Number Of Excitons Research Articles

Non-poissonian formation of multiple excitons in photoexcited CdTe colloidal quantum qots by femtosecond nonresonant two-photon absorption

Using direct multiexcitonic spectroscopy, we experimentally observe for the first time the non-Poissonian formation of multiple excitons by femtosecond nonresonant two-photon absorption process in semiconductor colloidal quantum dots (QDs). Each of the multiple excitons is individually generated via the absorption of a pair of photons during the femtosecond pulse irradiation. The non-Poissonian distribution of the generated excitons is reflected as a non-quadratic dependence on the pulse intensity of the average number of excitons per QD. This is the main observation of the present work. It is explained by a multiexcitonic formation model that is based on the phenomenon of intrapulse state filling of the few quantum electronic states accessed by the two-photon transitions. The experiments are conducted with 3.9-nm CdTe QDs in room-temperature hexane solution using the femtosecond pump-probe transient absorption technique, where an intense pump pulse generates the excitons and a weak probe pulse measures their number via intraband one-photon absorption.

Dynamics of dissipative multiple exciton generation in semiconductor nanostructures

The population dynamics of single exciton and biexciton states in a simple model of a spherical semiconductor nanostructure is modeled numerically in the presence of Coulomb coupling between single and two exciton states and a dissipation channel in order to study the transient biexciton population that occurs in an optically excited semiconductor nanocrystal. The results show that the system evolution strongly changes if the dissipation is included. In a certain range of parameters, the growth of the exciton number (multiple exciton generation process) is fast (on picosecond time scale) and the following decay (Auger process) is much slower (hundreds of picoseconds). In some cases, the maximum occupation of the biexciton state increases when dissipation is included. The dynamics of an ensemble of nanostructures with a certain size dispersion is studied by averaging over the energy of the biexciton state which can be different for each single nanostructure. The validity of Markov and secular approximation is also verified.

Terahertz-Induced Optical Emission of Photoexcited Undoped GaAs Quantum Wells

Intense terahertz (THz) pulse induces photoluminescence (PL) flash from undoped high-quality GaAs/AlGaAs quantum wells under continuous wave laser excitation. The number of excitons increases 10,000-fold from that of the steady state under only laser excitation. The THz electric field dependence and the relaxation dynamics of the PL flash intensity suggest that the strong electric field of the THz pulse ionizes impurity states during the 1 ps period of the THz pulse and release carriers from a giant reservoir containing impurity states in the AlGaAs layers.

Pre-equilibrium neutron-emission spectra of 238U with an effective nucleon–nucleon interactions

In this study, the initial exciton numbers for the target nucleus 238U were calculated through a new method offered by Tel et al. (Ann. Nucl. Energy, 35, 220, 2008). Neutron-emission spectra produced by (n,xn) reactions on 238U nuclei have been calculated by using Hartree–Fock method with effective nucleon–nucleon Skyrme interactions with SKM∗ parameters. Pre-equilibrium nuclear reactions have been used to investigate the effect of initial exciton numbers on the nucleon emission spectra. Calculations have been made in the framework of the hybrid, equilibrium and GDH models using ALICE/ASH computer code. The initial exciton numbers calculated with the theoretical neutron and proton densities have been obtained with SKM∗ for the 238U(n,xn) reaction at 14.0 and 18.0MeV incident neutron energies. We also compared the geometry-dependent hybrid model (GDH), newly evaluated with the initial exciton number calculations of the neutron emission spectra of the 238U(n,xn) reaction. The obtained results are discussed and compared with the available experimental data and are shown to be in agreement with each other. All calculated results have been compared with experimental data from Experimental Nuclear Reaction Data (EXFOR).

Photophysics in Single-Walled Carbon Nanotubes with (6,4) Chirality at High Excitation Densities: Bimolecular Auger Recombination and Phase-Space Filling of Excitons

The photophysics in single-walled carbon nanotubes (SWNTs) at high excitation densities has been attracting significant attention for exciton stability. Exciton–exciton interactions and an exciton Mott transition of quasi-one-dimensional excitons in SWNTs are of fundamental interest because the screening effect of the Coulomb interaction is suppressed in one-dimensional systems. In this study, we investigate exciton dynamics and a bimolecular Auger recombination process in SWNTs with (6,4) chirality using femtosecond transient absorption measurements at high excitation densities in the range of 2.4 × 1013–4.8 × 1015 photons cm–2 per pulse. Temporal evolutions of the change in the absorption coefficient are analyzed with an analytical expression for the bimolecular Auger recombination dynamics. We determine the Auger recombination coefficient and initial exciton number per tube, and the absorption cross section is also obtained. By using the phase-space filling model, the exciton size is estimated to be 3.1 ± 1.3 or 2.2 ± 1.0 nm, which depends on the corresponding wave function. Based on the estimated exciton size, the possibility of the Mott transition of quasi-one-dimensional excitons is discussed.

