Delocalized Exciton States Research Articles

Exciton transport in a moiré potential: From hopping to dispersive regime

The propagation of excitons in transition metal dichalcogenide (TMD) monolayers has been intensively studied revealing interesting many-particle effects, such as halo formation and nonclassical diffusion. Initial studies have investigated how exciton transport changes in twisted TMD bilayers, including Coulomb repulsion and Hubbard-like exciton hopping. In this work, we investigate the twist-angle dependent transition of the hopping regime to the dispersive regime of effectively free excitons. Based on a microscopic approach for excitons in the presence of a moir\'e potential, we show that the hopping regime occurs up to an angle of approximately ${2}^{\ensuremath{\circ}}$ and is well described by a moir\'e intercell tunneling model. At large angles, however, this nearest-neighbor approximation fails due to increasingly delocalized exciton states. Here, the quantum-mechanical dispersion of free particles with an effective mass determines the propagation of excitons. Overall, our work provides microscopic insights into the character of exciton propagation in twisted van der Waals heterostructures.

Strain-Enhanced Formation of Delocalized Exciton States in Phthalocyanine Crystalline Thin Films

Strain-Enhanced Formation of Delocalized Exciton States in Phthalocyanine Crystalline Thin Films

Scaling relations of exciton diffusion in linear aggregates with static and dynamic disorder.

Exciton diffusion plays an important role in many opto-electronic processes and phenomena. Understanding the interplay of intermolecular coupling, static energetic disorder, and dephasing caused by environmental fluctuations (dynamic disorder) is crucial to optimize exciton diffusion under various physical conditions. We report on a systematic analysis of the exciton diffusion constant in linear aggregates using the Haken-Strobl-Reinekermodel to describe this interplay. We numerically investigate the static-disorder scaling of (i) the diffusion constant in the limit of small dephasing rate, (ii) the dephasing rate at which the diffusion is optimized, and (iii) the value of the diffusion constant at the optimal dephasing rate. Three scaling regimes are found, associated with, respectively, fully delocalized exciton states (finite-size effects), weakly localized states, and strongly localized states. The scaling powers agree well with analytically estimated ones. In particular, in the weakly localized regime, the numerical results corroborate the so-called quantum Goldilocks principle to find the optimal dephasing rate and maximum diffusion constant as a function of static disorder, while in the strong-localization regime, these quantities can be derived fully analytically.

Realization of high-quality RF sputtered ZnMgO (x=15%) thin films by post-growth annealing treatment

ZnMgO, a direct wide-bandgap material with high exciton binding energy, has received widespread research attention in optoelectronics. However, its inherent n-type doping facilitates defect-related visible emission, which hampers the required luminescence properties in the UV region. Also, ZnMgO requires post-growth treatment which releases accumulated strain energy, modulates defect states, expands crystallite size, and augments ad-atoms interdiffusion, thereby affecting its bandgap. This work reports the influence of post-deposition annealing parameters like temperature and time on RF sputtered Zn0.85Mg0.15O thin films using rapid thermal annealing. The study showed that high temperature annealing aided Mg incorporation in ZnO lattice together with an increase in the grain size due to aggregation of grains. Highest activation energy, 78 meV, and narrowest line width, 34 meV, for D°X peak of 900 °C annealed film was obtained from photoluminescence study. Increasing annealing temperature also showed dissociation of bound exciton peaks into corresponding localized and delocalized exciton states.

Multiscale simulations of singlet and triplet exciton dynamics in energetically disordered molecular systems based on many-body Green's functions theory

We present a multiscale model based on many-body Green’s functions theory in the GW approximation and the Bethe–Salpeter equation (GW-BSE) for the simulation of singlet and triplet exciton transport in molecular materials. Dynamics of coupled electron–hole pairs are modeled as a sequence of incoherent tunneling and decay events in a disordered morphology obtained at room temperature from molecular dynamics. The ingredients of the rates associated to the events, i.e. reorganization energies, site energies, lifetimes, and coupling elements, are determined from a combination of GW-BSE and classical polarizable force field techniques. Kinetic Monte Carlo simulations are then employed to evaluate dynamical properties such as the excitonic diffusion tensor and diffusion lengths. Using DCV5T-Me(3,3), a crystalline organic semiconductor, we demonstrate how this multiscale approach provides insight into the fundamental factors driving the transport processes. Comparing the results obtained via different calculation models, we investigate in particular the effects of charge-transfer mediated high exciton coupling and the influence of internal site energy disorder due to conformational variations. We show that a small number of high coupling elements indicative of delocalized exciton states does not impact the overall dynamics perceptively. Molecules with energies in the tail of the excitonic density of states dominate singlet decay, independent of the level of disorder taken into account in the simulation. Overall, our approach yields singlet diffusion lengths on the order of 10 nm as expected for energetically disordered molecular materials.

