On the calculation of mean lifetimes in the preequilibrium exciton model

An explicit and exact calculation method of mean lifetimes in the nuclear exciton model describing preequilibrium decay is presented. A simple algorithm is suggested. Relations of existing approaches with the analytical solution of the time-integrated master equation are discussed.     

Exact evaluation of the kernel of the generalized master equation for the coupled coherent and incoherent exciton motion

The kernel of the Nakajima-Zwanzig generalized master equation is calculated exactly for the coupled coherent and incoherent exciton motion according to the Haken-Strobl model. As a simple application the resulting master equation is used for the determination of the mean square displacement of exciton transport.     

Pre-equilibrium decay in the exciton model

A new closed-form formulation of the statistical exciton model which gives results very similar to the total pre-equilibrium spectrum obtained by solving a set of Boltzmann-like master equations that describe the nuclear equilibration process, is suggested. The effect of including the loss of strength from each exciton state of a composite nucleus due to the transitions of Δn = 0 (Δp = Δh = 0) in both the master equations and closed-form formalisms is discussed.     

Nakajima-Zwanzig's generalized master equation: Evaluation of the kernel of the integro-differential equation

A method is presented, which allows the exact elimination of the projection operator from the kernel of the Nakajima-Zwanzig generalized master equation without using perturbational expansions. Expressions for kernels of generalized master equations using several frequently occuring types of projection operators are derived explicitly. The application of this method for the exact derivation of generalized master equations describing the coherent and the coupled coherent and incoherent exciton motion is proposed. As another application, the derivation of the Smoluchowski equation is suggested.     

Pre-equilibrium effect in heavy ion induced fusion

The exciton model is applied to a multi-nucleon emission process after fusion caused by heavy-ion collisions. The master equations for the probability distribution are successively solved by assuming an initial distribution calculated by using a simple Fermi gas model. The equilibration process is discussed, and the results are compared with those of the equilibrium statistical theory and experimental data.     

Is the coupled coherent and incoherent exciton motion characterized by its mean square displacement?

The relevance of the mean square displacement for the comparison between different microscopic models is discussed starting from the Nakajima-Zwanzig generalized master equation. The time dependences of its kernel and of the mean square displacement are explicitly given for the Haken-Strobl model of exciton motion.     

Effect of binding energy on the kinetics of radiative and nonradiative recombination of bound excitons

Radiative and nonradiative lifetimes of bound excitons in GaSe with various binding energies are measured by the magnetooptical method using time-resolution spectroscopy. It is shown that both the radiative and nonradiative lifetimes of an exciton bound at an ionized center increase with the binding energy of an exciton with a defect. Possible reasons for this dependence are considered. The results of calculations obtained in the framework of the proposed model are in good agreement with experimental data.     

Energy spectra and lifetimes of quasiparticles in an open quantum dot surrounded by identical barriers in a cylindrical quantum wire

A theory of quasi-stationary states and lifetimes of electrons, holes, and excitons in an open cylindrical semiconductor quantum wire containing a quantum dot surrounded by two identical antidots (with potential barriers of finite height) is developed using the scattering matrix method. The energy spectra and lifetimes of electrons, holes, and excitons in a β-HgS/β-CdS/β-HgS/β-CdS/β-HgS nanoheterosystem are calculated and analyzed as functions of the geometric parameters of the quantum dot involved. It is demonstrated that an increase in the height of the quantum dot leads to a decrease in the energy of quasi-stationary exciton states of the Breit-Wigner type and to an increase in their lifetimes. The lifetime of exciton states is long enough for these states to be observed in the experiment.     

Master equation techniques for exciton motion,relaxation, capture,and annihilation

Techniques developed recently for the study of exciton dynamics in molecular solids are discussed. They include master equation methods for the analysis of prerelaxation energy transfer, the generalized master equation approach to coherence in exciton motion, and the defect technique as applied to exciton trapping and annihilation.     

Temperature and density dependence of exciton lifetimes in GaAs/AlGaAs multiple quantum wells

The photoluminescence linewidths and excition lifetimes of free excitons in GaAs/AlGaAs multiple quantum wells were systematically investigated as a function of temperature, quantum well width, and carrier density. The experimental results showed that the exciton decay processes were strongly related to the linewidth of the exciton and the exciton binding energy.     

Luminescence decay and the absorption cross section of individual single-walled carbon nanotubes

The absorption cross section of highly luminescent individual single-walled carbon nanotubes is determined using time-resolved and cw luminescence spectroscopy. A mean value of approximately 1 x 10(-17) cm2 per carbon atom is obtained for (6,5) tubes excited at their second optical transition, and corroborated by single tube photothermal absorption measurements. Biexponential luminescence decays are systematically observed, with short and long lifetimes around 45 and 250 ps. This behavior is attributed to the band edge exciton fine structure with a dark level lying a few meV below a bright one.     

Radiative and non-radiative relaxation of excitons in strain-compensated quantum dots

We have investigated the population dynamics of excitons in strain-compensated InAs quantum dots (QDs) using a pump–probe technique under resonant excitation. Precise control of polarization directions of incident pulses enabled us to selectively estimate population lifetimes for two orthogonally polarized exciton ground states according to polarization selection rules. Measured decay times of the probe transmissions were highly dependent on the polarization directions of the exciton states. We found that the ratio of the decay times for the orthogonally polarized states is in quantitative agreement with the ratio of square of the transition dipole moments. This indicates that radiative recombination processes have a dominant effect on the population dynamics and that non-radiative and spin relaxations are negligible in our QDs. As a result, we can estimate the radiative lifetimes to be 1.0±0.1 and 1.7±0.2 ns for orthogonally polarized exciton ground states.     