Multiexciton Absorption Cross Sections of CdSe Nanocrystals at Band-Edge Energy

Picosecond transient absorption signals of two kinds of cadmium selenide quantum dots were measured at various excitation intensities. The average number of excitons per quantum dot was calculated from a Poisson model, which together with kinetic parameters was used to determine exciton population kinetics. Exciton and multiexciton absorption cross sections were determined and analyzed in terms of the electronic states of the quantum dots.

Numerical simulation of exciton dynamics in Cu2O at ultra-low temperatures within a potential trap

We have studied theoretically the relaxation behaviour of excitons in cuprous oxide (Cu2O) at ultra-low temperatures when excitons are confined within a potential trap by solving numerically the Boltzmann equation. As relaxation processes, we have included in this paper deformation potential phonon scattering, radiative and non-radiative decay and Auger decay. The relaxation kinetics has been analysed for temperatures in the range between 0.3 and 5 K. Under the action of deformation potential phonon scattering only, we find for temperatures above 0.5 K that the excitons reach local equilibrium with the lattice, i.e. that the effective local temperature is coming down to the bath temperature, while below 0.5 K a non-thermal energy distribution remains. Interestingly, for all temperatures the global spatial distribution of excitons does not reach the equilibrium distribution, but stays at a much higher effective temperature. If we include further a finite lifetime of the excitons and the two-particle Auger decay, we find that both the local and the global effective temperature do not come down to the bath temperature. In the first case we find that a Bose–Einstein condensation (BEC) occurs for all temperatures in the investigated range. Comparing our results with the thermal equilibrium case, we find that BEC occurs for a significantly higher number of excitons in the trap. This effect could be related to the higher global temperature, which requires an increased number of excitons within the trap to observe the BEC. In the case of Auger decay, we do not find a BEC at any temperature due to the local heating of the exciton gas.

Nearly Temperature‐Independent Threshold for Amplified Spontaneous Emission in Colloidal CdSe/CdS Quantum Dot‐in‐Rods

Nearly Temperature‐Independent Threshold for Amplified Spontaneous Emission in Colloidal CdSe/CdS Quantum Dot‐in‐Rods

Advanced theory of multiple exciton generation effect in quantum dots

The theoretical aspects of the effect of multiple exciton generation (MEG) in quantum dots (QDs) have been analysed in this work. The statistical theory of MEG in QDs based on Fermi’s approach is presented, taking into account the momentum conservation law. According to Fermi this approach should give the ultimate quantum efficiencies of multiple particle generation. The microscopic mechanism of this effect is based on the theory of electronic “shaking”. According to this approach, the wave function of “shaking” electrons can be selected as Plato’s functions with effective charges depending on the number of generated excitons. From the theory it is known increasing the number of excitons leads to enhancement of the Auger recombination of electrons which results in reduced quantum yields of excitons. The deviation of the averaged multiplicity of the MEG effect from the Poisson law of fluctuations has been investigated on the basis of synergetics approaches. In addition the role of interface electronic states of QDs and ligands has been considered by means of quantum mechanical approaches. The size optimisation of QDs has been performed to maximise the multiplicity of the MEG effect.

On the nuclear properties of 32S, 64Zn, 67Zn, 89Y, 90Zr and 153Eu targets used for production of 32P, 64Cu, 67Cu, 89Sr, 90Y and 153Sm therapeutic radionuclides

We have calculated the nuclear binding energy per particle, root mean square (rms) charge radii, neutron and proton densities, and density dependent initial exciton numbers of the target nuclei 32S, 64Zn, 67Zn, 89Y, 90Zr and 153Eu used for production of the therapeutic radionuclides 32P, 64Cu, 67Cu, 89Sr, 90Y and 153Sm. The calculations are performed using the Skyrme Hartree–Fock (SHF) and Skyrme–Hartree–Fock–Bogoliubov (SHFB) approximations based on an effective Skyrme force. The calculated results are discussed and compared with experimental data and with results obtained using relativistic mean field theory.

