Radiative Decay Time Research Articles

Secondary and activated X($$\gamma$$) radiation of SPHIC particle therapy facility

We report the secondary X( $$\gamma$$ ) radiation from the accelerator in a normal operating state and activated X( $$\gamma$$ ) radiation from the accelerator devices when the accelerator stops operating in the cancer treatment facility of the Shanghai Proton and Heavy Ion Center (SPHIC). These radiation measurements show us the structural radiation distribution along the beam lines and devices inside the accelerator room when the beam is on and off and can support the radiation protection design of the accelerator facility used for cancer treatment and help evaluate the accumulated radiation dose in the case of an emergency, such as a personal safety system failure or a radiation accident. The radiation dose rate measured in this facility shows that the facility is safe from the radiation protection point. After shooting the quality assurance (QA) beam, the radiation dose rate in the treatment room was also measured to investigate the radiation dose space distribution and decay time dependence. In addition, the time period before safely entering the treatment room after determining the shooting of the QA beam is recommended to be approximately 5 min.

Development of Na2O-MO-Bi2O3-B2O3-Sm2O3 glasses (MO=Ba/Mg) for laser and scintillation application.

Na2O-MO-Bi2O3-B2O3-Sm2O3 glasses (MO=Ba/Mg) were developed for the light emitting materials applications. Four broad emission peaks were observed at transitions 4G5/2 → 6HJ (J=5/2, 7/2, 9/2 and 11/2). The maximum transition occur at 600 nm. Concentration quenching were observed at 0.5 mol% of Sm3+ ion for both glasses. The X-ray luminescence spectra were found similar to the photoluminescence spectra. The Judd Ofelt (JO) parameters for Sm3+ in the present glasses were procure from the absorption and emission spectra. The important parameters for the laser materials like quantum efficiency, stimulated emission cross section and radiative decay time were explained in detail in the present study. The lifetime were found decreasing with Sm3+-concentration which is the evident of energy transfer between Sm3+ ions. The energy transfer process were further explained by applying IH model. The orange emission from the present glasses were confirmed from CIE chromoticity diagram. According to the present results it is suggested that at 404 nm excitation and 600 nm emission the NMgBiBSm1.0 glass is suitable for the orange emitting devices and from X-ray luminescence emission spectra the present glass might be used for scintillation purpose.

Temperature-Dependent Stimulated Emission Cross-Section in Nd3+:YLF Crystal.

Spectroscopic properties of neodymium-doped yttrium lithium fluoride were measured at different temperatures from 35 K to 350 K in specimens with 1 at% Nd3+ concentration. The absorption spectrum was measured at room temperature from 400 to 900 nm. The decay dynamics of the 4F3/2 multiplet was investigated by measuring the fluorescence lifetime as a function of the sample temperature, and the radiative decay time was derived by extrapolation to 0 K. The stimulated-emission cross-sections of the transitions from the 4F3/2 to the 4I9/2, 4I11/2, and 4I13/2 levels were obtained from the fluorescence spectrum measured at different temperatures, using the Aull–Jenssen technique. The results show consistency with most results previously published at room temperature, extending them over a broader range of temperatures. A semi-empirical formula for the magnitude of the stimulated-emission cross-section as a function of temperature in the 250 K to 350 K temperature range, is presented for the most intense transitions to the 4I11/2 and 4I13/2 levels.

PbS/CdS Quantum Dot Room-Temperature Single-Emitter Spectroscopy Reaches the Telecom O and S Bands via an Engineered Stability.

We synthesized PbS/CdS core/shell quantum dots (QDs) to have functional single-emitter properties for room-temperature, solid-state operation in the telecom O and S bands. Two shell-growth methods-cation exchange and successive ionic layer adsorption and reaction (SILAR)-were employed to prepare QD heterostructures with shells of 2-16 monolayers. PbS/CdS QDs were sufficiently bright and stable to resolve photoluminescence (PL) spectra representing both bands from single nanocrystals using standard detection methods, and for a QD emitting in the O-band a second-order correlation function showed strong photon antibunching, important steps toward demonstrating the utility of lead chalcogenide QDs as single-photon emitters (SPEs). Irrespective of type, few telecom-SPEs exist that are capable of such room-temperature operation. Access to single-QD spectra enabled a direct assessment of spectral line width, which was ∼70-90 meV compared to much broader ensemble spectra (∼300 meV). We show inhomogeneous broadening results from dispersity in PbS core sizes that increases dramatically with extended cation exchange. Quantum yields (QYs) are negatively impacted at thick shells (>6 monolayers) and, especially, by SILAR-growth conditions. Time-resolved PL measurements revealed that, with SILAR, initially single-exponential PL-decays transition to biexponential, with opening of nonradiative carrier-recombination channels. Radiative decay times are, overall, longer for core/shell QDs compared to PbS cores, which we demonstrate can be partially attributed to some core/shell sizes occupying a quasi-type II electron-hole localization regime. Finally, we demonstrate that shell engineering and the use of lower laser-excitation powers can afford significantly suppressed blinking and photobleaching. However, dependence on shell thickness comes at a cost of less-than-optimal brightness, with implications for both materials and experimental design.