Observation of long lifetime lines in photoluminescence from Si: In

We report the first observation of lines with lifetimes of about 200 μs in silicon samples doped with indium. These long lifetimes are comparable to the radiative lifetime for the indium bound exciton and suggest that these lines are due to isoelectronic centers in silicon.     

Random-walk model of precompound decay II: Stochastic uncertainties in the lifetimes and cross-sections

The uncertainties arising from the stochastic nature of precompound decay nuclear reactions are analyzed in the framework of the preequilibrium exciton and random-walk models. It is demonstrated that the standard deviations and the mean values of the exciton-state lifetimes are of the same order of magnitude. Their correlations are weakly positive, except for exciton states near the equilibrium number, where the correlations are significant. The usefulness and the limitations of the never-come-back approximation are discussed. A general proof is presented of the conditions under which the master-equation and random-walk approaches to Markov processes are equivalent. Connections between different preequilibrium models, e.g. the multi-step compound model and the microscopic statistical theory of precompound decay, are pointed out. It is shown that the waiting time between subsequent collisions is governed by a Poisson process, suggesting that the variance associated with the nucleon mean free path in nuclear matter, as estimated from preequilibrium models, is considerable. The stochastic uncertainties in the emission cross-sections correspond to those of a Bernoulli process.     

Picosecond excitonic luminescence in ZnO and other wide-gap semiconductors

Radiative lifetimes of free-atom transitions, scaled by ω³ for comparison at 368 nm, are not faster than about 6.9 ns. BaF₂ core-valence luminescence, scaled in the same way from 220 to 368 nm, corresponds to 4.1 ns. In contrast, excitonic transitions in wide-gap semiconductors display subnanosecond radiative lifetimes, and in particular ZnO has radiative lifetimes measured at 50–300 ps for D^o,X and 400–900 ps for free excitons. The “giant oscillator strength” corresponding to these lifetimes can be explained by theories developed initially for defect-bound excitons, then quantum wells, and nanoparticles. An exciton is a coherent array of N dipoles, where N is the number of sites covered by coherent translational motion of the exciton. This is not essentially a phenomenon of multiple excitons, but applies as well to single-exciton decay. It differs in that regard from the more familiar Dicke giant dipole of N coherently excited atoms lacking translational periodicity. The phenomenon suggests possibilities for achieving ultrafast scintillators and high light yield.     

Two-component equilibration in the exciton model of nuclear reactions

A generalized two-components master-equation approach for a nuclear system towards equilibration is described, two-components being the proton and the neutron components. An approximate closed form solution of the two-component equation is discussed. Further an effective one-component master equation is derived from it. Explicit expressions for effective transition and emission rates are derived under a binomial model. The mean free path of an exciton has been found to be reduced in accounting for the two-component equilibration.     

Analytic solution of the pauli master equation for the exciton motion in the infinite linear chain with a single trap

An explicit analytic solution of the Pauli master equation for the incoherent exciton motion in the infinite linear chain with a single trap is presented. The total probability to find the exciton in the chain is also calculated.     

Theory of nonlinear dispersive waves and selection of the ground state

A theory of time-dependent nonlinear dispersive equations of the Schr?dinger or Gross-Pitaevskii and Hartree type is developed. The short, intermediate and large time behavior is found, by deriving nonlinear master equations (NLME), governing the evolution of the mode powers, and by a novel multitime scale analysis of these equations. The scattering theory is developed and coherent resonance phenomena and associated lifetimes are derived. Applications include Bose-Einstein condensate large time dynamics and nonlinear optical systems. The theory reveals a nonlinear transition phenomenon, "selection of the ground state," and NLME predicts the decay of excited state, with half its energy transferred to the ground state and half to radiation modes. Our results predict the recent experimental observations of Mandelik et al. in nonlinear optical waveguides.     

Energy transfer in europium doped yttrium vanadate crystals

The absorption, fluorescence and excitation spectra and fluorescence lifetimes were obtained at temperatures from 8 K to room temperature, for single crystals of yttrium vanadate doped with several concentrations of europium. A model is proposed to explain the luminescence properties of YVO₄ crystals based on transitions within the VO^3-₄ ion. The host sensitized energy transfer in YVO₄: Eu³⁺ is explained by amodel based on thermally activated exciton migration. The model predicts the observed temperature and concentration dependences for the fluorescence intensities and lifetimes. The diffusion coefficient, diffusion length, exciton hopping time and trapping rate are determined and compared with similar data obtained on other systems.     

MgxZn1-xO薄膜的生长及其光学性能

利用金属有机化学气相沉积(MOCVD)法在蓝宝石(1120)衬底上生长高质量的Mg_xZn_1-xO合金薄膜,实验研究了各种不同Mg组分对样品光学性能的影响。通过光致发光荧光衰减特性对激子发光寿命的研究表明,激子的寿命随Mg组分的增加而增加,当Mg组分达到50%时,激子的寿命为872 ps。