Radiative Decay Time Research Articles

Perceiving impressive optical properties of ternary SiO2-TiO2-ZrO2:Eu3+ sol-gel glasses with high reluctance for concentration quenching: An experimental approach

Eu3+-doped ternary SiO2-TiO2-ZrO2 glasses were prepared through a non-hydrolytic sol-gel process. Fourier transform infrared spectroscopy was employed to identify the vibrational modes present in the ternary glass. The phonon energies were calculated using excitation spectrum and complemented well with the vibrational modes in the Raman spectrum. Photoluminescence spectrum was used to assess the luminescent augmentation. The uniform distribution of dopant ions in the as prepared ternary glass aided in achieving a comparatively unmatched doping concentration of Eu3+ion (17.5wt%) without luminescence quenching and it can be attributed to reduced cluster formation. High concentration of rare earth ion incorporation leads to intense and tunable emission with higher brightness. Excellent color purity of the emission by means of CIE color gamut for the red color was observed, which is close to the National Television Standard Committee (NTSC) value. Theoretical investigations of the photophysical parameters and radiative decay time were carried out with Judd-Ofelt analysis. The double exponentially fitted decay curve gave a satisfactory experimental lifetime. These results indicate that tailor made europium doped ternary SiO2-TiO2-ZrO2 glasses have potential applications in the areas of photonic devices as well as biological sensors because of its intense tunable and efficient emission, high purity, and low toxicity.

Internal quantum efficiency in polar and semipolar (11–22) InxGa1-xN/InyGa1-yN quantum wells emitting from blue to red

In this work, we investigate the impact of the quantum confined Stark effect and of the carrier localization on the internal quantum efficiency of polarized single or multiple InxGa1-xN/GaN quantum well(s), and semi-polar (11–22) multiple InxGa1-xN/InyGa1-yN quantum well. We find that increasing the influence of the quantum confined Stark effect at constant indium content with increasing the well-width induces a reduction of the internal quantum efficiency onsets for a decreasing value of the photoexcitation density. Similar but no so dramatic trend is reported when increasing the indium content and thus when increasing the localization of carriers to localized fluctuations of the chemical composition of the alloy. In addition, a change of the electric field internal to active layers (quantified by using time-resolved photoluminescence spectroscopy) realized by growing samples along a semi-polar orientation leads to experimental observation of a substantial enhancement of the threshold of photoexcitation density at which onsets the reduction of the internal quantum efficiency. A correlation is found through several orders of magnitude between the photoexcitation density PT for the onset of the collapse of IQE and the values of the photoluminescence radiative decay time τrad. A scaling law is found in the investigated samples: PT ∼ τrad-n with n = 3/2 ± 0.15 which evidences that quantum confined Stark effect is the main origin for the efficiency droop in nitride light-emitting diodes based on InxGa1-xN active layers.

Thermally Activated Delayed Fluorescence from Ag(I) Complexes: A Route to 100% Quantum Yield at Unprecedentedly Short Decay Time.

