Exciton Distribution Research Articles

Core molecule dependence of energy migration in phenylacetylene nanostar dendrimers: Ab initio molecular orbital–configuration interaction based quantum master equation study

The core molecule dependence of energy (exciton) migration in phenylacetylene nanostar dendrimers is investigated using the ab initio molecular orbital (MO)-configuration interaction based quantum master equation approach. We examine three kinds of core molecular species, i.e., benzene, anthracene, and pentacene, with different highest occupied MO-lowest unoccupied MO (HOMO-LUMO) gaps, which lead to different orbital interactions between the dendron parts and the core molecule. The nanostars bearing anthracene and pentacene cores are characterized by multistep exciton states with spatially well-segmented distributions: The exciton distributions of high-lying exciton states are spatially localized well in the periphery region, whereas those of low-lying exciton states are done in the core region. On the other hand, for the nanostar bearing benzene core, which also has multistep exciton states, the spatial exciton distributions of low-lying exciton states are delocalized over the dendron and the core regions. It is found that the former nanostars exhibit nearly complete exciton migration from the periphery to the core molecule in contrast to the latter one, in which significant exciton distribution remains in the dendron parts attached to the core after the exciton relaxation, although all these dendrimers exhibit fast exciton relaxation from the initially populated states. It is predicted from the analysis based on the MO correlation diagrams and the relative relaxation factor that the complete exciton migration to the core occurs not only when the HOMO-LUMO gap of the core molecule is nearly equal to that of the dendron parts attached to the core (anthracene case) but also when fairly smaller than that (pentacene case), whereas the complete migration is not achieved when the HOMO-LUMO gap of the core is larger than that of the dendron parts (benzene case). These results suggest that the fast and complete exciton migration of real dendrimers could be realized by adjusting the HOMO-LUMO gap of the core molecule to be smaller than that of dendron parts, although there exist more complicated relaxation processes as compared to simple dendritic aggregate models studied so far.

Ultrafast tailoring of the exciton distribution in quantum wells

AbstractBy excitation with two, consecutive, picosecond pulses we demonstrate that the distribution and temperature of a photocreated exciton population in semiconductor quantum wells can be dynamically tailored. The changes in the exciton distribution result in abrupt alterations of the photoluminescence dynamics, in particular in the appearance of a sudden decrease in the heavy‐hole excitonic emission when the second pulse impinges upon the sample. We present a quasi‐equilibrium thermodynamical model that includes the energy distribution of excitons and electron–hole plasma, which quantitatively accounts for the sharp dips observed in the exciton emission dynamics. (© 2008 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Influence of Semiconductor Bands Bending on Exciton Photoluminescence

A theoretical model for calculation of the Wannier–Mott exciton distribution in semiconductor sample accounting for the energy bands bending near semiconductor surface is presented. It is shown that the exciton distribution essentially depends on the surface potential under certain sample surface and bulk parameters. Changing the surface potential value we can study the exciton photoluminescence from the illuminated surface and from a thin layer near this surface.

Theoretical investigation of terahertz optical sidebands in a single quantum dot

We present a theoretical investigation on the dynamics of excitons, excitonic distribution functions, and absorption spectra of a single quantum dot driven by terahertz electric fields using the femtosecond laser pulse. In particular, we observe optical sidebands at frequencies of $2n\ensuremath{\hbar}{\ensuremath{\omega}}_{L}$ away from the exciton resonance due to coherent quantum superposition of Floquet states induced by the terahertz field. These frequency-doubled sidebands can occur at twice the frequency typically observed in bulk, quantum well, and superlattice structures. We have also obtained the oscillating excitonic densities for Floquet states as a function of the terahertz field strengths.

Intramolecular Exciton Relaxation and Migration Dynamics in Poly(3-hexylthiophene)

Exciton dynamics are investigated for size selected poly(3-hexylthiophene) samples in dilute chloroform solutions using time-resolved fluorescence. The sizes range from an average of 39 monomers (Mw = 6490 Da) to an average of 168 monomers (Mw = 27860 Da). Both isotropic emission transients, which monitor downhill energy migration, and time-resolved emission depolarization, used to measure orientational migration of the excitons, are reported as a function of emission energy. Downhill energy migration accelerates significantly as the chains become longer. While amplitude of the initial (sub-100 fs) depolarization increases with chain length, the subsequent rate of exciton reorientation is relatively insensitive to chain length for times less than 30 ps and then slows as the chains become longer. The chain length dependence provides additional insight into the connection between spectral diffusion and exciton spatial migration. The results are considered in terms of the distribution of accessible exciton s...