Dynamics of excitons in a potential trap at ultra-low temperatures: paraexcitons in Cu2O

We present experiments on the luminescence of excitons confined in a potential trap at sub-Kelvin temperatures after nanosecond pulsed laser excitation. Analysis of the experimental results with a rate model shows that the so-called Auger decay of yellow excitons, which in previous studies led to a rapid decay of the excitons at high densities and thus prevented reaching the critical density for Bose–Einstein condensation (BEC), is greatly reduced for paraexcitons. We demonstrate that exciton numbers well above 1010 can be collected in a potential trap, albeit at temperatures in the 10 K range. During their lifetime of about 500 ns the paraexcitons cool down to the temperature of the He bath. This opens up the possibility to observe a BEC of paraexcitons provided that the bath temperature can be reduced to below 100 mK.

The Effect of Ageing on Exciton Dynamics, Charge Separation, and Recombination in P3HT/PCBM Photovoltaic Blends

AbstractA study of how light‐induced degradation influences the fundamental photophysical processes in the active layer of poly(3‐hexylthiophene)/[6,6]‐phenyl C61‐butyric acid methyl ester (P3HT/PCBM) solar cells is presented. Non‐encapsulated samples are systematically aged by exposure to AM 1.5 illumination in the presence of dry air for different periods of time. The extent of degradation is quantified by the relative loss in the absorption maximum of the P3HT, which is varied in the range 0% to 20%. For degraded samples an increasing loss in the number of excitons within the P3HT domains is observed with longer ageing periods. This loss occurs rapidly, within the first 15 ps after photoexcitation. A more pronounced decrease in the population of polarons than excitons is observed, which also occurs on a timescale of a few picoseconds. These observations, complemented by a quantitative analysis of the polaron and exciton population dynamics, unravel two primary loss mechanisms for the performances of aged P3HT/PCBM solar cells. One is an initial ultrafast decrease in the polaron generation, apparently not related to the exciton diffusion to the polymer/fullerene interface; the second, less significant, is a loss in the exciton population within the photoexcited P3HT domains. The steady‐state photoinduced absorption spectra of degraded samples exhibits the appearance of a signal ascribed to triplet excitons, which is absent for non‐degraded samples. This latter observation is interpreted considering the formation of degraded sites where intersystem crossing and triplet exciton formation is more effective. The photovoltaic characteristics of same blends are also studied and discussed by comparing the decrease in the overall power conversion efficiency of solar cells.

Optical Properties of Nanostructures

We analyzed linear molecular chain with exciton excitations for the case when number of excitons is not conserved. The results obtained are in some sense amazing. The dispersion law of finite chain is surface depending on two independent angles. The same conclusion is valid for concentrations of exitons and exciton pairs. As it was expectable physical characteristics of the finite chain depend on spatial coordinates. All results are compared to corresponding results of infinite chain.

Auger-decay dynamics of germanium nano-islands in silicon

The decay dynamics of self-assembled germanium islands is studied by time-resolvedfluorescence spectroscopy. The scaling behavior of the decay rate with the number of excitonsin the islands is shown to agree with expectations for an Auger-recombination-dominatedprocess in the asymptotic limit of high exciton numbers. The multi-excitonic decay timeand spectral behavior are compared to theoretical estimates.

INVESTIGATION OF NUCLEAR STRUCTURE OF EVEN–EVEN 98–136Cd, 102–136Te AND 114–146Xe ISOTOPES

In this paper, the ground state properties of neutron rich even–even Cd, Te and Xe isotopes are investigated using the Skyrme–Hartree–Fock–Bogoliubov (SHFB) methods with SKM* and SLy4 force parameters. These isotopes are vital to the structural studies of unstable nuclei taking place at near Z = 50 closed shell. The main subject of this paper is to calculate the binding energies per particle, the rms nuclear charge radii, the rms nuclear proton density radii, and the rms nuclear neutron density radii with SHFB methods. The results are compared with experimental data and with the results of relativistic meanfield theory (RMFT). In addition to these calculations the initial neutron and proton exciton numbers for 112 Cd , 124 Te and 126 Xe isotopes, which have high cross sections for proton induced nuclear reactions (p, 2n) and used for radioisotopes production, will also be calculated. These exciton numbers can be used for pre-equilibrium proton induced (p, 2n) reactions on 112 Cd , 124 Te and 126 Xe isotopes.

