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The semiconducting behavior of two-dimensional (2D) metal nanostructures has recently attracted much interest for their possible applications in optoelectronics and others. In particular, tuning the bandgap of such nanostructures can open up a new avenue for fabricating functional nano-devices. In the present article, we report the synthesis of 2D metallic Zn nanosheets at room temperature using a ball mill, which is capable of producing large-scale materials in a single run. Initially, nanoplates were formed for ball milling the octahedral-shaped Zn nanoparticles for the time of milling of 6 h. Subsequent ball milling for another 6 h leads these nanoplates to nearly uniform nanosheets. The thickness of these 2D nanostructures was found to decrease with an increase in the time of milling. Visible photoluminescence (PL) emissions centered at ∼3, ∼2.9, and ∼2.75 eV were observed from all the Zn particles showing semiconductor behavior. The origin of such semiconductor behavior was explained based on the radiative transition of electrons from the sp band to the upper states of the 3d band. This argument was confirmed through the studies of photoelectron spectroscopy and the first principle calculations employing density functional theory (DFT). Furthermore, excitation-dependent PL studies indicated that the bandgap of the 2D Zn nanostructures decreased with the increase in the ball milling time. Therefore, a redshift in the bandgap was observed with the increase in the ball milling time. Such changes in the bandgap with the thickness of 2D Zn nanostructures were also verified from the studies of DFT. Thus, the present study demonstrated that the bandgap of 2D metallic Zn nanostructures could be effectively tuned by reducing the thickness. [ABSTRACT FROM AUTHOR]

Relativistic coupled-cluster calculations of the ionization potential, dissociation energy, and excited electronic states under 35 000 cm−1 are presented for the actinium monofluoride (AcF) molecule. The ionization potential is calculated to be IPe = 48 866 cm−1, and the ground state is confirmed to be a closed-shell singlet and thus strongly sensitive to the T , P -violating nuclear Schiff moment of the Ac nucleus. Radiative properties and transition dipole moments from the ground state are identified for several excited states, achieving a mean uncertainty estimate of ∼450 cm−1 for the excitation energies. For higher-lying states that are not directly accessible from the ground state, possible two-step excitation pathways are proposed. The calculated branching ratios and Franck–Condon factors are used to investigate the suitability of AcF for direct laser cooling. The lifetime of the metastable (1)3Δ1 state, which can be used in experimental searches of the electric dipole moment of the electron, is estimated to be of order 1 ms. [ABSTRACT FROM AUTHOR]

We study the spectroscopy of an ensemble of negatively charged nitrogen-vacancy centers in diamond at high temperatures between room temperature and 700 K under high vacuum conditions. Spin resonances are studied using optical detection of magnetic resonance (ODMR), and optical spectroscopy is employed to study radiative transitions. Upon increasing the temperature, the intensity of radiative decay in visible and infrared decreased. In addition, the ODMR resonance frequencies were decreased, and the phonon line emission shifted to higher wavelengths. Density functional theory calculation of the zero-field splitting parameter (D) revealed that thermal expansion is not enough to explain the shift in the ODMR frequencies. Fitting the measured intensity of photo-luminescence with the theoretical predictions of the Mott–Seitz model yields the value of 0.22 eV for the energy barrier associated with nonradiative decay. [ABSTRACT FROM AUTHOR]

Time-dependent density functional theory (TDDFT) based approaches have been developed in recent years to model the excited-state properties and transition processes of the molecules in the gas-phase and in a condensed medium, such as in a solution and protein microenvironment or near semiconductor and metal surfaces. In the latter case, usually, classical embedding models have been adopted to account for the molecular environmental effects, leading to the multi-scale approaches of TDDFT/polarizable continuum model (PCM) and TDDFT/molecular mechanics (MM), where a molecular system of interest is designated as the quantum mechanical region and treated with TDDFT, while the environment is usually described using either a PCM or (non-polarizable or polarizable) MM force fields. In this Perspective, we briefly review these TDDFT-related multi-scale models with a specific emphasis on the implementation of analytical energy derivatives, such as the energy gradient and Hessian, the nonadiabatic coupling, the spin–orbit coupling, and the transition dipole moment as well as their nuclear derivatives for various radiative and radiativeless transition processes among electronic states. Three variations of the TDDFT method, the Tamm–Dancoff approximation to TDDFT, spin–flip DFT, and spin-adiabatic TDDFT, are discussed. Moreover, using a model system (pyridine–Ag20 complex), we emphasize that caution is needed to properly account for system–environment interactions within the TDDFT/MM models. Specifically, one should appropriately damp the electrostatic embedding potential from MM atoms and carefully tune the van der Waals interaction potential between the system and the environment. We also highlight the lack of proper treatment of charge transfer between the quantum mechanics and MM regions as well as the need for accelerated TDDFT modelings and interpretability, which calls for new method developments. [ABSTRACT FROM AUTHOR]