Exciton warming in III–V semiconductors and microcavities

We present a time resolved experiment in which we dynamically tailor the occupation and temperature of a photogenerated exciton distribution in QWs by excitation with two delayed picosecond pulses. The modification of the excitonic distribution results in ultrafast changes in the PL dynamics. Our experimental results are well accounted by a quasiequilibrium thermodynamical model, which includes the occupation and momentum distribution of the excitons. We use this model and the two-pulse experimental technique to study the polariton dynamics in InGaAs-based microcavities in the strong coupling regime. In particular, we demonstrate that resonantly injected upper polaritons mainly relax to the lower polariton branch via scattering to large momentum polariton states, producing the warming of the polariton reservoir.

Formation of islands of condensed exciton phases in semiconductor quantum wells in inhomogeneous fields

The appearance and properties of the structures in the density distribution of indirect excitons in a quantum well plane in semiconductors in an electric field have been studied for the case where a metallic electrode has a circular orifice. It has been shown that the inhomogeneous structures in the exciton density (islands and rings with an increased exciton density) appear, because the condensed exciton phase is present and the system is nonequilibrium due both to the finiteness of the exciton lifetime and to pumping. The dependences of the structure on the system parameters (window sizes, temperature, and pumping intensity) are in agreement with the experimental results reported by A.V. Gorbunov and V.B. Timofeev, Pis’ma Zh. Eksp. Teor. Fiz. 83, 178 (2006) [JETP Lett. 83, 146 (2006)]; Usp. Fiz. Nauk 176, 652 (2006) [Phys. Usp. 49, 629 (2006)]; Pis’ma Zh. Eksp. Teor. Fiz. 84, 390 (2006) [JETP Lett. 84, 329 (2006)].

Interaction between a high-density polariton phase and the exciton environment in semiconductor microcavities

Spontaneous polariton coherence is observed following creation of a cool exciton distribution in a GaAs-based semiconductor microcavity. Interactions between the high density polariton state and high-$k$ excitons result in a blueshift in energy and are shown to limit the temporal coherence of the polariton phase to $10--20\phantom{\rule{0.3em}{0ex}}\mathrm{ps}$. The effect of exciton-polariton interaction is also revealed in two beam experiments where the coherence of the polariton optical parametric oscillator is strongly suppressed by the creation of a reservoir population. These interactions are likely to be important in any polariton high density phase created under conditions of nonresonant excitation.

Modelling of organic nanograting heterojunctions for photoelectric conversion

Organic nanograting heterojunctions for photoelectric conversion were studied based on a two-dimensional analytic exciton transport model. The photocurrent and external quantum efficiency were investigated as the function of materials and geometric parameters. It showed that the introduction of the complicated heterointerfaces changed the exciton distribution within the nanostructures and enhanced significantly the exciton dissociation probability. The photoelectric conversion was determined by the cooperation of the two mechanisms of light absorption and exciton migration. The possibility of the nanograting heterojunctions applied to photovoltaic devices was evaluated quantitatively with the recombination-dominated assumption at donor–acceptor interface. The results suggested that the photovoltaic performance in these nanograting heterojunctions might be improved significantly after optimization when compared to that in a planar heterojunction.

Simulation of exciton formation and transport in electrically driven polariton laser structures

A model for exciton formation, dissociation and transport is proposed for the simulation of an electrically pumped polariton laser with a geometry similar to that of a VCSEL and resonant cavity LEDs. We demonstrate how the strain effects and the geometry of the device influence the exciton distribution for a GaN/InGaN laser structure.