Surface driven nano-morphology of poly 3-hexylthiophene film, and their photophysical, spectral and electronic traits

The present paper compiles, the substrate driven nano-morphology of fluorescent semiconducting poly 3-hexylthiophene (P3HT) polymer films and their effects on photophysical, spectral, and electronic behaviour. Surface morphological study reveals regular long sized aggregation and large crystalline structure of polymer thin film in liquid surface grown (LSG) film as compared to hard surface grown (HSG) film. Experimentally observed and computationally simulated Raman spectra show that HJ coupling dominates in LSG film compared to HSG film. LSG film yields the structured electronic absorption and emission spectra with increased radiative decay time, in comparison to HSG film. These indicate a decrease of non-radiative transitions in LSG film and support more crystalline nature of the P3HT film as well as improve intrinsic mobility and device parameters in the sandwiched structure of Schottky diode configuration. The findings show that LSG based electronic/photonic devices have a high potential for enhanced performance at a little cost.

Enhanced red luminescent PBTNAEu glasses for solid state lasers

The luminescence characteristics of Eu3+ activated 30 PbO+(50–x) B2O3+5 TeO2+10 NaF+5 AlF3+x EuF3 (PBTNAEu) glasses prepared by melt-quenching process were studied for red laser applications. The phonon-side band mechanism of Eu3+: 5D2 emission state was discussed with the help of photoluminescence excitation and the Raman spectral features. The Judd-Ofelt intensity parameters that will be used to evaluate various laser characteristic parameters were determined from both the absorption and the emission spectra. The absorption from 7F0 ground and 7F1 first excited states was considered by applying the thermal correction. The emission spectra were used to study the luminescence enhancement and the asymmetric environment around the Eu3+ ions. The quantum efficiency of emitted luminescence was determined from the experimental and radiative decay times of 5D0 emission state. From the experimental results, it was suggested that the PBTNAEu glass containing 2.0 mol% of EuF3 has more potentiality for red laser applications.

Diode-pumped laser and spectroscopic properties of Yb,Ho:GGAG at 2 µm and 3 µm

In this paper, spectroscopy and laser results obtained with new active medium Yb,Ho:GGAG (gadolinium–gallium–aluminum–garnet) emitting laser radiation at 2.09 m and 2.94 m are shown. The absorption, emission cross-section, and radiative decay time for several energy levels Yb3+ (2F5/2), Ho3+ (5I6 and 5I7) are presented. Using longitudinal diode pumping the laser action at 2 m and 3 m together with the wavelength tunability at several spectral bands 2063–2113 nm, 2860 nm, and 2912–2942 nm were reached.

Investigation of the dimensionality using temperature-dependent decay times in InGaAs-coupled quantum well-quantum dots structures

The radiative temperature-dependent decay times were exploited to understand the confinement size analyzing the relaxation from thermally-excited states in InGaAs coupled quantum well (QW)–quantum dots (QDs) structures. We investigated the confinement size from coupled QW-QDs structures by measuring the decay time of radiative process from exciton states when the temperature is gradually increased. The photoluminescence (PL) decay time from the exciton states is amended with the PL intensity at low temperature (~5 K) to distinguish the radiative decay time. The zero-dimensional and the two-dimensional confinement size are obtained from the isolated QDs and QW. However, this assumption is no longer correct for optical coupling because of the direct tunneling when the distance between the QW and the QDs becomes less than 10 nm in the coupled QW-QDs structures. Below 6 nm between the QW and the QDs in the coupled QW-QDs structures, the power factor from the temperature variations increases from 0 to 0.3 (for QDs) and from 1 to 1.37 (for QW), which accords with a quasi-zero-dimensional density of states (~Tα=0.3) and a quasi-two-dimensional density of states (~Tα=1.37), respectively, because of the extended wave functions in the optical couplings and the existence of dark states in the coupled QW-QDs structures.