Pre-equilibrium Emission Spectra Calculations of 232Th(n,xn) Using New Evaluated Method

The knowledge of the thorium (232Th) cycle potentialities are required for the design of a fusion-fission (hybrid) reactor. Pre-equilibrium nuclear reactions have been used to investigate the effect of initial exciton numbers on the nucleon emission spectra. In this study, the initial exciton numbers for the target nucleus of 232Th were calculated through a method of offered by Tel et al. and then were used to obtain the effect cross section of the neutron emission spectra. Using this new method, a different way from the literature, the initial exciton numbers calculated with the theoretical neutron and proton densities have been obtained with SKM* on the 232Th(n,xn) reaction at 14.1 and 18.0 MeV incident neutron energies. The results were analyzed by comparing the empirical results in the literature.

Evidence for Multiple Trapping Mechanisms in Single CdSe/ZnS Quantum Dots from Fluorescence Intermittency Measurements over a Wide Range of Excitation Intensities

Fluorescence intermittency of single CdSe/ZnS core/shell quantum dot particles is investigated over a wide range of excitation intensities and at two excitation wavelengths. Deviation from previously observed power law behavior in both the on- and off-duration probability distributions is observed at both excitation wavelengths, one near the band gap and one 350 meV above the band gap. Increasing the excitation intensity modifies the off-duration probability distribution such that the probability of long off-time events decreases, an effect that is observed to saturate at an average number of excitons created per pulse of one, ⟨N⟩ ≈ 1. The on-duration probability distribution is well-described by a power law for short on-time events, crossing over to an exponential distribution for long on-time events. Increasing the excitation intensity induces this crossover to occur at earlier times, saturating at ⟨N ⟩ ≈ 2. The different intensity-dependent trends for the on- and off-duration probability distributions ...

Neutron skin effect of some Mo isotopes in pre-equilibrium reactions

The neutron skin effect has been investigated for even isotopes of molybdenum at 25.6 MeV 94 − 100Mo(p, xn) reaction using the geometry-dependent hybrid model of pre-equilibrium nuclear reactions. Here the initial neutron/proton exciton numbers were calculated from the neutron/ proton densities obtained from an effective nucleon–nucleon interaction of the Skyrme type. Initial exciton numbers from different radii of even Mo isotopes were used to obtain the corresponding neutron emission spectra. In this investigation the calculated results are compared with the experimental data as also with each other. The results using central densities in the geometry-dependent hybrid model are in better agreement with the experimental data.

The effect of the initial exciton numbers on 54,56Fe(p, xp) Pre-Equilibrium Reactions

In pre-equilibrium nuclear reactions, the geometry-dependent hybrid model is applied with the use of the neutron and proton densities to investigate the effect of initial exciton numbers on the nucleon emission spectra. The initial exciton numbers calculated with the theoretical neutron and proton densities have been obtained within the Skryme-Hartree-Fock method with SKM* and SLy4 forces on target nuclei in the 54,56Fe(p, xp) reaction at 61.5-MeV incident proton energy by using a new calculationmethod of Tel et al. Also, the differences between the initial exciton numbers for protons and neutrons as a function of nuclear radius, focusing on systematic discrepancies correlated to differences in the proton and neutron densities have been investigated.

Electron Correlation Effects on the Femtosecond Dephasing Dynamics ofE22Excitons in (6,5) Carbon Nanotubes

Highly nonlinear pump fluence dependence was observed in the ultrafast one-color pump-probe responses excited by 38 fs pulses resonant with the E(22) transition in a room-temperature solution of (6,5) carbon nanotubes. The differential probe transmission (ΔT/T) at the peak of the pump-probe response (τ = 20 fs) was measured for pump fluences from ∼10(13) to 10(17) photons/pulse cm(2). The onset of saturation is observed at ∼2 × 10(15) photons/pulse cm(2) (∼8 × 10(5) excitons/cm). At pump fluences >4 × 10(16) photons/pulse cm(2) (∼1.6 × 10(6) excitons/cm), ΔT/T decreases as the pump fluence increases. Analogous signal saturation behavior was observed for all measured probe delays. Despite the high exciton density at saturation, no change in the E(22) population decay rate was observed at short times (<300 fs). The pump probe signal was modeled by a third-order perturbation theory treatment that includes the effects of inhomogeneous broadening. The observed ΔT/T signal is well-fit by a pump-fluence-dependent dephasing rate linearly dependent on the number of excitons created by the pump pulse. Therefore, the observed nonlinear pump intensity dependence is attributed to the effects of quasi-elastic exciton-exciton interactions on the dephasing rates of single carbon nanotubes. The low fluence total dephasing time is 36 fs, corresponding to a homogeneous width of 36 meV (290 cm(-1)), and the derived E(22) inhomogeneous width is 68 meV (545 cm(-1)). These results are contrasted with photon-echo-derived parameters for the E(11) transition.