Broadband Semiconductor Light Sources Operating at 1060 nm Based on InAs:Sb/GaAs Submonolayer Quantum Dots

In this paper, we investigate the potential of submonolayer-grown InAs:Sb/GaAs quantum dots as active medium for opto-electronic devices emitting in the 1060 nm spectral range. Grown as multiple sheets of InAs in a GaAs matrix, submonolayer quantum dots yield light-emitting devices with large material gain and fast recovery dynamics. Alloying these structures with antimony enhances the carrier localization and red shifts the emission, whereas dramatically broadening the optical bandwidth. In a combined experimental and numerical study, we trace this effect to an Sb-induced bimodal distribution of localized and delocalized exciton states. While the former do not participate in the lasing process, they give rise to a bandwidth broadening at superluminescence operation and optical amplification. Above threshold laser properties like gain and slope efficiency are mainly determined by the delocalized fraction of carriers.

Liberation of Charge Carriers by Optical Pumping Excitons in Poly(3-hexylthiophene) Aggregates

In conjugated polymers used in photovoltaics, charges may be produced on ultrafast time scales without requiring exciton diffusion to a donor-acceptor interface. To investigate the role of high-energy, delocalized exciton states in charge generation within polymer domains, we apply a pump-push-probe transient absorption technique to pristine poly(3-hexylthiophene) nanoparticles. The near-infrared push pulse induces exciton dissociation through the S-3 <- S-1 electronic transition, which is predicted to show intra-molecular charge-transfer character. We suggest that the spatial extent of the high-energy exciton, which induces electron-hole separation, is sufficient to overcome the intrinsic Coulombic attraction of the electron-hole pair. We observe that similar to 10% of the pushed excitons undergo dissociation to form free charges. The kinetics of charge recombination indicate that the electron and hole are separated by a distance of similar to 3 nm across the polymer domains.

Theoretical Model of Exciton States and Ultrafast Energy Transfer in Heliobacterial Type I Homodimeric Reaction Center.

A simple theoretical model of exciton dynamics was proposed to interpret the fast excitation energy-transfer process in the type I homodimeric reaction center of Heliobacterium modesticaldum (hRC); this structure was recently identified and shown to resemble that of the plant/cyanobacterial photosystem I (PSI) reaction center. The exciton state model, which mainly relies on the geometries of 54 bacteriochlorophyll (BChl) g, 4 BChl- g', and 2 chlorophyll (Chl) a on hRC and assumes constant site energy values for the pigments, reproduced the absorption spectrum of hRC rather well. The model also enabled numerical analysis of the exciton dynamics on hRC, which can be compared with the decay-associated spectra obtained by the laser spectroscopy experiments. The model indicates that the stronger transition-dipole moment on BChl- g contributes to the faster energy transfer due to the higher coherency of the delocalized exciton states on hRC compared to that on PSI that arranges Chl- a at almost homologous locations.

Reconstruction of exciton wave functions of coupled quantum emitters including spin with ultrafast spectroscopy using localized nanooptical fields

Coulomb-induced resonance energy transfer mechanisms between coupled nanostructures lead to the formation of new, delocalized exciton states. Their hybrid wave functions can be decomposed into the basis of local states of the uncoupled system by the use of coherent, spatially resolved spectroscopy. The suggested quantum state tomography protocol combines nanooptical fields with a four-wave mixing technique: At least one pulse of the pulse sequence is spatially localized at a specific quantum emitter. It was suggested to use nanoplasmonic structures together with pulse-shaped fields for the localization. In this paper, the method is applied to a system of two coupled semiconductor quantum dots. The basic reconstruction concept first proposed in Richter et al. (Phys Rev B 86:085308, 2012) and Schlosser et al. (New J Phys 15:025004, 2013) is extended to the case of including different spin states of the excitons in the quantum dots. For this purpose, the theoretical scheme has to be modified and the localized fields need the ability to change their polarization. We show that the application of the developed reconstruction scheme to two-dimensional spectra gives full access to the internal structure of the interacting quantum states.

Intrachain exciton dynamics in conjugated polymer chains in solution.