Influence of exciton distribution on external quantum efficiency in bilayer organic solar cells

AbstractThe spatially resolved photogeneration of excitons in bilayer organic solar cells (OSC) was simulated by a detailed optical model. A layering scheme involving up to six different ultrathin films on the substrate was taken into account, including the optical constants, interference effects and resulting standing waves. Thus a strong dependency of local exciton generation on the specific geometry and the wavelength was shown. To determine the distribution of the generated excitons the continuity equation was solved. By defining the quenching properties of cell interfaces the influence of interface exciton dissociation was demonstrated. With given exciton diffusion lengths the generation of charge carriers and the quantum efficiency of phthalocyanine‐C60 OSCs was calculated. The applicability of the model is discussed by comparison with experimental data. (© 2006 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Nonlinear optical approach to multiexciton relaxation dynamics in quantum dots

Unlike the majority of molecular systems quantum dots can accommodate multiple excitations, which is a particularly important attribute for potential lasing applications. We demonstrate in this work the concept of using nth order nonlinear spectroscopies in the transient grating configuration as a means of selectively exciting (n-1)/2 excitons in a semiconductor and probing the subsequent relaxation dynamics. We report a direct observation of multiparticle dynamics on ultrashort time scales through comparison of third and fifth order experiments for CdSe colloidal quantum dots. Time constants associated with multiexciton recombination and depopulation dynamics are reported. Deviation from a Poisson model for the distribution of photoexcited excitons, biexcitons, and triexcitons is also discussed.

Exciton migration in organic thin films

Limitations of the analytical method for calculating the exciton distribution in organic thin films, attributed to the improper boundary conditions when the organic film approaches the exciton diffusion length, were analyzed by comparison with an exciton random walk simulation. The random walk simulation results are in better agreement with in situ photoluminescence measurements than predictions based on the one-dimensional (1D) diffusion equation, especially for thin films (<15nm). The three-dimensional exciton diffusion length in tris(8-hydroxyquinoline) aluminum is determined to be 26nm, equivalent to 15nm upon projection to 1D. The result is not sensitive to the molecular size, a parameter arbitrarily set in the simulation. In addition, the exciton distribution in operating organic light emitting devices was also simulated.

Transient absorption microscopic study of triplet excitons in organic crystals

We have constructed a transient absorption microscope to study triplet excitons in organic crystals. Using the microscope, transient absorption of small crystals can be measured and the spatial distribution of triplet excitons can be obtained. Additionally, it was found that the contrast of the image becomes much better than that of a conventional bright-field microscope. These characteristic features of the transient absorption microscope will be discussed. We also report on the absorption spectra in benzophenone, anthracene and α- and β-perylene crystals. Absorption spectra due to triplet excitons in the crystals were similar to the corresponding solution spectra, although there were some minor differences.

Optical Properties of Simple Bilayer Polymer Light Emitting Diode

Optical properties of bilayer organic light-emitting diodes (OLEDs) based on MEH-PPV (2-methoxy, 5-(2′-ethyl-hexoxy)-1,4-phenylene vinylene) and Alq3 (tris(8-hydroxy-quinolinato) aluminum) are investigated in this paper. Numerical method solving 1D model based on driftdiffusion, continuity and Poisson`s equations extended by boundary conditions in treatment of internal organic interface is applied. The obtained space charge densities were employed in the emissive singlet exciton continuity equation resulting in the singlet exciton distribution and the emission zone profile. Dipole methods were utilized for the simulation of light emission in a layered medium. The electroluminescence (EL) spectra of OLED are considered by taking into account dominant interference effects. The exciton quenching near electrodes, allowing for a dominant nonradiative decay, is treated through effective emission zone narrowing.

Microscopic Spatial Distribution of Bound Excitons in High-Quality ZnO

The surface morphology of the ZnO layers is dominated by a distinct hexagonal domain structure. While the laterally integrated cathodoluminescence spectrum shows intense and narrow I8 luminescence, a distinct emission line at spectral position of I0/I1 emerges in the local spectra taken at domain boundaries. In contrast, no I0/I1 emission is found inside the domains. Monochromatic images further evidence the selective incorporation of impurities at the grain boundaries of domains. Monochromatic images of the I8 peak wavelength directly visualize the strain relaxation across the domains towards their very center, where a drop in quantum efficiency indicates enhanced defect concentration.