The semiconductor–metal heteronanocrystals (HNCs) that possess a perfect epitaxial interface can accommodate novel and interesting physical phenomena owing to the strong interaction and coupling between the semiconductor excitons and metal plasmons at the interface. Here, we fabricate the pyramidal ZnO–Au HNCs and study their unique photophysical properties. Several Au nanospheres are perfectly epitaxially bound with a single ZnO NC owing to the small lattice mismatch between them and there are also ZnO–Au–ZnO sandwiched HNCs. There is a strong coupling between the green defect-associated recombination in the ZnO NC and the localized surface plasmon resonance (LSPR) of the Au nanosphere at the interface of the HNC. This leads to resonant defect recombination–LSPR energy transfer and resultant nearly complete quenching of the green defect luminescence of the ZnO NCs in the HNCs, leaving only the UV exciton luminescence. The lifetimes of both the green and UV emission bands decrease significantly in the ZnO–Au HNCs relative to that of the pure ZnO NCs owing to the combined effect of resonance energy transfer and surface plasmon enhanced radiative transition. The exponent of the luminescence intensity–excitation intensity power function for the green emission band is remarkably smaller than unity, and this suggests that the involved defects have an intermediate concentration. [ABSTRACT FROM AUTHOR]

Color tunability from red to orange to yellow has been demonstrated in GaN-based LED devices with Eu-doped GaN layers as the active region. Under current injection, this is achieved by varying the current density and the pulse conditions. The underlying mechanism behind this color tunability is a redistribution of energy among the 5DJ states of a Eu3+ ion. This energy shuffling is facilitated by a local defect that has been neglected in previous modeling work. Including this defect allows for a quantitative prediction of the relative time-averaged populations of the Eu3+ ion's 5D0 and 5D1 states. Extracting, from experimental results, the red and yellow/green emission spectra due to radiative transitions from the respective levels and mixing them allows the overall chromaticity of the emission to be determined for varied excitation conditions. In addition, the model allows us to determine the optimal injection conditions to maximize the gamut of color tunability while minimizing power consumption. These simulations pave the way for practical, systematic color tuning from a single-contact pixel. [ABSTRACT FROM AUTHOR]

ErF3-doped TeO2–Ga2O3–BaF2–AlF3–Y2O3 (TGBAY) glasses with high fluorescence efficiency and a high thermal damage threshold were developed for potential mid-infrared fiber laser applications. A model 2.7-μm fiber laser based on this material was analyzed using rate and propagation equations. Under 808 and 980 nm laser pumping, fluorescence emissions with central wavelength at 1.55 and 2.73 μm were detected. Based on the Judd–Ofelt (J–O) theory, the intensity parameters (Ωλ, λ = 2, 4, and 6) and radiative transition property were calculated and characterized through absorption and emission spectra. The results indicated that tellurite–gallium oxyfluoride glass had a high glass transition temperature (Tg, ∼391 °C), large emission cross sections at 1.55 μm (6.32 × 10−21 cm2) and 2.73 μm (9.68 × 10−21 cm2) as well as a longer fluorescence lifetime (6.84 ms at 1.55 μm and 262 μs at 2.73 μm) relative to the conventional Er3+-doped tellurite glass. The temperature dependence of the emission spectra indicated that TGBAY-2Er glass was more favorable to achieve infrared emission at low temperatures. Numerical simulation revealed the feasibility of achieving a ∼2.7 μm fiber laser operation based on the developed Er3+-doped tellurite–gallium oxyfluoride glass fiber. [ABSTRACT FROM AUTHOR]

The role of porosity and surface-termination on the radiative and the nonradiative relaxation processes of luminescent porous silicon is investigated using temperature-dependent, time-resolved photoluminescence spectroscopy. We show that, for porous silicon having low- to mid-porosity, radiative relaxation times should be associated with the quantum confinement of excitons (the confined photo-excited electron–hole pairs), while nonradiative relaxation processes are related to the state of the surface (e.g., the surface chemistry), in agreement with previous reports. However, for high-porosity films of porous silicon, we have found much faster low-temperature relaxation times, which are associated with radiative transitions from the triplet excitonic state. This state becomes partially allowed due to a strong coupling and mixing with the singlet state in high-porosity films of porous silicon containing fairly small silicon nanocrystallites. [ABSTRACT FROM AUTHOR]

We report on the unusually large blue shift of the electroluminescence spectrum with an increase of the drive current at 77 K in a double-barrier nanoheterostructure with a deep AlSb/InAs0.83Sb0.17/AlSb quantum well grown by metal-organic vapor phase epitaxy on the n-GaSb:Te substrate. The rise of the drive current from 20 mA to 220 mA led to a shift of the electroluminescence spectrum maximum toward higher photon energies by 100 meV. It was shown that this effect is due to indirect (tunneling) radiative transitions between electrons in InAsSb quantum well and heavy holes localized near the AlSb/p-GaSb heterointerface. The energy of radiative transition was linearly dependent on the applied voltage. In the drive current range of 50–220 mA, an electroluminescence blue shift was accompanied by spectrum narrowing by 40 meV and a noticeable change of the spectrum shape. With a rise in the drive current, a superlinear increase of the electroluminescence intensity caused by the nonlinear dependence of tunneling radiative recombination rate on transition energy was observed at 300 K and 77 K. [ABSTRACT FROM AUTHOR]

