High Excitation Effects Research Articles

High-excitation effect on photoluminescence of sol-gel ZnO nanopowder

We studied the power-dependent photoluminescence of ZnO nanopowder grown by sol-gel method at low temperature. At moderate optical pumping intensity, two nonlinear emission bands due to the radiative recombination of free biexciton and the inelastic exciton–exciton scattering were detected. We found that the threshold of the excitation density for the emergence of the nonlinear emission bands is low (<18 W/cm2) in sol-gel ZnO nanopowders.

Effects of excitonic diffusion on stimulated emission in nanocrystalline ZnO

We present optically pumped emission data for ZnO, showing that high excitation effects and stimulated emission/lasing are observed in nanocrystalline ZnO thin films at room temperature, although such effects are not seen in bulk material of better optical quality. A simple model of exciton density profiles is developed which explains our results and those of other authors. Inhibition of exciton diffusion in nanocrystalline samples compared to bulk significantly increases exciton densities in the former, leading—via the nonlinear dependence of emission in the exciton bands on the pump intensity—to large increases in emission and to stimulated emission.

ZnO rediscovered — once again!?

In an introductory chapter we shortly review the history of ZnO research and the motivations for the present renaissance of the worldwide interest in this II–VI compound. Then we concentrate on the following topics: band structure, excitons and polaritons, luminescence dynamics, high excitation effects like biexcitons or the transition to an electron–hole plasma, and finally lasing. We finish with a short conclusion and outlook.

Calculation of excited-state properties using general coupled-cluster and configuration-interaction models

Using string-based algorithms excitation energies and analytic first derivatives for excited states have been implemented for general coupled-cluster (CC) models within CC linear-response (LR) theory which is equivalent to the equation-of-motion (EOM) CC approach for these quantities. Transition moments between the ground and excited states are also considered in the framework of linear-response theory. The presented procedures are applicable to both single-reference-type and multireference-type CC wave functions independently of the excitation manifold constituting the cluster operator and the space in which the effective Hamiltonian is diagonalized. The performance of different LR-CC/EOM-CC and configuration-interaction approaches for excited states is compared. The effect of higher excitations on excited-state properties is demonstrated in benchmark calculations for NH(2) and NH(3). As a first application, the stationary points of the S(1) surface of acetylene are characterized by high-accuracy calculations.

Spectroscopy of CdTe Microcavity Polaritons: Temperature and High Excitation Effects

Spectroscopy of CdTe Microcavity Polaritons: Temperature and High Excitation Effects

Spectroscopy of CdTe Microcavity Polaritons: Temperature and High Excitation Effects

The effect of temperature on microcavity polariton stimulation threshold is presented along with measurement of the occupation number, in wave vector space, for the lower polariton dispersion curve. The polariton state population is deduced from angle-resolved photoluminescence. Those measurements are performed on CdTe-based microcavity samples containing 16 and 24 quantum wells in order to maintain the strong exciton photon coupling regime under high excitation and high temperature conditions in spite of bare exciton state broadening.

High Excitation Effects and Relaxation Dynamics in CdS/ZnSe Single Quantum Wells

High Excitation Effects and Relaxation Dynamics in CdS/ZnSe Single Quantum Wells

High Excitation Effects and Relaxation Dynamics in CdS/ZnSe Single Quantum Wells

Photoluminescence of the ground and excited states of type II CdS/ZnSe single quantum wells with different width is investigated for high electron densities and by means of time resolved photoluminescence measurements. For high excitation intensities the filling of states in the QW up to energies of the second excited state can be observed. The PL decay times of the ground state vary from 2–36 ns for different samples and depend strongly on the QW width. This dependence can be explained with the overlap of electron and hole wavefunctions. The PL rise time of the ground state is influenced by inter- and intrasubband relaxation, which is induced by the cooling of hot carriers.

Spectroscopy of CdTe Microcavity Polaritons: Temperature and High Excitation Effects

Spectroscopy of CdTe Microcavity Polaritons: Temperature and High Excitation Effects

High-Excitation Effects in the Optical Properties of ?-Doped ZnSe Quantum Wells

The influence of high-density optical excitation on n-doped ZnSe/(Zn,Mg)(S,Se) quantum wells is studied by quasi-stationary photoluminescence and pump-probe experiments. In photoluminescence, the trion and exciton transitions saturate weakly with increasing excitation density. This is accompanied by superlinearly growing low- and high-energy wings attributed to collisions in the dense system of excitons, trions and carriers. For non-resonant excitation, a strong heating of the background carrier gas takes place and collisions lead to an effective recombination already during the relaxation of the electron-hole pairs down to the bottom of the band. This process limits reachable exciton and trion densities under quasi-stationary excitation.