We investigate exciton dynamics on a polymer chain in solution induced by the Brownian rotational motion of the monomers. Poly(para-phenylene) is chosen as the model system and excitons are modeled via the Frenkel exciton Hamiltonian. The Brownian fluctuations of the torsional modes were modeled via the Langevin equation. The rotation of monomers in polymer chains in solution has a number of important consequences for the excited state properties. First, the dihedral angles assume a thermal equilibrium which causes off-diagonal disorder in the Frenkel Hamiltonian. This disorder Anderson localizes the Frenkel exciton center-of-mass wavefunctions into super-localized local exciton ground states (LEGSs) and higher-energy more delocalized quasi-extended exciton states (QEESs). LEGSs correspond to chromophores on polymer chains. The second consequence of rotations-that are low-frequency-is that their coupling to the exciton wavefunction causes local planarization and the formation of an exciton-polaron. This torsional relaxation causes additional self-localization. Finally, and crucially, the torsional dynamics cause the Frenkel Hamiltonian to be time-dependent, leading to exciton dynamics. We identify two distinct types of dynamics. At low temperatures, the torsional fluctuations act as a perturbation on the polaronic nature of the exciton state. Thus, the exciton dynamics at low temperatures is a small-displacement diffusive adiabatic motion of the exciton-polaron as a whole. The temperature dependence of the diffusion constant has a linear dependence, indicating an activationless process. As the temperature increases, however, the diffusion constant increases at a faster than linear rate, indicating a second non-adiabatic dynamics mechanism begins to dominate. Excitons are thermally activated into higher energy more delocalized exciton states (i.e., LEGSs and QEESs). These states are not self-localized by local torsional planarization. During the exciton's temporary occupation of a LEGS-and particularly a quasi-band QEES-its motion is semi-ballistic with a large group velocity. After a short period of rapid transport, the exciton wavefunction collapses again into an exciton-polaron state. We present a simple model for the activated dynamics which is in agreement with the data.

Fluorescence depolarization in poly[2-methoxy-5-((2-ethylhexyl)oxy)-1,4-phenylenevinylene]: Sites versus eigenstates hopping

We report upon a theoretical study of singlet exciton migration and relaxation within a model conjugated polymer chain. Starting from poly[2-methoxy-5-((2-ethylhexyl)oxy)-1,4-phenylenevinylene] polymer chains, we assume that the pi-conjugation is disrupted by conformational disorder of the chain itself, giving rise to a localized Frenkel exciton basis. Electronic coupling between segments as determined by the coupling between the transition densities of the localized excitons gives rise to delocalized exciton states. Using a kinetic Monte Carlo approach to compute the exciton transfer kinetics within the manifold of either the dressed chromophore site basis or dressed eigenstate basis, we find that the decay of the polarization anisotropy of the exciton is profoundly affected by the delocalization of the exciton over multiple basis segments. Two time scales emerge from the exciton migration simulations: a short, roughly 10 ps, time scale corresponding to rapid hopping about the initial excitation site followed by a slower, 180 ps, component corresponding to long range hopping. We also find that excitations can become trapped at long times when the hopping rate to lower-energy states is longer than the radiative lifetime of the exciton.

Radiative and phonon-induced dephasing in double quantum dots

A simple method for describing the evolution of a quantum state of a double quantum dot system interacting simultaneously with the electromagnetic environment and with the lattice modes is developed. It is shown that the joint action of the two reservoirs leads to nontrivial effects in the system dephasing. As an example, the impact of phonon-induced initial dephasing on the radiative decay of delocalized exciton states is discussed.

Coexistence of free and self‐trapped excitons in disordered J‐aggregates

AbstractCoexistence of free and self‐trapped exciton in amphi‐PIC J‐aggregates is proved by the numerous experimental data presented and analyzed. The self‐trapping states are shown to originate from delocalized exciton states exhibiting collective nature of polaronic relaxation. The strong correlation between the efficiency of exciton self‐trapping and static disorder degree in J‐aggregates has been shown. In amphi‐PIC J‐aggregates the polaronic state formation found out to require the overcoming an energy barrier which appears for quasi‐one‐dimension system as a result of combination of one‐dimensional exciton motion and three‐dimension lattice deformation. (© 2006 WILEY‐VCH Verlag GmbH &amp; Co. KGaA, Weinheim)

Heterogeneous Exciton Dynamics Revealed by Two-Dimensional Optical Spectroscopy

We show that optical two-dimensional (2D) spectroscopy can recover ultrafast heterogeneous dynamics of closely spaced delocalized exciton states from a molecular exciton manifold characterized by a single absorption band. The complete experimental third-order nonlinear optical response from room-temperature J-aggregates in liquid phase is reproduced for the first time with self-consistent Frenkel exciton theory combined with modified Redfield theory. We show that exciton relaxation between the exciton states and nuclear-motion-induced exchange-narrowed energy fluctuations of individual delocalized exciton states can be distinguished because these two processes lead to a distinctively different evolution of the absolute 2D spectrum. Our technique also allows recovery of the variation of the exciton relaxation rates as well as the degree of exciton delocalization across the absorption band.

