Exponential Decay Components Research Articles

Ultrafast carrier dynamics and transient nonlinear absorption in chalcogenide perovskite BaZrS3

The unique combination of excellent semiconducting properties in halide perovskites and the high stability and nontoxicity of oxide perovskites has led to a recent surge in interest in chalcogenide perovskite BaZrS3 for optoelectronic applications. However, to realize its potential in future device technologies, a comprehensive understanding of photoexcited carrier dynamics and transient optical response is imperative, yet it remains largely unexplored for BaZrS3. In this work, employing transient absorption spectroscopy, we have revealed that photoexcited carriers in epitaxial BaZrS3 nanofilms exhibit two exponential decay components relating to optical phonon cooling and interband recombinations. Meanwhile, our investigation unveils an intriguing transient nonlinear absorption phenomenon in BaZrS3, characterized by the ultrafast switching of the pump-induced transparency (i.e., the saturable absorption) to the absorption enhancement within a timescale commensurate with the measurement resolution (hundreds of femtosecond). This study provides crucial dynamic insights essential for leveraging chalcogenide perovskites, such as BaZrS3, in the development of advanced optoelectronic devices.

A room-temperature ultrafast carrier dynamical study and thickness-dependent investigation of WTe2 thin films on a flexible PET substrate

In the current era of increasing demand for optoelectronic-based devices with ultra-rapid response, it is important to understand the processes associated with the relaxation dynamics of hot carriers and transient electrical properties of WTe2 material under photoexcitation of charge carriers. In this work, using femtosecond laser pump–probe spectroscopy at room temperature we performed the transient absorption measurement on sputtered deposited WTe2 thin films having four different thicknesses to study dynamics associated with the relaxation of their hot carriers. The relaxation dynamics of photoexcited charge carriers undergo three exponential decay components associated with electron–phonon thermalization in the conduction band and phonon-assisted electron–hole recombination between the electron and hole pocket. The thickness-dependent investigation of WTe2 thin films reveals that the electron–hole recombination process is more prominent in thicker films than in thinner films, supporting previously published theoretical and experimental conclusions. The Ultrafast study of WTe2 thin films suggests that it is a suitable material for future ultrafast optoelectronic-based device applications.

Size‐Dependent Optical Properties and Exciton Self‐Trapping Emission in Carbon Quantum Dots

AbstractCarbon quantum dots (CQDs), synthesized through controlled microplasma treatment of orange juice, exhibit unique luminescent characteristics. The impact of synthesis time on particle size was investigated. Deconvoluted emission spectra displayed broad bands, peaking at an excitation wavelength of 350 nm for CQD sizes of 2.06, 2.4, and 3.09 nm, indicating multiple fluorescence peaks. This suggests a mixture of CQDs with different excitation energies on their surfaces. Luminescence kinetics unveiled three exponential decay components with time constants of 2.73, 4.61, and 3.27 ns, indicating multiple luminescent centers. Emission behavior affected by excitation wavelengths ranging from 350 to 490 nm suggests that quantum transition probabilities are influenced by impurities or defects. These findings emphasize the size‐dependent optical properties and exciton self‐trapping (EST) emission of CQDs. The calculation of the Huang‐Rhys factor S values underscores the profound influence of carrier‐phonon coupling and the Huang‐Rhys factor on the emergence of ESTs within CQDs. Understanding these interactions is crucial for fully utilizing the capabilities of CQDs in applications spanning optoelectronics, sensors, bioimaging, and various technological fields.

TSL, OSL and scintillation properties of Tb-doped barium fluoride translucent ceramics

We report on the fabrication of Tb-doped barium fluoride translucent ceramics, and their scintillation and dosimetric characteristics. The Tb-doped barium fluoride ceramics were fabricated by spark plasma sintering. Scintillation derived from the 4f→4f transitions of Tb3+ under X-rays was observed from the Tb-doped barium fluoride ceramics. Their decay times under X-rays were about 10 ms. Further, thermally-stimulated luminescence originating from the transitions was detected, and their dose responses were observed in the range of 0.1–100 mGy. Moreover, the Tb-doped barium fluoride ceramics exhibited optically-stimulated luminescence (OSL), and three exponential decay components were detected in the OSL decay curves.