Perspectives of Unicolored Phosphor‐Sensitized Fluorescence

AbstractUnicolored phosphor‐sensitized fluorescence (UPSF) is a dual emitting concept proposed for improving efficiencies and operational lifetimes of blue organic light emitting diodes (OLEDs). To overcome the limitations of the individual emitters, it uses a phosphorescent donor to sensitize a fluorescent acceptor. To quantify the potential of the concept, a multiscale model of a UPSF OLED is developed. It starts from atomistic morphologies, the rates of all processes on the available experimental data are parameterized, and the respective master equation is solved with the help of the kinetic Monte Carlo algorithm. The simulations show that the energy transfer between donor molecules is essential to reproduce the results of the time‐resolved photoluminescence experiment. The scope of the experiment is expanded by studying the effect of the acceptor concentration, as well as Förster and (parasitic) Dexter energy transfer from the donor to acceptor, on the characteristics of the UPSF OLED. The study shows that an appropriate material design can further improve efficiency by more than 30% and at the same time achieve radiative decay times below 0.02 µs, thus significantly extending OLED operational lifetime.

Investigation of Li2O–Gd2O3–MO–B2O3–Nd2O3 (MO=Ba/Bi) glasses for laser applications by Judd–Oflet (J–O) theory

In this work, we synthesize Li2O–Gd2O3-MO-B2O3–Nd2O3 glasses (where MO = Ba/Bi) by melt-quenching technique for laser applications; characterize their physical properties, absorption and luminescence spectra; and determine their decay time and Judd-Oflet (J-O) parameters. We confirm the glass phase of the samples by XRD pattern and obtain the J-O parameters for Nd3+ in the glasses from the absorption and photoluminescence emission spectra, while measuring the intensity of the electric dipole transitions and oscillator strengths from absorption spectra and the radiative parameters from photoluminescence emission spectra. We also determine some key parameters of the laser materials, such as radiative decay time, quantum efficiency and stimulated emission cross section. The lifetime decreases with increasing Nd3+ concentration that is indicative of energy transfer between Nd3+ ions. We also demonstrate that the physical properties and absorption spectra are independent of the gadolinium ions (Gd2O3), while the luminescence properties are functions of the Gd2O3 due to the efficient energy transfer from the Gd3+ ions to the incorporated activator (Nd3+). These results suggest that the 50Li2O-15Gd2O3–5Bi2O3-28.5B2O3-1.5Nd2O3 glass is useful for laser applications at 577-nm excitation and at 1.07-μm emission.

Radiative decay time as a function of temperature in double GaAs quantum rings

This study examined the temperature-dependent, time-integrated/-resolved photoluminescence of the exciton states in double GaAs quantum rings. The density of states was analyzed from the temperature-dependent radiative decay time of exciton states to better understand the intra-relaxation process from thermally excited exciton states. To distinguish the radiative decay time from a non-radiative process, the decay time from the exciton states was calibrated by a comparison with the photoluminescence intensity at low temperatures, where the non-radiative process can be negligible. In ideal quantum ring structures, the ring width becomes almost zero. Therefore, one-dimensional nature would be expected. On the other hand, this assumption is not valid because widthless ring structures cannot be grown using current growth techniques. In other words, the quasi-one-dimensional nature (power order of the temperature dependence from radiative decay times ∼0.8 between 1.0 (two-dimensional nature) and 0.5 (one-dimensional nature)) was observed in quantum ring structures when the ring width was comparable to the radius of the ring structures. In the case of double quantum rings, however, the power order of the temperature dependence was obtained from the radiative decay times, which gave rise to 1.0, corresponding to a two-dimensional nature. This increased dimensional nature can be attributed to the appreciable ring width and ring size, extended wave function due to optical coupling, and the presence of dark exciton states.

Unicolored phosphor-sensitized fluorescence for efficient and stable blue OLEDs

Improving lifetimes and efficiencies of blue organic light-emitting diodes is clearly a scientific challenge. Towards solving this challenge, we propose a unicolored phosphor-sensitized fluorescence approach, with phosphorescent and fluorescent emitters tailored to preserve the initial color of phosphorescence. Using this approach, we design an efficient sky-blue light-emitting diode with radiative decay times in the submicrosecond regime. By changing the concentration of fluorescent emitter, we show that the lifetime is proportional to the reduction of the radiative decay time and tune the operational stability to lifetimes of up to 320 h (80% decay, initial luminance of 1000 cd/m2). Unicolored phosphor-sensitized fluorescence provides a clear path towards efficient and stable blue light-emitting diodes, helping to overcome the limitations of thermally activated delayed fluorescence.