We design thermal emitters based on gold micrograting structures with VO2-filled slots for enhanced far-field thermal rectification. We numerically calculate the thermal rectification ratio for two different approaches, peak extinction and peak shift. In peak extinction, the VO2 phase transition switches the radiative coupling of the surface plasmon on and off. In peak shift, the phase transition shifts the wavelength of the radiatively-coupled surface plasmon. We vary the extinction coefficient of VO2 to determine the effect on rectification for each approach. In both cases, the rectification ratio can be increased by increasing the VO2 loss in the metallic state. This suggests that highly efficient, microstructured thermal rectifiers can be achieved via suitable manipulation of the optical properties of VO2. [ABSTRACT FROM AUTHOR]

A photoluminescence (PL) method to characterize phase transition processes of Li+/Er3+:BaTiO3 (BLET) ferroelectric ceramics by infrared fluorescence intensities has been proposed. The fluorescence intensities are applied to detect the phase transition of BLET ceramics in multiple phase transition regions. It proves that the change in fluorescence intensities at the phase transition is due to a variation in the radiative transition probability of the 4I13/2 level. A physical mechanism is elucidated considering the relationship between fluorescence properties and the phase structure. Our research benefits an in-depth understanding of the relationship between the PL characteristics and the phase structure of ferroelectric materials and further develops the application potential of PL methods in ferroelectric phase transition detection. [ABSTRACT FROM AUTHOR]

We study the radiative hydrodynamics flow of radiation-driven heat waves in hot dense plasmas, using approximate self-similar solutions. Specifically, we have focused on the intermediate regime between the pure radiative supersonic flow and the pure subsonic regime. These two regimes were investigated using both exact self-similar solutions and numerical simulations; however, most of the study used numerical simulations, mainly because the radiative heat wave and the shock regions are not self-similar altogether. In a milestone work [Garnier et al., "Self-similar solutions for a nonlinear radiation diffusion equation," Phys. Plasmas 13, 092703 (2006)], it was found that for a specific power law dependency temperature profile, a unique exact self-similar solution exists that is valid for all physical regimes. In this work, we approximate Garnier's exact solution for a general power-law temperature-dependency, using simple analytical considerations. This approximate solution yields a good agreement compared to numerical simulations for the different thermodynamic profiles within the expected range of validity. In addition, we offer an approximate solution for the energies absorbed in the matter, again, for a general power-law temperature profile. Our approximate self-similar solution for the energy yields very good results compared to exact numerical simulations for both gold and Ta 2 O 5 . We also set a comparison of our self-similar solutions with the results of an experiment for radiation temperature measurement in a Hohlraum in low-density foams that is addressed directly to the intermediate regime, yielding a good agreement and similar trends. The different models as well as the numerical simulations are powerful tools to analyze the supersonic–subsonic transition region. [ABSTRACT FROM AUTHOR]

A controlled manipulation of defects in zinc oxide (ZnO) and the understanding of their electronic structure can be a key issue towards the fabrication of p-type ZnO. Zn vacancy (VZn), Zn interstitials (IZn), and O vacancy (VO) are mainly native point defects, determining the optoelectronic properties of ZnO. The electronic structure of these defects still remains controversial. Here, we experimentally demonstrate that the green emission in ZnO comes from VZn-related deep acceptor and VZn-VO clusters, which is accompanied by the radiative transition between the triplet and the ground singlet state with the excited singlet state located above the CB minimum. Moreover, the IZn is identified to be a shallow donor in ZnO, being mainly responsible for the n-type conductivity of non-intentionally doped ZnO. [ABSTRACT FROM AUTHOR]

Photoluminescence (PL) of Eu3+ ions in single layers of ZnO and ZnMgO and in short-period ZnO/ZnMgO and ZnO/MgO superlattices (SLs) grown by molecular beam epitaxy was investigated, and the results have been presented. Single ZnO and ZnMgO layers were grown on GaN/sapphire substrates, whereas the SLs on c-oriented crystalline ZnO. All samples were implanted with 300 keV Eu ions, and then rapid thermal annealing was performed at 800 °C for 5 min in an oxygen atmosphere to activate the intra-4f-shell PL of Eu3+ ions. After annealing, the red emission due to the 5D0 → 7F2 radiative transitions of Eu3+ was observed at excitation above the energy gap. The results clearly show that Mg is essential for obtaining the high 4f–4f PL intensity of Eu3+ ions, as in ZnMgO layers and short period SLs, the red emission is several times more intense than in ZnO. The PL excitation spectra were examined to confirm the energy transfer from the bandgap to the Eu3+ ions. Clear evidence is presented that in the studied short period SLs, the excitation of the Eu3+ PL occurs via generation of excitons in the barrier layers. [ABSTRACT FROM AUTHOR]