The four new Ag(I) complexes Ag(phen)(P2-nCB) (1), Ag(idmp)(P2-nCB) (2), Ag(dmp)(P2-nCB) (3), and Ag(dbp)(P2-nCB) (4) with P2-nCB = bis(diphenylphosphine)-nido-carborane, phen = 1,10-phenanthroline, idmp = 4,7-dimethyl-1,10-phenanthroline, dmp = 2,9-dimethyl-1,10-phenanthroline, and dbp = 2,9-di-n-butyl-1,10-phenanthroline were designed to demonstrate how to develop Ag(I) complexes that exhibit highly efficient thermally activated delayed fluorescence (TADF). The substituents on the 1,10-phenanthroline ligand affect the photophysical properties strongly (i) electronically via influencing the radiative rate of the S1 → S0 transition and (ii) structurally by rigidifying the molecular geometry with respect to geometry changes occurring in the lowest excited S1 and T1 states. The oscillator strength of the S1 ↔ S0 transition f(S1 ↔ S0)-an important parameter for the TADF efficiency being proportional to the radiative rate-can be increased from f(S1 ↔ S0) = 0.0258 for Ag(phen)(P2-nCB) (1) to f(S1 ↔ S0) = 0.0536 for Ag(dbp)(P2-nCB) (4), as calculated for the T1 state optimized geometries. This parameter governs the radiative TADF decay time (τr) at ambient temperature, found to be τr = 5.6 μs for Ag(phen)(P2-nCB) (1) but only τr = 1.4 μs for Ag(dbp)(P2-nCB) (4)-a record TADF value. In parallel, the photoluminescence quantum yield (ΦPL) measured for powder samples at ambient temperature is boosted up from ΦPL = 36% for Ag(phen)(P2-nCB) (1) to ΦPL = 100% for Ag(dbp)(P2-nCB) (4). This is a consequence of a cooperative effect of both decreasing the nonradiative decay rate and increasing the radiative decay rate in the series from Ag(phen)(P2-nCB) (1), Ag(idmp)(P2-nCB) (2), and Ag(dmp)(P2-nCB) (3) to Ag(dbp)(P2-nCB) (4). Another parameter important for the TADF behavior is the activation energy of the S1 state from the state T1, ΔE(S1-T1). Experimentally it is determined for the complexes Ag(dmp)(P2-nCB) (3) and Ag(dbp)(P2-nCB) (4) to be of moderate size of ΔE(S1-T1) = 650 cm-1.

Negatively Charged and Dark Excitons in CsPbBr3 Perovskite Nanocrystals Revealed by High Magnetic Fields.

The optical properties of colloidal cesium lead halide perovskite (CsPbBr3) nanocrystals are examined by time-resolved and polarization-resolved spectroscopy in high magnetic fields up to 30 T. We unambiguously show that at cryogenic temperatures the emission is dominated by recombination of negatively charged excitons with radiative decay time of 300 ps. The additional long-lived emission, which decay time shortens from 40 down to 8 ns and in which the decay time shortens and relative amplitude increases in high magnetic fields, evidences the presence of a dark exciton. We evaluate g-factors of the bright exciton gX = +2.4, the electron ge = +2.18, and the hole gh = -0.22.

Optical spectroscopy and emission properties of Ho3 +-doped gadolinium calcium silicoborate glasses for visible luminescent device applications

The Ho3+-doped gadolinium calcium silicoborate glasses with the composition of 25Gd2O3+10CaO+10SiO2+(55−x)B2O3+xHo2O3, where x=0.0, 0.5, 1.0, 1.5, 2.0 and 2.5mol%, were synthesized by well-known melt quenching method. These glasses have been characterized through physical, structural, optical, visible emission and decay curve analysis. From the absorption spectrum, Judd-Ofelt intensity parameters for 1.0mol% Ho2O3 doped glass have been carried out and were used to examine radiative properties for the important visible emitting levels of Ho3+ ions. The present Ho3+:glasses show strong green emission at 545nm and red emission at 655nm under excitation at 452nm. The radiative decay time and stimulated emission cross-section for the (5S2,5F4)→5I8 transition are found to be 280 μs and 0.63×10−20cm2, respectively. Decay curves for all the concentration of Ho3+ ions exhibit non-exponential nature for (5S2,5F4)→5I8 transition. The decrease in decay times is due to the resonant energy transfer among the excited Ho3+ ions. Hence, these results confirm that the present silicoborate glass system could be considered as a promising candidate for visible green laser applications.