Effects of high excitation on localized excitons in cubic ZnCdS lattice matched to GaAs

Effects of high excitation on localized excitons have been studied in cubic Zn0.42Cd0.58S lattice-matched to GaAs. The time-integrated photoluminescence (PL) spectrum at 2 K under weak excitation conditions (0.025 μJ/cm2) was composed of a single emission peaking at 2.863 eV and its LO-phonon replica. The linewidth of the main peak was 18.5 meV. With increasing excitation energy density (Iex), the spectra broadened, especially on the lower energy side and a small shoulder appeared at about 2.85 eV for Iex ≳ 2 μJ/cm2. In order to study the detailed mechanism, the nonlinear luminescence component was studied by the population mixing technique. The positive signals were detected on the lower energy side (2.85 eV) at an Iex value in the range between 1.8 and 2.9 μJ/cm2. This nonlinear component was attributed to localized excitonic molecules.

High excitation effects in type II GaAs/AIAs superlattices

High intensity photoluminescence on GaAs/AIAs superlattices of type II at low temperature reveals the rǒle of Hartree energy which tends to open the gap. Pre,iminary analysis provides a satisfactory fit with Hartree-Fock model. The kinetics of luminescence and total decay in the electron-hole plasma regime is measured.

Excitonic luminescence and the effect of high excitation in ZnSeZnS strained-layer superlattices grown on ZnS substrates

Excitonic properties of the ZnSeZnS MQW strained-layer superlattices (SLSs), fabricated on (100) ZnS and GaAs substrates by a low-pressure metalorganic chemical vapour deposition method, have been investigated by means of photoluminescence and absorption spectra. With increasing excitation density by a N 2 pulsed laser, it was found that a dominant exciton band moves toward higher photon energy. In order to understand the dissociation mechanisms of excitons, the temperature dependence of the linewidth and emission intensity obtained under high excitation was explicitly measured. The linewidth on temperature can be analyzed by considering three contributions which are related to scatterings of excitons with LO and acoustic phonons, and with donor impurities. The activation energy of about 71 meV for the thermal liberation of excitons can be obtained and is close to approximately 4 E B,3D (the binding energy E B,3D=19 meV in ZnSe bulk).

Theoretical study of the dipole moment function of OH(X 2Π)

We present a theoretical study of the sensitivity of the dipole moment function (DMF) of the X 2Π ground state of OH to basis set saturation and to refinements in the correlation treatment. Emphasis is placed on determining the slope of the DMF at re and the r value at which the maximum occurs. We consider the effect of oxygen polarization functions up through h type, expansion of the active orbital space to include the O 3dδ orbital, the effect of higher excitations using the averaged coupled-pair functional method, and the effect of evaluating the dipole moment as an energy derivative rather than as an expectation value. Our theoretical DMFs, which should be the most accurate to date, differ markedly from an empirical DMF of Turnbull and Lowe that is based on experimentally derived intensity ratios. The theoretical DMFs agree better with a recently published DMF of Nelson et al., but suggest that this empirical DMF is also inaccurate for r>2.3 a0.

Excitons in layered BiI 3 single crystals

This paper reviews the various exciton spectra in the absorption edge of BiI 3 crystals, which show many aspects of exciton behavior. The band edge exciton transition takes place with a very large oscillator strength and is ascribed to rather localized s→p transition in Bi 3+ ions. Its anisotropy and surface effects are characterized by a van der Waals bonded layered lattice. A series of very sharp absorption lines near the edge is attributed to direct exciton transitions in a stacking fault plane, and the resonant emission lines can be observed with no Stokes' shift. Their conspicuous properties, including the relaxation process, high excitation effects, propagation along the plane and so on, are summarized and discussed in terms of an exciton system confined in a nearly perfect two-dimensional lattice. The necessity and desirability of more complete studies of the stacking fault exciton system are emphasized.