Energy Transfer in Unsymmetrical Phenylene Ethynylene Dendrimers

We have applied the fluorescence upconversion technique to explore the electronic excitation energy transfer in unsymmetrical phenylene ethynylene dendrimers. Steady-state emission spectra show that the energy transfer from the dendrons to the core is highly efficient. Ultrafast time-resolved fluorescence measurements are performed at various excitation wavelengths to explore the possibility of assigning absorption band structures to exciton localizations. We propose a kinetic model to describe the time-resolved data. Independent of the excitation wavelength, a typical rise-time value of 500 fs is measured for the fluorescence in the dendrimer without an energy trap, indicating initial delocalized excitation. While absorption is into delocalized exciton states, emission occurs from localized states. When an energy trap such as perylene is introduced on the dendrimer, varying the excitation wavelength yields different energy-transfer rates, and the excitation energy migrates to the trap through two channels. The interaction energy between the dendrimer backbone and the trap is estimated to be 75 cm(-1). This value is small compared to the vibronic bandwidth of the dendrimer, indicating that the monodendrons and the energy trap are weakly coupled.

Co-existence of free and self-trapped excitons in J-aggregates

Nature of excited electronic states of amphi-PIC J-aggregates, which are the source of the self-trapping states, have been investigated using low-temperature site-selective, time-resolved spectroscopy techniques. The self-trapping states are shown to evolve from the delocalized exciton states within the J-band. The strongly localized electronic states located on the low-frequency edge of the J-band, are not able to form polaronic states and, hence, the polaronic relaxation process is particularly collective one. The exciton self-trapping is more effective in J-aggregates with strong disorder, requires overcoming a self-trapping barrier.

An exciton in a quantum well in the presence of crossed electric and magnetic fields

An analytical approach to the problem of the Wannier–Mott exciton in a semiconductor quantum well (QW) in the presence of external magnetic and electric fields is developed. The magnetic field is taken to lie in the heteroplanes while the electric field is directed perpendicular to the heteroplanes. Explicit dependencies of the energy levels and wave-functions of the exciton on the magnitudes of the fields for a wide range of the width of the QW are obtained. For the narrow QW, the results are valid for arbitrary electron and hole effective masses. In the case of intermediate and wide QWs, the adiabatic approximation implying the extreme difference of the electron and hole masses is used. In the intermediate QW, the states of the relative motion are the standard Coulomb states affected by the external fields while the states of the centre of mass are the size-quantized states in the QW. We focus particularly on the delocalized states caused by the external electric field and the motion of the excitons centre of mass in the magnetic field. These states are localized far away from the Coulomb centre. A strong influence of the boundaries of the wide QW on the delocalized exciton states is found to occur. Estimates of the expected values are made using typical parameters associated with GaAs QW.

Ultrafast exciton relaxation in the B850 antenna complex of Rhodobacter sphaeroides.

Spectral changes were measured with femtosecond resolution following low-intensity, broad-band excitation of the peripheral antenna complex of the purple photosynthetic bacterium Rhodobacter sphaeroides. Absorption anisotropy decays also were measured. We identified a 35-fs relaxation of the absorption and emission spectra of the excited state, as well as a 20-fs anisotropy decay. We interpret these results as interlevel relaxation and dephasing, respectively, of extensively delocalized exciton states of the circular bacteriochlorophyll aggregate.

Exciton Delocalization Length in the B850 Antenna of Rhodobacter sphaeroides

The properties of elementary excited states in the B850 band of the peripheral light-harvesting antenna (LH2) of the photosynthetic purple bacterium Rhodobacter sphaeroides has been studied at room temperature by means of femtosecond transient absorption experiments combined with computer simulations. Polarized pump−probe kinetics have a fast component of 100 and 65 fs for the anisotropic and isotropic decays, respectively. Direct numerical simulations show that for incoherent hopping-like excitation transfer in the B850 ring of 18 Bchl a molecules at room temperature the fast component of the anisotropy decay is 3 times longer than the corresponding component of the isotropic decay, strongly suggesting that delocalized exciton states are involved in the observed dynamics. To estimate the coherence length of the exciton we have measured absorption difference spectra of LH2 from 810 to 880 nm 2 ps after the excitation into the B800 band with 75 fs laser pulses. Exciton calculations where also monomeric dou...

Free-carrier effect on exciton dynamics in GaAs/AlxGa1-xAs quantum wells.

We report on a strong modulation of the resonantly excited (e1:hh1)1S exciton photoluminescence and LO-phonon Raman scattering by an additional, weak, above-gap radiation at low temperatures. These effects are attributed to the interaction between excitons and cold, free carriers (in the e-h pair density range of ${10}^{5}$n${10}^{7}$ ${\mathrm{cm}}^{\mathrm{\ensuremath{-}}2}$) that causes an exciton population redistribution and an increased dephasing rate. The analysis indicates that localized and delocalized exciton states span the same spectral range. It also yields the spectral dependence of the additional exciton dephasing that is due to the interaction with free carriers.