Energy Level Structure and Multiple 4f125d1 Emission Bands for Tm2+ in Halide Perovskites: Theory and Experiment

Rapid nonradiative relaxation from high-excited electronic states to lower-excited electronic states in luminescent materials leads to radiative transitions only from the lowest excited state. This behavior is observed in most luminescent materials. One notable exception is Tm2+, which is known to show luminescence from up to three 4f125d1 excited states. Here we report a study that explains the deviant behavior of Tm2+. Using ab initio wave function-based embedded-cluster calculations in Tm2+ -doped CsCaBr3 and CsCaCl3, we show that the manifold of 4f125d1 excited states shows various energy gaps for states with parallel potential energy surfaces. This strongly limits nonradiative relaxation, enabling emission from highly excited states in the case of a large energy gap to the next lower state. We also compared calculated and measured absorption and emission spectra and radiative decay times of the emitting states, which show a remarkably good agreement between theory and experiment. Based on the new ins...

Design of nonlinear optical response of multipole-type excitons by film thickness and incident pulse width

We theoretically investigate the nonlinear optical pulse responses of excitons in a thin film where the excitonic center-of-mass motion is confined. A large interaction volume between excitons and radiation yields particular coupled states with radiative decay times reaching several femto-seconds. By considering two polarization directions of light, we reveal that these fast-decay modes dominantly survive in an optical Kerr spectra even under a massive nonradiative damping $\Gamma=30$ meV. The results clearly show that there is an optimal combination of the incident pulse width and the film thickness for maximizing the integrated intensity of nonlinear signals.

Influence of the substrate material on the optical properties of tungsten diselenide monolayers

Monolayers of transition-metal dichalcogenides such as WSe2 have become increasingly attractive due to their potential in electrical and optical applications. Because the properties of these 2D systems are known to be affected by their surroundings, we report how the choice of the substrate material affects the optical properties of monolayer WSe2. To accomplish this study, pump-density-dependent micro-photoluminescence measurements are performed with time-integrating and time-resolving acquisition techniques. Spectral information and power-dependent mode intensities are compared at 290 K and 10 K for exfoliated WSe2 on SiO2/Si, sapphire (Al2O3), hBN/Si3N4/Si, and MgF2, indicating substrate-dependent appearance and strength of exciton, trion, and biexciton modes. Additionally, one CVD-grown WSe2 monolayer on sapphire is included in this study for direct comparison with its exfoliated counterpart. Time-resolved micro-photoluminescence shows how radiative decay times strongly differ for different substrate materials. Our data indicates exciton–exciton annihilation as a shortening mechanism at room temperature, and subtle trends in the decay rates in correlation to the dielectric environment at cryogenic temperatures. On the measureable time scales, trends are also related to the extent of the respective 2D-excitonic modes’ appearance. This result highlights the importance of further detailed characterization of exciton features in 2D materials, particularly with respect to the choice of substrate.

Optical and spectroscopic study of erbium doped calcium borotellurite glasses

In this study, 10CaF2 – (29.9–0.4x)CaO – (60-0.6x)B2O3 – xTeO2 – 0,1Er2O3 (x = 10, 16, 22, 30 and 50 mol %) glasses were synthesized, and their optical and spectroscopic properties were investigated. X-ray diffraction, density, glass transition temperature (Tg), crystallization temperature (Tx), refraction index, luminescence, radiative lifetime and optical absorption measurements were carried out. Molar volume (Vm), thermal stability (Tx-Tg), electronic polarizability (αm), optical bang gap energy (Eg) and Judd-Ofelt (JO) parameters Ωt (2,4,6) were also calculated. The results are discussed in terms of tellurium oxide content. The increase of TeO2 in the glasses composition increases density, refractive index and electronic polarizability. The optical band gap energy decreases varying from 3.37 to 2.71 eV for the glasses with 10 and 50 mol% of TeO2, respectively. The optical absorption coefficient spectra show characteristic bands of Er3+ ions. Furthermore, these spectra in NIR region show a decrease of hydroxyl groups as a function of TeO2 addition. Luminescence intensity and radiative lifetimes at 1530 nm show an increasing with the TeO2 content. The JO parameters of Er:CaBTeX glasses follow the trend Ω2 > Ω4 > Ω6 and the quality factor values (Ω4/Ω6) were between 1.37 and 3.07. By comparing the measured lifetime with the calculated radiative decay time, quantum efficiency was calculated. The luminescence emission intensity at 1530 nm decreases with the increase of temperature. The lifetime values show a slight trend to decrease with the temperature increase, from 300 to 420 K, for all the samples.