A huge-amplitude white-light superflare on a L0 brown dwarf discovered by GWAC survey

ABSTRACT White-light superflares from ultra-cool stars are thought to be resulted from magnetic reconnection, but the magnetic dynamics in a fully convective star is not clear yet. In this paper, we report a stellar superflare detected with the ground wide angle camera (GWAC), along with rapid follow-ups with the F60A, Xinglong 2.16-m, and LCOGT telescopes. The effective temperature of the counterpart is estimated to be 2200 ± 50 K by the BT-Settl model, corresponding to a spectral type of L0. The R-band light curve can be modelled as a sum of three exponential decay components, where the impulsive component contributes a fraction of 23 per cent of the total energy, while the gradual and the shallower decay phases emit 42 per cent and 35 per cent of the total energy, respectively. The strong and variable Balmer narrow emission lines indicate the large amplitude flare is resulted from magnetic activity. The bolometric energy released is about 6.4 × 1033 erg, equivalent to an energy release in a duration of 143.7 h at its quiescent level. The amplitude of Δ R = −8.6 mag (or Δ V = −11.2 mag), placing it one of the highest amplitudes of any ultra-cool star recorded with excellent temporal resolution. We argue that a stellar flare with such rapidly decaying and huge amplitude at distances greater than 1 kpc may be false positive in searching for counterparts of catastrophic events such as gravitational wave events or gamma-ray bursts, which are valuable in time-domain astronomy and should be given more attention.

Resonance-enhanced excitation and relaxation dynamics of coherent phonons in Fe1.14Te.

Lattice dynamics plays a significant role in manipulating the unique physical properties of materials. In this work, femtosecond transient optical spectroscopy is used to investigate the generation mechanism and relaxation dynamics of coherent phonons in Fe1.14Te-a parent compound of chalcogenide superconductors. The reflectivity time series consist of the exponential decay component due to hot carriers and damped oscillations caused by the A1g phonon vibration. The vibrational frequency and dephasing time of the A1g phonons are obtained as a function of temperature. With increasing temperature, the phonon frequency decreases and can be well described with the anharmonicity model. Dephasing time is independent of temperature, indicating that the phonon dephasing is dominated by phonon-defect scattering. The impulsive stimulated Raman scattering mechanism is responsible for the coherent phonon generation. Owing to the resonance Raman effect, the maximum photosusceptibility of the A1g phonons occurs at 1.590 eV, corresponding to an electronic transition in Fe1.14Te.

Model of conversion of flow from confined to unconfined with the generalized Mittag-Leffler process

We considered in the paper a mathematical model used to replicate the conversion from confined to unconfined flow due to groundwater abstraction. We argued that the existing model, while very useful, however, does not capture the effect of the aquitard with fractures, as in such media there is a crossover behavior from stretched exponential to power law. We modified the model by replacing the exponential decay component with the generalized Mittag-Leffler function, which yields a model with nonlocal properties. To solve the system, we consider two different approaches, the first consists of discretizing the model directly using the forward Euler method, and the second consists of converting the model to an integral equation and deriving the solution by employing a polynomial interpolation approach. We presented using the Von Neumann stability approach for the confined part and both yield stability unconditionally. Some numerical simulations are performed.

Synthesis and elemental analysis of gadolinium halides (GdX3) in glass matrix for radiation detection applications

Numerous researchers were intrigued to glasses for radiation detection and scintillation applications in nuclear and high-energy physics research. To mitigate the requirements of density and luminescence enhancement, gadolinium (Gd) compounds are widely practiced. Gd halides (GdX3) are hygroscopic salts that can function as excellent sensitizers in a glass network to improve activator (Ce) luminescence. To ensure the elemental abundance of the constituents in glass samples after high temperature combustion, EDX reports at both the intermediate and final phases of the glass samples can be useful. However, EDX analysis was unable to detect halogens in glass substances in their intermediate and final forms. The implications were examined on the basis of prior literature indicating the feasibility of high temperature combustion in an air environment. Nevertheless, it was discovered that transmittance spectra are essential for the performance of radioluminescence and scintillation features. Two samples that were previously enriched with GdI3 had transmittance values more than 55% at the X-ray generated emission peak point. α -peaks with an energy resolution (FWHM) of 29% and 31% were obtained utilizing these samples. The scintillation decay projections from these two samples were fitted with three exponential decay components, with the shortest components being 30.6 ns and 29.5 ns, contributing 38% and 37.4%, respectively.