To experimentally identify the character of radiative transitions in trigonal Cu2BaSnS4, we conduct temperature and excitation intensity dependent photoluminescence (PL) measurements in the temperature range of 15–300 K. The low-temperature near band edge PL spectrum is interpreted as the free exciton at 2.11 eV and the bound exciton at 2.08 eV, coupled with associated phonon-assisted transitions. In the low energy region, we assign the dominant defect emission at 1.96 eV to donor–acceptor-pair recombination and the weak broad emission at 1.6 eV to the free-to-bound transition. The activation energies and temperature shift for the radiative transitions are determined and discussed. Above 90 K, the free exciton recombination becomes the dominant radiative transition, with its energy shift mainly governed by the contribution of optical phonons. [ABSTRACT FROM AUTHOR]

We report here ab initio calculated electronic transition dipole moments for the sodium dimer ion pair states of ¹Σg+ g symmetry. They vary strongly as a function of internuclear distance because of the effect of the Na+ + Na- ion pair potential, which also causes the formation of additional wells and shoulders in the molecular potential energy curves. We also present a computational study of the transition dipole moment matrix elements and lifetimes for these ion-pair states. [ABSTRACT FROM AUTHOR]

Non-adiabatic dynamics, where systems non-radiatively transition between electronic states, plays a crucial role in many photo-physical processes, such as fluorescence, phosphorescence, and photoisomerization. Methods for the simulation of non-adiabatic dynamics are typically either numerically impractical, highly complex, or based on approximations which can result in failure for even simple systems. Recently, the Semiclassical Monte Carlo (SCMC) approach was developed in an attempt to combine the accuracy of rigorous semiclassical methods with the efficiency and simplicity of widely used surface hopping methods. However, while SCMC was found to be more efficient than other semiclassical methods, it is not yet as efficient as is needed to be used for large molecular systems. Here, we have developed two new methods: the accelerated-SCMC and the accelerated-SCMC with re-Gaussianization, which reduce the cost of the SCMC algorithm up to two orders of magnitude for certain systems. In most cases shown here, the new procedures are nearly as efficient as the commonly used surface hopping schemes, with little to no loss of accuracy. This implies that these modified SCMC algorithms will be of practical numerical solutions for simulating non-adiabatic dynamics in realistic molecular systems. [ABSTRACT FROM AUTHOR]

Eu+3 was incorporated into the lattice of a lead-free ferroelectric Na1/2Bi1/2TiO3 (NBT) as per the nominal formula Na0.5Bi0.5-xEuxTiO3. This system was investigated with regard to the Eu+3 photoluminescence (PL) and structural behaviour as a function of composition and electric field. Electric field was found to irreversibly change the features in the PL spectra and also in the x-ray diffraction patterns below the critical composition x=0.025. Detailed analysis revealed that below the critical composition, electric field irreversibly suppresses the structural heterogeneity inherent of the host matrix NBT and brings about a long range ferroelectric state with rhombohedral (R3c) distortion. It is shown that the structural disorder on the nano-scale opens a new channel for radiative transition which manifests as a new emission line branching off from the main 5D0→7F0 line along with a concomitant change in the relative intensity of the other crystal field induced Stark lines with different J values. The study suggests that Euþ3 luminescence can be used to probe the relative degree of field induced structural ordering in relaxor ferroelectrics and also in high performance piezoelectric alloys where electric field couples very strongly with the lattice and structural degrees of freedom. [ABSTRACT FROM AUTHOR]

Eu+3 was incorporated into the lattice of a lead-free ferroelectric Na1/2Bi1/2TiO3 (NBT) as per the nominal formula Na0.5Bi0.5-xEuxTiO3. This system was investigated with regard to the Eu+3 photoluminescence (PL) and structural behaviour as a function of composition and electric field. Electric field was found to irreversibly change the features in the PL spectra and also in the x-ray diffraction patterns below the critical composition x=0.025. Detailed analysis revealed that below the critical composition, electric field irreversibly suppresses the structural heterogeneity inherent of the host matrix NBT and brings about a long range ferroelectric state with rhombohedral (R3c) distortion. It is shown that the structural disorder on the nano-scale opens a new channel for radiative transition which manifests as a new emission line branching off from the main 5D0→7F0 line along with a concomitant change in the relative intensity of the other crystal field induced Stark lines with different J values. The study suggests that Euþ3 luminescence can be used to probe the relative degree of field induced structural ordering in relaxor ferroelectrics and also in high performance piezoelectric alloys where electric field couples very strongly with the lattice and structural degrees of freedom. [ABSTRACT FROM AUTHOR]