Recombination dynamics of excitons in ZnO/ZnMgO multiple quantum wells grown on silicon substrate

Time-resolved photoluminescence measurements were taken at various temperatures to investigate the behavior of excitons in ZnO/Zn0.9Mg0.1O multiple quantum wells (MQWs) grown on Si substrate using epitaxial Lu2O3 buffer layer by pulsed laser deposition. The temperature dependence of the radiative decay times of the emissions from both the ZnO wells and the Zn0.9Mg0.1O barriers is analyzed. The radiative lifetime of the well excitons exhibits linear dependence till room temperature, indicating that the excitons are effectively confined in the quantum wells and do not thermalize to 3D excitons. The short radiative lifetime of ~1.15 ns indicates high radiative recombination rate at room temperature. The radiative lifetime of the barrier excitons shows approximately T1/2 dependence on temperature. In combination with the spectral dependence of the PL lifetime, it indicates localization of excitons in the barrier due to compositional fluctuation. The short radiative lifetime at room temperature and the weak localization energy of ~9 meV suggest that our MQWs are of high optical quality.

Coronal Loops Heating in the Atmosphere of the Ad Leo Red Dwarf

We study the possible origin of long-lasting soft X-ray flares on the AD Leo star, which were observed onboard the Extreme Ultraviolet Explorer (EUVE) spacecraft for the period of 1993-2000 [1]. These flares have relatively long rise and decay times of the radiation intensity (τ R ≈ 104 s and τ d ≈ 5 · 103, respectively), as well as a relatively large emission measure EM ≈ 1051cm−3, which exceeds by 1–3 orders of magnitude the emission measure of soft X-ray flares on the Sun. Assuming that the radiation appears in magnetic loops and basing on the observed values of the emission measure and radiation decay time, the authors of [1] determined the typical length $$ \overline{l}\approx 1.5\cdot {10}^{10} cm $$ , electron number density $$ \overline{n}\approx 3\cdot {10}^{11}c{m}^{-3} $$ , and plasma temperature $$ \overline{T}\approx 2.5\cdot 107 $$ K of the loops. This paper considers plasma heating due to dissipation of the electric currents in the coronal magnetic loops of the star induced by the photospheric convection. The large inductance of the loop as an equivalent electric circuit determines the long time of the current rise in the source and explains the observed time of plasma heating and the rise time of the X-ray radiation intensity. It is shown that the parameters of the X-ray sources in the AD Leo atmosphere agree with the parameters calculated under the assumption of simultaneous emission of a great number of loops (about 50) with electric currents greater than 1013 A, which exceeds the electric currents in the solar coronal magnetic loops by 1–3 orders of magnitude. Such an exceeding can be related to the higher photospheric convection velocities on the late-type stars compared with the Sun.

Reductive exfoliation of substoichiometric MoS2 bilayers using hydrazine salts.

Substoichiometric molybdenum disulphide (MoSx) nanosheets are successfully synthesised following a novel reductive route using hydrazine salts. The resulting two dimensional crystals are found to be highly monodispersed in thickness, forming exclusively 1.9 ± 0.2 nm thick bilayers. The lateral dimensions of the nanosheets are governed by the precursor bulk particle's size. Exploring a range of hydrazine derivatives with various degrees of steric hindrance leads to the conclusion that intercalation does not occur during the process and that exfoliation is instead facilitated by the reduction of Mo centres leading to the exfoliation of substoichiometric bilayers with distorted lattices. The lattice distortion is found to be persistent across all samples with XPS analysis pointing towards a S to Mo ratio of 1.2. The resulting material features an electronic bandgap of 2.1 eV, which is wider than that of pristine monolayer MoS2 with relatively longer radiative decay time.