Luminescence from very small semiconductor particles — high excitation effects

Whereas under low irradiance excitation, only the large clusters of a polydisperse population luminesce, via recombination at defects, under high irradiance this path is saturated and clusters of all sizes show band edge emission, but at a higher energy than in bulk samples. The shift is satisfactorily accounted for by the confinement of the electrons and holes in zero dimensional space.

Time resolved luminescence of CdS at high excitation

We report a study at low temperature of the time resolved luminescence of CdS, excited by two photon absorption. Concerning the so called EHP-LO and P bands, we confirm our results previously obtained on CdSe[1, 2]. (a) At high excitation a broad band (peak position at λ > 495 nm) occurs due to radiative recombination in an electron hole plasma, assisted by the emission of one LO phonon. (b) The simultaneous kinetics of the P line (λ ≅ 490.5 nm) and A-LO line (λ ≅ 492.5 nm) are conflicting with the interpretation of the P line as resulting of radiative exciton-exciton collisions. We interpret the P line as due to biexciton recombination. We have studied the luminescence in the ( I 2- I 1) region (486nm < λ < 490nm) at low excitation. We observe clearly the following, (a) A broad band (488 nm < λ < 490.5 nm) which corresponds to the gain observed in previous experiments of pulse and probe spectroscopy and interpreted as direct recombination in an electron hole liquid (EHL). (b) After the disappearance of the EHL band, one single line ( M D ), which shifts continuously towards the I 2 position during the time resolved kinetics. We suggest it to be connected with the high excitation effect on donor impurities (bound polyexcitons).

The nonlinear optical susceptibility χ (2) near the bandgap in CdS

We have measured the second harmonic intensity generated in ultrapure CdS crystals using a tunable dye laser. The fundamental frequency corresponding to the bandgap was tuned over the various exciton energy levels. An analysis shows that at laser intensities necessary to observe second harmonic signals high excitation phenomena are unavoidable. At these intensities the excitonic contributions to the nonlinear optical susceptibility χ (2) are quenched by high excitation effects.

Analytic configuration interaction gradient studies of SH4, sulfurane

The potential energy surface of SH4, the prototype sulfurane, has been examined in light of previous theoretical studies suggesting that there may be two distinct energy minima. The first of these has the four H atoms in roughly octahedral positions, as in the experimentally known SF4 molecule, and belongs to point group C2v. The second isomer is square pyramidal, point group C4v. Molecular electronic structure theory has been applied using basis sets of triple zeta (TZ) and triple zeta plus polarization (TZ+P) quality. Configuration interaction (CI) including in some cases all single and double excitations was carried out and geometries optimized via newly developed analytic CI gradient techniques. At the TZ SCF and TZ CI levels of theory there are both C2v and C4v relative minima, separated by +2.4 and −1.6 kcal, respectively. However, using the more complete TZ+P SCF and TZ+ P CI methods, the C2v minimum disappears, leaving only the C4v minimum. Geometries corresponding to the expected C2v minimum lie about 6 kcal above the square pyramid structure. The C4v mimimum lies 74.6 kcal above separated H2S+H2 at the TZ+ P CI level of theory, and this is reduced to 72.4 kcal when the effects of higher excitations (unlinked clusters) are considered. Harmonic vibrational frequencies have been predicted at the three simpler levels of theory to prove the proposed designations of the various stationary points. For the square pyramidal SH4, the predicted frequencies are quite sensitive to basis set, as might be expected for an extremely flat potential energy surface such as this. The square planar geometry lies only slightly above the square pyramid, which faces a barrier of 2.3 kcal to inversion. The transition state for dissociation via SH4→SH2+H2 was also located at several levels of theory and has an interesting structure. The barrier to dissociation is ∼46 kcal and the activation energy ∼42 kcal, suggesting that SH4 should be a ’’makable’’ molecule.

Analytic Approximations for Integrated Electron?Atom Excitations

Accurate calculations of atomic excitations require estimates of the effect of higher excitations on the effective (optical) potential coupling various reaction channels. The total cross section for a particular excitation is proportional to the maximum contribution of that excitation to the imaginary part of the elastic momentum-space optical potential, and is typical of the contribution to the potential in general. Analytic expressions relevant to the calculation of optical potentials are given Their validity is estimated by comparison with more-accurate calculations and with experimental excitation cross sections.