of the free induction. The optically detected signal increases in time and oscillates with increasing amplitude until damped by radiative and dephasing processes. The expanding area of coherent exciton polarization (inflating antenna) produced by the exciting pulse is the underlying physical mechanism. The formalism developed can be applied to various biexciton transients

Exciton ionization induced by an electric field in a strongly coupled GaAs/AlxGa1-xAs superlattice.

We study exciton ionization induced by an axial electric field in a strongly coupled GaAs/${\mathrm{Al}}_{0.3}$${\mathrm{Ga}}_{0.7}$As superlattice at low temperatures. The field-induced ionization times of the heavy-hole 1s exciton in the miniband field regime are determined from transient four-wave-mixing experiments and theoretical model calculations. They are found to decrease with increasing field, and lie in the picosecond and subpicosecond ranges for fields on the order of a few kV/cm. They are considerably longer than those calculated for a 1s exciton in bulk GaAs. We explain this difference as a result of the nonparabolicities in the superlattice miniband dispersion. In addition to studying the field-induced ionization of the 1s resonance, we also investigate the field dependence of excitonic wave packets composed of both bound and continuum states of the exciton. It is found that the continuum components of the wave packet are more sensitive to low electric fields than the 1s component. \textcopyright{} 1996 The American Physical Society.

Time-resolved optical investigations of bloch oscillations in semiconductor superlattices

We present a complementary study of the interband and intraband dynamics of optically excited Bloch oscillations in GaAs Al xGa 1−xAs superlattices. Distinct differences in the frequency and dephasing of the oscillations give evidence for Bloch oscillations performed by electrons in continuum states and by excitonic wavepackets. The dephasing time of the electronic continuum states is found to be exceptionally long as compared to the excitonic dephasing time under identical excitation conditions. These observations are confirmed by THz emission experiments, where Bloch oscillations are detected under optical excitation well above the fundamental band gap. The experimental observations suggest that the electronic coherence may be partially maintained during relaxation and momentum scattering processes.

Coherent dynamics of excitonic and biexcitonic wave packets in semiconductor superlattices

We report on the observation of an electric-field-induced electric-dipole moment of biexcitonic molecules in a GaAs/Al0.3Ga0.7As superlattice. The macroscopic oscillating electric field, associated with the microscopic wave packet dipole moments, is monitored by time-resolved transmittive electro-optic sampling. The field dependence of the biexcitonic binding energy is detected by transient four-wave mixing. From these experiments, an ultrafast non-resonant excitation mechanism of biexcitons is inferred.