Structural parameters effect on the electrical and electroluminescence properties of silicon nanocrystals/SiO2 superlattices

The effect of the oxide barrier thickness (tSiO2) reduction and the Si excess ([Si]exc) increase on the electrical and electroluminescence (EL) properties of Si-rich oxynitride (SRON)/SiO2 superlattices (SLs) is investigated. The active layers of the metal–oxide–semiconductor devices were fabricated by alternated deposition of SRON and SiO2 layers on top of a Si substrate. The precipitation of the Si excess and thus formation of Si nanocrystals (NCs) within the SRON layers was achieved after an annealing treatment at 1150 °C. A structural characterization revealed a high crystalline quality of the SLs for all devices, and the evaluated NC crystalline size is in agreement with a good deposition and annealing control. We found a dramatic conductivity enhancement when the Si content is increased or the SiO2 barrier thickness is decreased, due to a larger interaction of the carrier wavefunctions from adjacent layers. EL recombination dynamics were studied, revealing radiative recombination decay times of the order of tens of microseconds. Lower lifetimes were found at higher [Si]exc, attributed to exciton confinement delocalization, whereas intermediate barrier thicknesses present the slowest decay. The electrical-to-light conversion efficiency increases monotonously at thicker barriers and smaller Si contents. We ascribe these effects mainly to free carriers, which enhance carrier transport through the SLs while strongly quenching light emission. Finally, the combination of the different results led us to conclude that tSiO2 ∼ 2 nm and [Si]exc from 12 to 15 at% are the ideal structure parameters for a balanced electro-optical response of Si NC-based SLs.

The photoluminescence properties of QWs with asymmetrical step-like InGaN/GaN quantum barriers

The asymmetrical structures were created by inserting a low-indium-content layer between the QW and barrier to form a step-like quantum barrier (QB) at one side of QW. The optical effects of the inserting layer on QW emission were investigated with low-temperature photoluminescence (PL) and time-resolved PL (TRPL). The inserted layer partially relaxed the strain within QW layer and induced about 25nm red-shift in the PL emission compared with conventional QW, while the presence of localization centers around QW affected the emission mechanism and increased the radiative decay time. Furthermore, the position of the inserted layer played different roles in the changed structures, and whilst the n-side step-barrier exhibited strong localization in the energy levels of the inserted layer, the p-side step-barrier showed stronger localization center for the QW levels.

Recombination dynamics of excitons with low non-radiative component in semi-polar (10-11)-oriented GaN/AlGaN multiple quantum wells

Optical properties of GaN/Al0.2Ga0.8N multiple quantum wells grown with semi-polar (10-11) orientation on patterned 7°-off Si (001) substrates have been investigated. Studies performed at 8 K reveal the in-plane anisotropic behavior of the QW photoluminescence (PL) intensity for this semi-polar orientation. The time resolved PL measurements were carried out in the temperature range from 8 to 295 K to deduce the effective recombination decay times, with respective radiative and non-radiative contributions. The non-radiative component remains relatively weak with increasing temperature, indicative of high crystalline quality. The radiative decay time is a consequence of contribution from both localized and free excitons. We report an effective density of interfacial defects of 2.3 × 1012 cm−2 and a radiative recombination time of τloc = 355 ps for the localized excitons. This latter value is significantly larger than those reported for the non-polar structures, which we attribute to the presence of a weak residual electric field in the semi-polar QW layers.

Temperature dependence of the radiative recombination time in ZnO nanorods under an external magnetic field of 6 T.

The Temperature dependence of the exciton radiative decay time in ZnO nanorods has been investigated, which is associated with the density of states for the intra-relaxation of thermally excited excitons. The photoluminescence decay time was calibrated by using the photoluminescence intensity in order to obtain the radiative decay time. In the absence of an external magnetic field, we have confirmed that the radiative decay time increased with temperature in a similar manner to that seen in bulk material (∼ T1.5). Under an external magnetic field of 6 T parallel to the c-axis, we found that the power coefficient of the radiative decay time with temperature decreased (∼ T1.3) when compared to that in the absence of a magnetic field. This result can be attributed to an enhancement of the effective mass perpendicular to the magnetic field and a redshift of the center-of-mass exciton as a consequence of perturbation effects in the weak-field regime.