Temperature dependence of the radiative recombination time in laterally coupled GaAs quantum dots

We performed temperature dependent photoluminescence measurements of exciton states in GaAs laterally coupled quantum dots. The temperature dependence of the radiative decay time was used to investigate the density of states to study the intra-relaxation of thermally excited exciton states. We analyzed the density of states in laterally coupled quantum dots by observing the radiative decay time of exciton states as a function of temperature, where the decay time of exciton photoluminescence was calibrated with respect to photoluminescence intensity to extract the radiative decay time. In the case of laterally coupled quantum dots, a one-dimensional structure can be formed by elongation along the direction of coupling between two quantum dots. However, this assumption is not valid when the size in the uncoupled direction is comparable to that in the coupled direction. From experimental measurements, the power order of the temperature dependence was found to be 0.8, which corresponds to a quasi-one-dimensional system (∼Tα=0.8), which is in between a two-dimensional (α = 1) and a one-dimensional (α = 0.5) density of states. This is due to several factors including a large size along the uncoupled direction, an extended wave function owing to optical coupling, and the presence of dark states.

Band-Edge Exciton in CdSe and Other II–VI and III–V Compound Semiconductor Nanocrystals − Revisited

In this Mini Review, we summarize major corrections to the dark-bright exciton theory [ Efros et al. Phys. Rev B 1996 , 54 , 4843 - 4856 ], which should be used for quantitative description of the band edge exciton in II-VI and III-V compound quantum-dot nanocrystals (NCs). The theory previously did not take into account the long-range exchange interaction, resulting in the under-estimation of the splitting between the upper bright and lower dark or quasi-dark exciton, as reported by several experimental groups. Another type of correction originates from the closeness in energy of the ground, 1S3/2, and the first excited, 1P3/2, hole levels in a spherical NC, resulting in significant energetic overlap of the levels from the 1S3/21Se and 1P3/21Se exciton manifolds connected with the ground 1Se electron level. The thermal occupation of the optically forbidden 1P3/21Se exciton levels changes the radiative decay time of the NCs at both helium and room temperatures. We demonstrate the role of both effects in CdSe NCs and compare our predictions with available experimental data.

Optical and luminescence characteristics of Sm3+ doped B2O3-GeO2-Gd2O3 glasses

Glasses with the composition of 30B2O340GeO2-(30-x)Gd2O3-xSm2O3 (x = 0.25, 0.5, 1 and 1.5 mol%) have been prepared by melt quenching technique. Their optical and luminescence properties were analyzed via absorption, photoluminescence and decay time measurements. Judd–Ofelt intensity parameters (Ωλ, λ = 2,4,6) obtained from the absorption spectrum follow the trend Ω4 > Ω2 > Ω6. Photoluminescence spectra revealed a dominant emission band at 597 nm which was attributed to 4G5/2 → 6H7/2 transition. From the emission spectra and Judd–Ofelt intensity parameters radiative properties including transition probabilities (A), radiative decay times (τR), branching ratio (βR) and stimulated emission cross-section (σpE) were calculated. The decay curves of 4G5/2 level exhibited non-exponential behavior and well fitted to Inokuti-Hirayama model for S = 6. The analyzed spectroscopic properties viz. A, βR, σpE and CIE chromaticity coordinates demonstrate the suitability of BGGS1.00 glass for visible reddish-orange laser applications among the prepared glasses.

Temperature-dependent radiative and non-radiative dynamics of photo-excited carriers in extremely high-density and small InGaN nanodisks fabricated by neutral-beam etching using bio-nano-templates