We report electroluminescence originating from L-valley transitions in n-type Ge/Si0.15Ge0.85 quantum cascade structures centered at 3.4 and 4.9 THz with a line broadening of Δ f / f ≈ 0.2. Three strain-compensated heterostructures, grown on a Si substrate by ultrahigh vacuum chemical vapor deposition, have been investigated. The design is based on a single quantum well active region employing a vertical optical transition, and the observed spectral features are well described by non-equilibrium Green's function calculations. The presence of two peaks highlights a suboptimal injection in the upper state of the radiative transition. Comparison of the electroluminescence spectra with a similar GaAs/AlGaAs structure yields one order of magnitude lower emission efficiency. [ABSTRACT FROM AUTHOR]

A unified framework for describing dense plasma screenings from warm dense matter to hot dense matter, including gradient correction and exchange-correlation, has been recently proposed by Stanton and Murillo [Phys. Rev. E 91,033104 (2015)]. With this unified description of linear screening, the energy levels, absorption oscillator strengths, and radiative transition probabilities of hydrogen-like plasmas are reported for a wide range of critical screening parameters (plasma temperatures and densities), where the bound level nl merges into continuum. The importance of both finite temperature gradient correction and exchange correlation increases with the increasing plasma coupling strength or the Coulomb coupling parameter Γ, and the effect of exchange correlation is generally greater than that of finite temperature gradient correction. Further results from the weakly coupled and strongly coupled limits or Debye plasmas and dense quantum plasmas also show consistency with those from the unified framework, respectively. [ABSTRACT FROM AUTHOR]

The absolute quantum cutting efficiency of Tb3+-Yb3+ co-doped glass was quantitatively measured by an integrating sphere detection system, which is independent of the excitation power. As the Yb3+ concentration increases, the near infrared quantum efficiency exhibited an exponential growth with an upper limit of 13.5%, but the visible light efficiency was reduced rapidly. As a result, the total quantum efficiency monotonically decreases rather than increases as theory predicted. In fact, the absolute quantum efficiency was far less than the theoretical value due to the low radiative efficiency of Tb3+ (<61%) and significant cross-relaxation nonradiative loss between Yb3+ ions. [ABSTRACT FROM AUTHOR]

An efficient three-photon near-infrared (NIR) quantum cutting (QC) is reported in Tm3+ singly doped YVO4 polycrystalline phosphors, where an optimized content of Tm3+ is determined to be 1.0 mol. %. Upon the absorption of a visible photon around 473 nm, three NIR photons emitting at 1180, 1479, and 1800 nm can be obtained efficiently by the sequential three-step radiative transitions of Tm3+. The underlying mechanisms are analyzed in terms of the steady and dynamic fluorescence spectra measurements. Internal quantum yield is calculated to be 161.8% as a theoretical value when luminescence quenching due to defect species can be overcome. In addition, the broadband ultraviolet (UV)-excited [VO4]3- can strongly sensitize the 1G4 level of Tm3+ in the wavelength range likely from 250 to 360 nm, greatly increasing the UV photo-response and NIR fluorescent intensity of Tm3+. The further development of this broadband three-photon NIR QC material would explore the new route to improve the photo-response of novel photoelectronic devices, particularly in 250-360 nm. [ABSTRACT FROM AUTHOR]

In recent years, semiconductor quantum dots have demonstrated their potential to reach the goal of being an ideal source of single and entangled photon pairs. Exciting reports of near unity entanglement fidelity, close to unity photon indistinguishability, and high collection efficiency in nanophotonic structures have been demonstrated by several distinct groups, showing unequivocally the maturity of this technology. To achieve the required complexity and scalability in realistic quantum photonic implementations, two-photon interference of photons from multi-sources must be reached. While high indistinguishability values have been observed for photons generated from the same source within a relatively short time separation, achieving similar visibility for larger time separation or in multi-source experiments still requires intensive efforts. In fact, the coupling to the particular mesoscopic environment of charge carriers confined in the quantum dot leads to decoherence processes, which limit the quantum interference effects to a short time window. Here, we discuss the progress in studying the dynamics of this decoherence, which crucially depends on the evolution of line broadening in high-quality self-assembled InGaAs quantum dots. Characterization of line broadening mechanisms is the first fundamental step to be able to counteract them. Optimization of the growth and active and passive control of the radiative transitions are crucial for the technological readiness of non-classical light sources based on semiconductor platforms. [ABSTRACT FROM AUTHOR]