Eu3+ ion doped sodium–lead borophosphate glasses for red light emission

Sodium–lead borophosphate glasses doped with different concentrations of Eu3+ ion are prepared by using the melt quenching technique to study their physical, XRD FTIR and luminescence properties to understand the lasing potentialities of these glasses. The XRD studies confirm the amorphous nature of the glasses. FTIR spectrum indicates the presence of BO3 and PO4 structural units. From the emission spectra, Judd–Ofelt (J–O) intensity parameters have been evaluated from the transition 5D0→7FJ (J=2 and 4). The evaluated (J–O) intensity parameters have been used to calculate the radiative transition probabilities, luminescence branching ratio and radiative decay times. The intensity ratio (R) value due to 5D0→7F2/5D0→7F1 transition intensity of Eu3+ ions, increase with increasing concentrations suggesting higher asymmetry and covalent bonding character between rare earth ion and oxygen ligands. The chromaticity coordinates were calculated and analyzed with Commission International deI’Eclairage color diagram. The lifetimes of 5D0 metastable state for the samples with different concentrations were also measured and discussed. The predicted and experimental lifetimes for the 5D0 level in sodium lead borophosphate glasses were compared and discussed in detail.

Excitonic recombination dynamics in non-polar GaN/AlGaN quantum wells

The optical properties of GaN/Al0.15Ga0.85N multiple quantum wells are examined in 8 K–300 K temperature range. Both polarized CW and time resolved temperature-dependent photoluminescence experiment are performed so that we can deduce the relative contributions of the non-radiative and radiative recombination processes. From the calculation of the proportion of the excitonic population having wave vector in the light cone, we can deduce the variation of the radiative decay time with temperature. We find part of the excitonic population to be localized in concert with the report of Corfdir et al. (Jpn. J. Appl. Phys., Part 2 52, 08JC01 (2013)) in case of a-plane quantum wells.

Nanotemplate-directed InGaAs/GaAs single quantum dots: Toward addressable single photon emitter arrays

The lattice mismatch strain-driven coherent three-dimensional InGaAs island self-assembled quantum dots (SAQDs) formed on GaAs (001) and buried in an appropriate optical cavity have been shown to be effective single photon sources at low temperature. However, SAQDs suffer from (1) structural and chemical inhomogeneity resulting in large variation in emission wavelengths and (2) being randomly positioned, virtually insurmountable obstacles for input-output communication in any systems architecture. In this paper, the authors exploit the engineered surface stress-directed atom migration approach to create single flat InGaAs/GaAs quantum dot (QD) on nanotemplated mesas in regular arrays with control on QD size, shape and physical location. Photoluminescence studies on this class of QDs show an exciton coherence time longer than 1 ps and radiative decay time ∼0.8 ns, which is comparable to the findings on SAQDs but has higher uniformity than SAQDs.

Two-photon interference from remote quantum dots with inhomogeneously broadened linewidths

In this paper, we study the influence of quasiresonant and nonresonant excitation on the interference properties of single photons emitted from quantum dots (QDs). The quasiresonant excitation scheme leads to an increase of interference visibility of photons emitted from the same QD to 69% compared to 12% for nonresonant excitation. Furthermore, we demonstrate quantum interference of photons emitted from separate QDs which are simultaneously excited into their $p$ shell. We can readily extract a two-photon interference visibility as high as (39 \ifmmode\pm\else\textpm\fi{} 2)% for nonpostselected coincidences exceeding the predicted value based on coherence and radiative decay times of the quantum dot emission ($\ensuremath{\sim}$25%). We account for this observation by treating the emission of both quantum dots as inhomogeneously broadened ensembles of Fourier-limited photons and observe good congruence between experiment and model.