Excitonic Relaxation in a Conjugated Polymer: Initial Coherence beyond Energy-Dispersive Population Transfer

In this contribution, we deal with the pathways of femtosecond excitonic relaxation in conjugated polymers, focusing on the prominent, ground-state nondegenerate model system poly(p-phenylenevinylene) (PPV). After a brief discussion of the molecular exciton picture of optical excitations in organic semiconductors, we present exemplary results, categorized in two regimes. On early picosecond time scales, the dynamics of incoherent population relaxation is investigated by combining selectively tuned femtosecond excitation pulses with the technique of fluorescence up-conversion. As an initially prepared excitonic distribution gradually lowers its energy by diffusion to the bottom of the density of states (DOS), an excitation energy-dependent bathochromic shift dynamics is observed, accompanied by spectral intensity rearrangements. Motivated by the perception that intersite electronic coupling is absent in the lowest energy regime of the DOS, we further devise a strategy that enables us to measure the very early electronic oscillations and their quantum-stochastic relaxation within approximately the first 200 fs. By using femtosecond wave packet interferometry with appropriately tuned but otherwise freely propagating pulses, we observe fluorescence interferograms with strongly damped, low-frequency beatings, which stem from the spatial interference of two wave packets launched by two-pulse excitation. The results can be explained, semiquantitatively, in terms of a second-order perturbational approach and open up a new perspective on the complex puzzle of PPV optical dynamics.

Observation of ortho and para-excitons by time-resolved excitonic Lyman spectroscopy

A review of our recent experiments on the detection of ortho- and para-excitons in Cu 2O by the induced absorption associated with an internal excitonic transition (1s to 2p) is presented. In order to evaluate the temporal and spatial dynamics of the excitons, we developed a time-resolved mid-infrared femtosecond pump-probe spectroscopy analysis technique. Because of the large difference in the electron–hole exchange energy between the 1s and 2p excitons (12 and nearly 0 meV, respectively), we can distinguish ortho-excitons from para-excitons in mid-infrared pump probe spectra. Since the transition energy depends on the momenta of excitons because of the difference between the 1s and 2p exciton masses, we can obtain information on the thermal distribution of excitons. We successfully detected a signal associated with para-excitons. Based on the experimental results, we discuss the basic problems to be solved for the observation of exciton BEC, such as para-exciton mass, Auger recombination and ortho to para exciton conversion.

Investigation of excitons in AlGaN/GaN multiple quantum wells by lateral photocurrent and photoluminescence spectroscopies

We report on the investigation of excitons in Al0.2Ga0.8N/GaN multiple quantum wells (MQWs) by lateral photocurrent, and photoluminescence (PL) spectroscopies over the temperature range from 9 to 300 K. The MQWs were deposited homoepitaxially by plasma-assisted molecular-beam epitaxy on a (0001) GaN template grown by hydride vapor phase epitaxy. Excitonic peaks in the photocurrent spectra due to the bulk GaN template and the MQW structure were observed up to room temperature. The PL excitonic peak in the MQWs was strongly Stokes shifted over the whole temperature range, a result attributed to recombination via disorder-induced excitonic band-tail states in the MQWs, due primarily to well/barrier interface roughness fluctuations. A theoretical calculation estimates these fluctuations to be 0.7±0.2 monolayers. The temperature dependence of the Stokes shift indicates that, at higher temperatures, the excitons in the PL experiments are in thermal equilibrium with the lattice before recombining. At lower temperatures, the data suggest a nonthermal exciton distribution, which we attribute to exciton trapping in local potential minima. The PL intensity was found to be strongly quenched by two thermally activated mechanisms with activation energies of 33±6 meV and 165±33 meV. The splitting of the exciton, which gives rise to the photocurrent, was correlated with these two quenching mechanisms, and a model was developed to account for the temperature dependence of the excitonic photocurrent in the MQWs.

Spatial breathing of the exciton distribution in ZnSe quantum wells

AbstractWe measured the transport dynamics of excitons in ZnSe quantum wells with help of a time‐resolved nanophotoluminescence setup. Right after picosecond excitation, the excitons move out of the laser spot, but then they reverse the direction of propagation, and finally spread out again. We attribute this “spatial breathing” of the exciton population to acoustic phonon emission, which is a predominantly backward scattering event. This permits us to detect the first inelastic scattering event after the fast excitation generation, causing the end of the coherent transport regime. Using this method, we can determine simultaneously the coherence time and length of excitons in an 8nm well to 29 ps and 800 nm, respectively. We can reproduce the results with help of a Monte‐Carlo simulation of the exciton dynamics in the well. The obtained values for coherence length and time are consistent with other, independent experiments. (© 2004 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)