The spin forbidden transition a1Δ → X3Σ- in CH- has been studied using the Breit-Pauli Hamiltonian for a large number of geometries. This transition acquires intensity through spin-orbit coupling with singlet and triplet Π states. The transition moment matrix including more than one singlet and triplet Π states was calculated at the multi-reference configuration interaction/aug-cc-pV6Z level of theory. The computed radiative lifetime of 5.63 s is in good agreement with the experimental (5.9 s) and other theoretical (6.14 s) results. Transition moment values of the spin allowed A3Π → X3Σ- transition have also been calculated at the same level of theory. Calculations show that the corresponding radiative lifetime is considerably low, 2.4 × 10-7 s. Complete basis set extrapolated potential energy curves for the ground state of CH and the ground state and six low lying excited states (a1Δ, b1Σ+, two 3Π, and two 1Π) of CH- are reported. These curves are then used to calculate the vibrational bound states for CH and CH-. The computed electron affinity of CH supports the electron affinity bounds reported by Okumura et al. [J. Chem. Phys. 85, 1971 (1986)]. [ABSTRACT FROM AUTHOR]

Sub-monolayer quantities of Mg are introduced in multilayer stacked ZnMgTe quantum dots (QDs) embedded in ZnSe barriers in order to reduce the hole confinement energy by controlling the bandgaps and band-offsets of ZnTe/ZnSe system having type-II band alignment. The photoluminescence (PL) emission from such ZnMgTe/ZnSe QD structure is found to be a broad band centered at 2.35 eV. The higher energy side of the PL band shows a larger blue-shift with increasing excitation intensity and a faster life-time decay due to a greater contribution of the emission from the smaller size dots and the isoelectronic bound excitons. It is found that the characteristic decay time of the PL evolves along the band with a value of 129 ns at 2.18 eV to 19 ns at 2.53 eV. The temperature dependent PL emission is controlled by two thermally activated processes: ionization of electrons away from QD state to the barrier (EA1 ∼ 3 meV) by breaking the type-II excitons and thermal escape of the holes from the ground state to the barrier (EA2 ∼ 114-116 meV). We propose a modified band diagram and energy levels for this ZnMgTe/ZnSe multilayer QD system by determining the composition of Mg inside the QDs and solving the 1-D Schrodinger's equation and show that Mg incorporation lowers the hole activation energy via modification of the valence band offset without changing the barrier significantly. [ABSTRACT FROM AUTHOR]

In the classical modeling of decay rates for molecules interacting with a nontrivial environment, it is well known that two alternate approaches exist which include: (1) a mechanical model treating the system as a damped harmonic oscillator driven by the reflected fields from the environment; and (2) a model based on the radiative and nonradiative energy transfers from the excited molecular system to the environment. While the exact equivalence of the two methods is not trivial and has been explicitly demonstrated only for planar geometry, it has been widely taken for granted and applied to other geometries such as in the interaction of the molecule with a spherical particle. Here we provide a rigorous proof of such equivalence for the molecule-sphere problem via a direct calculation of the decay rates adopting each of the two different approaches. [ABSTRACT FROM AUTHOR]

The authors theoretically investigate quantum confinement and transition energies in quantum wells (QWs) asymmetrically positioned in wrinkled nanomembranes. Calculations reveal that the wrinkle profile induces both blue- and redshifts, depending on the lateral position of the QW probed. Relevant radiative transitions include the ground state of the electron (hole) and excited states of the hole (electron). Energy shifts as well as stretchability of the structure are studied as a function of wrinkle amplitude and period. Large tunable bandwidths of up to 70 nm are predicted for highly asymmetric, wrinkled QWs. [ABSTRACT FROM AUTHOR]

A systematic analysis of the S1-trans (A1Au) state of acetylene, using IR-UV double resonance along with one-photon fluorescence excitation spectra, has allowed assignment of at least part of every single vibrational state or polyad up to a vibrational energy of 4200 cm-1. Four observed vibrational levels remain unassigned, for which no place can be found in the level structure of the trans-well. The most prominent of these lies at 46 175 cm-1. Its 13C isotope shift, exceptionally long radiative lifetime, unexpected rotational selection rules, and lack of significant Zeeman effect, combined with the fact that no other singlet electronic states are expected at this energy, indicate that it is a vibrational level of the S1-cis isomer (A1A2). Guided by ab initio calculations [J. H. Baraban, A. R. Beck, A. H. Steeves, J. F. Stanton, and R. W. Field, J. Chem. Phys. 134, 244311 (2011)] of the cis-well vibrational frequencies, the vibrational assignments of these four levels can be established from their vibrational symmetries together with the 13C isotope shift of the 46 175 cm-1 level (assigned here as cis-3161). The S1-cis zero-point level is deduced to lie near 44 900 cm-1, and the ν6 vibrational frequency of the S1-cis well is found to be roughly 565 cm-1; these values are in remarkably good agreement with the results of recent ab initio calculations. The 46 175 cm-1 vibrational level is found to have a 3.9 cm-1 staggering of its K-rotational structure as a result of quantum mechanical tunneling through the isomerization barrier. Such tunneling does not give rise to ammonia-type inversion doubling, because the cis and trans isomers are not equivalent; instead the odd-K rotational levels of a given vibrational level are systematically shifted relative to the even-K rotational levels, leading to a staggering of the K-structure. These various observations represent the first definite assignment of an isomer of acetylene that was previously thought to be unobservable, as well as the first high resolution spectroscopic results describing cis-trans isomerization. [ABSTRACT FROM AUTHOR]