Temperature-dependent radiative and non-radiative dynamics of photoexcited carriers were studied in In0.3Ga0.7N nanodisks (NDs) fabricated from quantum wells (QWs) by neutral-beam etching using bio-nano-templates. The NDs had a diameter of 5 nm, a thickness of 2 and 3 nm, and a sheet density of 2 × 1011 cm–2. The radiative decay time, reflecting the displacement between the electron and hole wavefunctions, is about 0.2 ns; this value is almost constant as a function of temperature in the NDs and not dependent on their thickness. We observed non-exponential decay curves of photoluminescence (PL) in the NDs, particularly at temperatures above 150 K. The thermal activation energies of PL quenching in the NDs are revealed to be about 110 meV, corresponding to the barrier heights of the valence bands in the disks. Therefore, hole escape is deemed responsible for the PL quenching, while thermal activation energies of 12 meV due to the trapping of carriers by defects were dominant in the mother QWs. The above-mentioned non-exponential PL decay curves can be attributed to variations in the rate of hole escape in the NDs because of fluctuations in the valence-band barrier height, which, in turn, is possibly due to compositional fluctuations in the QWs. We found that non-radiative trapping, characteristic of the original QW, also exists in about 1% of the NDs in a form that is not masked by other newly formable defects. Therefore, we suggest that additional defect formation is not significant during our ND fabrication process.

Spectroscopic study of oscillator strength and radiative decay time of colloidal CdSe quantum dots

Characterization of samples of cadmium selenide quantum dots (CdSe) QDs dissolved in toluene colloidal solutions at a concentration of 1.4 mg/ml was carried out through UV–Vis absorption and photoluminescence (PL) spectroscopy. The size-dependent absorption and red-shifted PL emission peak wavelengths could be tuned between 510–576 and 545–606 nm respectively. Optical absorption spectral measurements yielded CdSe QDs having diameters about ~ 2.44–3.69 nm with energy gaps 2.32–2.08 eV which are higher than the bulk CdSe (1.74 eV) reminiscent of quantum confinement. This is found to be in good agreement with the semi-empirical pseudopotential model. In addition, the first excitonic absorption transition 1S(e)1S3/2(h) oscillator strength and the corresponding fluorescence radiative decay time of CdSe QDs are assessed using relevant Einstein relations for absorption and emission in a two-level system. The elaborated calculations would anticipate that the transition oscillator scale with the CdSe QD radius as ~ R2.54. Correspondingly, the calculated radiative decay times decrease from 56.4 to 23.2 ns which scale with CdSe QDs radius as ~ R−2.155 in fairly good agreement with experimental values reported in the literature.

Interband transitions dependent on indium concentration in Ga1−xInxAs/GaAs asymmetric triple quantum wells

In this paper, the optical and electronic properties of asymmetric triple quantum well (ATQW) structures are studied depending on the indium concentrations while quantum well (QW) thicknesses and barrier widths are kept constant. Calculation of electronic properties are done within the framework of the effective mass approximation. The indium concentrations in left quantum well (LQW) and right quantum well (RQW) are varied in order to see the change of energy levels. Then, interband transition energies, wavelengths, oscillator strengths and radiative decay times are determined depending on barrier height. The scope of this study, for the first time in the literature, covers converged interband transition energies for the asymmetric quantum well structures.

Photoluminescence of the Mg2Al4Si5O18-Al2O3-MgAl2O4-SiO2 ceramic system containing Fe3+ and Cr3+ as impurity ions

This work presents the results of photoluminescence, excitation and radiative decay time for a ceramic system containing Mg2Al4Si5O18-Al2O3-MgAl2O4-SiO2 with Fe3+ and Cr3+ as impurity ions. Emission data were obtained using several excitation wavelengths and the excitation data were acquired for the most intense emission bands. The optical results were analyzed according to the Tanabe-Sugano (TS) theory from which the crystalline field parameter Dq and Racah parameters B and C were obtained for the Fe3+ and Cr3+ sites. The results indicate that the Fe3+ and Cr3+ ions occupy tetrahedral and octahedral sites, respectively. The emission from Fe3+ and Cr3+ ions causes an intense and broad band ranging between 350 nm and 850 nm, showing that this material is a potential tunable radiation source at room temperature.

Spectral-luminescent and laser properties of the (Y1-x,Ybx)2O3−Al2O3−B2O3 glasses

Yttrium-alumina-borate glasses activated by the Yb3+ ions with compositions close to the huntite-like (Y1−xLnx)Al3(BO3)4 crystals have been synthesized by conventional melt-quenching technique in a platinum crucible, and their spectral-luminescent and laser properties have been investigated. It is established that this activator forms in the given glass one type of optical centers having the radiation decay time of 870 ± 40 μs. The limiting quantum yield of its luminescence in the case of complete dehydration of the glass will amount to ≈94%. The main laser parameters have been calculated and the nonlinearity of the refractive index and the threshold of laser-induced destruction of the glass surface have been determined. The lasing has been obtained on the glass plate of 2.1 mm thickness with a threshold of ≈60 W/mm2 estimated by specific absorbed power.