Mid-infrared laser lines observed in hydrogen/rare gas discharges are assigned to three-body recombination processes involving an electron, a rare gas (He or Ne) atom, and the triatomic hydrogen ion (H3+). Calculations of radiative transitions between neutral H3 Rydberg states support this interpretation, and link it to recent results for hydrogenic/rare gas afterglow plasmas. A mechanism for the population inversion is proposed, and the potential generality and astrophysical implications of such molecular recombination laser systems are briefly discussed. [ABSTRACT FROM AUTHOR]

We herein report on the occurrence of bright and efficient electrophosphorescence from a simple organic light-emitting diode (OLED) with a single organic layer comprised of a small molecular ionic iridium compound, formed using a solution process. The studied small molecular ionic iridium compound is [Ir(dfppy)2(bpy)]+PF6-, which exhibits excellent film-forming properties, bright green photoluminescence, and efficient bipolar carrier transport with balanced electron and hole mobilities of about 10-5 cm2/(V s). A high performance of the device was achieved by using a phosphorescent OLED (PHOLED) that was fabricated using the [Ir(dfppy)2(bpy)]+PF6- compound, with a peak brightness of about 18 000 cd/m2 and a peak current efficiency of 12 cd/A. A peak power efficiency of 2.5 lm/W was measured at 2800 cd/m2. These results suggest that the small molecular ionic iridium compound is a promising material for bright and efficient PHOLEDs manufactured using a simple solution process. [ABSTRACT FROM AUTHOR]

We employ a Monte Carlo method to investigate the carrier dynamics in the terahertz quantum cascade lasers with vertical and diagonal radiative transition designs. Electron-electron and electron-phonon scattering are included in the calculations and their effects on the temperature dependence of electron transport are evaluated. The simulation shows that the degradation of temperature performance is mainly due to the rapid electron relaxation from upper to lower laser levels, in which the electron-phonon interaction is the dominant scattering mechanism. The parasitic coupling between laser levels is weakened in the diagonal design, resulting in better device performance such as lower current density, higher operating temperature, and less hot electron effects. The calculations are in good agreement with experimental results. [ABSTRACT FROM AUTHOR]

Copper atoms and molecules are laser ablated into bulk liquid and solid helium, and the emission spectra of the laser excited D→X, B→X, and a→X transitions of Cu2 are observed to exhibit clearly resolved vibrational bands. Surprisingly, for the D→X and the B→X transitions, no differences were observed for superfluid He at 1.5 K, for the normal liquid at 2.65 K, or for the 1.5 K solid at higher pressures of about 30 bars. An interpretation based on the bubble model indicates that the interaction with the He matrix is much weaker than in the case of the alkali atoms. Compared to other solid rare gas matrices, the line shifts and line widths in condensed helium are much smaller by nearly an order of magnitude. [ABSTRACT FROM AUTHOR]

The first metastable triplet state of HeH+ was found to be present in ion beam experiments, with its lifetime estimated to be between hundreds of milliseconds and thousand of seconds. In this work, we use ab initio methods to evaluate the radiative lifetimes of the six vibrational levels of the a 3Σ+ of HeH+. The transition a 3Σ+→X 1Σ+ is spin-forbidden, but acquires intensity through spin-orbit interaction with the singlet and triplet Π states. Large scale CASSCF/MRCI calculations using an adapted basis set were performed to determine the potential energy curves of the relevant states of HeH+ as well as the matrix elements of the dipole and spin-orbit operators. The wave functions and energies of the vibrational levels of the a 3Σ+ and X 1Σ+ states are obtained using a B-spline method and compared to previous works. We find that the radiative lifetime of the vibrational levels increases strongly with v, the lifetime of the v=0 state being 150 s. We also analyze the contributions from discrete and continuum parts of the spectrum. With such a long lifetime, the a 3Σ+ state could have astrophysical implications. [ABSTRACT FROM AUTHOR]

Holmium doped lead–germano–tellurite glass was prepared by the melt quenching technique. The Judd–Ofelt intensity parameters were estimated as Ω2=7.6×10-20, Ω4=12.9×10-20, and Ω6=2.5×10-20 cm2. Radiative transition probabilities and lifetimes were also determined for some of the levels. Room temperature upconversion emissions have been observed from Ho3+ at 497 nm under 532 nm laser excitation, and at 557 and 668 nm under 762 nm laser excitation. The upconversion emission mechanisms were found to be due to a step wise excitation process. Upconversion emission intensity enhanced in a heat treated glass. [ABSTRACT FROM AUTHOR]

We have studied the dynamics of strong radiative shocks generated with the high-energy subnanosecond iodine laser at Prague Asterix Laser System facility (Prague) over long time scales, up to 100 ns. These shock waves are characterized by a developed radiative precursor, a radiation driven ionization wave in front of the density jump of the shock. Electronic density profiles are measured at different times after the laser pulse and at different distances from the axis of the shock tube. A new feature, described as a split precursor, has been observed. Comparisons with two-dimensional computations are shown. [ABSTRACT FROM AUTHOR]

In expansion cooling phase of pinched nitrogen plasma generated by fast capillary discharge, it might be possible to realize lasing a Blamer α recombination SXRL, which requires a rapid cooling of nonequilibrium plasma. It is effective to decrease the discharge current rapidly in reducing the additional heating caused by the joule heating and the magnetic compression of plasma as quickly as possible. The shaping of discharge current waveform was demonstrated with a transmission line and its effect on expanding plasma dynamics were investigated through magnetohydrodynamics (MHD) calculation, and validity of the MHD calculation in the expansion phase was shown using the discharge photographs taken by using a high speed camera. As a result, strong radiation from the H-like N ion at the maximum pinch, which is in the current decay phase of the triangular current with peak amplitude of over 70 kA and pulse width of 60 ns, has been confirmed in x-ray photodiode signals at wavelength of less than 2.5 nm, to clarify the existence of the Lyman series and continuum of the H-like N ion. Without additional heating by the discharge current after the generation of the fully stripped nitrogen ions, it might be possible to generate the population inversion between the principal quantum number n=2 and 3. [ABSTRACT FROM AUTHOR]

One of the general challenges for the evaluation and interpretation of optical emission spectroscopy measurements is the occurrence of radiation trapping, which is also named self-absorption or opacity. A convenient technique to treat radiation trapping is the introduction of a parameter, which is called escape factor and gives a measure for the amount of radiation trapping. In this paper evaluations on the concept of escape factors are presented for an inductively coupled plasma in argon. Especially, the strong argon line at 811.53 nm, which arises from the transition of 2p9-1s5, is under consideration. To estimate escape factors for this line, a particular method is proposed and presented here. First experimental results are obtained under the restrictive assumptions that transitions into the resonant levels 1s2 and 1s4 are sufficiently optically thin and ratios of population densities are constant. [ABSTRACT FROM AUTHOR]

Oscillator strengths of 23 electric-dipole-allowed bands of 14N2 in the 86.0–89.7 nm (111 480–116 280 cm-1) region are reported from synchrotron-based photoabsorption measurements at an instrumental resolution of 6.5×10-4 nm (0.7 cm-1) full width at half maximum. The absorption spectrum comprises transitions to vibrational levels of the cn 1Πu (n=3,4), o3 1Πu, and cn+1′ 1Σu+(n=3,4) Rydberg states as well as the b 1Πu and b′ 1Σu+ valence states. The J dependences of band f-values derived from the experimental line f-values are reported as polynomials in J(J+1) and are extrapolated to zero nuclear rotation in order to facilitate comparisons with the results of coupled Schrödinger equation calculations. Many bands in this study are characterized by a strong J dependence of the band f-values and display anomalous P-, Q-, and R-branch intensity patterns. Predissociation line widths are reported for six bands. The experimental f-value and line-width patterns inform current efforts to develop comprehensive spectroscopic models for N2 that incorporate rotational effects and predissociation mechanisms, and are critical for the construction of realistic atmospheric radiative-transfer models. [ABSTRACT FROM AUTHOR]

The blue-green photoluminescence emitted by pure and electron-doped strontium titanate under intense pulsed near-ultraviolet excitation is studied experimentally as a function of excitation intensity and temperature. Both emission spectra and time-resolved decays of the emission are measured and analyzed in the framework of simple phenomenological models. We find an interesting blue-to-green transition occurring for increasing temperatures in pure samples, which is absent in doped materials. The luminescence yield and decay rate measured as a function of temperature can be modeled well as standard activated behaviors. The leading electron-hole recombination process taking place in the initial decay is established to be second order, or bimolecular, in contrast to recent reports favoring a third-order interpretation as an Auger process. The temporal decay of the luminescence can be described well by a model based on two interacting populations of excitations, respectively identified with interacting defect-trapped (possibly forming excitons) and mobile charges. Finally, from the measured doping and sample dependence of the luminescence yield, we conclude that the radiative centers responsible for the luminescence are probably intrinsic structural defects other than bulk oxygen vacancies. [ABSTRACT FROM AUTHOR]

We present an experimental study of the radiative recombination dynamics in size-controlled anatase TiO2 nanoparticles in the range 20–130 nm. From time-integrated photoluminescence spectra and picosecond time-resolved experiments as a function of the nanoparticle size, excitation density, and temperature, we show that photoluminescence comes out from a bulk and a surface radiative recombination. The spectral shift and the different time dynamics provide a clear distinction between them. Moreover, the intrinsic nature of the emission is also proven, providing a quantitative evaluation of volume and surface contributions. [ABSTRACT FROM AUTHOR]