Dynamical evolution of anisotropic response of type-II Weyl semimetal TaIrTe4 under ultrafast photoexcitation

Layered type-II Weyl semimetals, such as WTe2, MoTe2, and TaIrTe4 have been demonstrated as a supreme photodetection material with topologically enhanced responsivity and specific sensitivity to the orbital angular momentum of light. Toward future device applications with high performance and ultrafast response, it is necessary to understand the dynamical processes of hot carriers and transient electronic properties of these materials under photoexcitation. In this work, mid-infrared ultrafast spectroscopy is performed to study the dynamical evolution of the anisotropic response of TaIrTe4. The dynamical relaxation of photoexcited carriers exhibits three exponential decay components relating to optical/acoustic phonon cooling and subsequent heat transfer to the substrate. The ultrafast transient dynamics imply that TaIrTe4 is an ideal material candidate for ultrafast optoelectronic applications, especially in the long-wavelength region. The angle-resolved measurement of transient reflection reveals that the reflectivity becomes less anisotropic in the quasi-equilibrium state, indicating a reduction in the anisotropy of dynamical conductivity in presence of photoexcited hot carriers. The results are indispensable in material engineering for polarization-sensitive optoelectronics and high field electronics.

Visualizing cation vacancies in Ce:Gd3Al2Ga3O12 scintillators by gamma-ray-induced positron annihilation lifetime spectroscopy

To clarify the existence of cation vacancies in Ce-doped Gd3Al2Ga3O12 (Ce:GAGG) scintillators, we performed gamma-ray-induced positron annihilation lifetime spectroscopy (GiPALS). GiPALS spectra of GAGG and Ce:GAGG comprised two exponential decay components due to positron annihilation at bulk and defect states. By an analogy with Ce:Y3Al5O12, the defect-related component was attributed to Al/Ga–O divacancy complexes. This component was weaker for Ce,Mg:GAGG, which correlated with the suppression of electron traps responsible for phosphorescence. O vacancies were charge compensators for Al/Ga vacancies. The lifetime of the defect-related component was changed by Mg co-doping. This was explained by the formation of vacancy clusters.

Pulse shape analysis of individual gamma events—Correlation to energy resolution and the possibility of its improvement

The measurement in 2014 of the nonproportionality of each decay component in CsI:Tl found opposite slopes of the fast and tail nonproportionality curves above about 10 keV. Somewhat earlier experiments on nonproportionality and resolution versus shaping time in NaI:Tl and CsI:Tl showed that proportionality and intrinsic resolution could be improved by including the slower “tail” component of the scintillation pulse. The observed opposing nonproportionality trends of fast and tail components constitute a basis for improvement of scintillator nonproportionality if they are added in a suitable linear combination. We examine whether combining the rise and decay components pulse by pulse with an algorithm of optimized proportions may also improve energy resolution. The premise is that a scintillation pulse carries more information about the particle stopping event than is conveyed in a simple measurement of the pulse height. In this work, we measured pulse shapes of individual gamma events in CsI:Tl and other scintillators using an eMorpho multichannel analyzer as a digital oscilloscope. Decomposition of every scintillation pulse into a rise and three exponential decay components allowed us to represent the pulse height spectrum as a linear combination of them. We found that energy resolution of CsI:Tl can be altered and improved through changing the weight of decay components in the linear combination.

Scintillation Properties of TlGd2Cl7 (Ce3+) Single Crystal

A novel TlGd2Cl7 single crystal with scintillation properties is presented. TlGd2Cl7 single crystals with different Ce3+-doping concentrations (0, 0.5, 1, and 5 mol%) are grown using the vertical Bridgman technique. All doped samples exhibit typical Ce3+ emission under X-ray excitation between 350 and 550 nm. Among all grown samples, the best light yield (LY) of ~49 700 photons/MeV and good energy resolution of 6% (full-width at half-maximum) are observed for the 5-mol%-Ce3+-doped TlGd2Cl7 crystal under an excitation of 662-keV $\gamma $ -rays at room temperature. Three exponential decay components are observed for all Ce3+- doped samples at room temperature. Faster decays are observed with the increase in the Ce3+ concentration. The effective atomic (Z) number of the proposed crystal is 64; therefore, efficient X- and $\gamma $ -ray detections could be performed with the proposed scintillator. The high LY, high effective Z number, good energy resolution, and fast scintillation response suggest that this scintillator could be employed in various applications. Further investigations are ongoing for the improvement of its scintillation properties with a higher Ce3+ concentration.

Precision measurements of the scintillation pulse shape for low-energy recoils in liquid xenon

We present measurements of the scintillation pulse shape in liquid xenon for nuclear recoils (NR) and electronic recoils (ER) at electric fields of 0 to 0.5 kV/cm for energies <15 keV and <70 keV electron-equivalent, respectively. The average pulse shapes are well-described by an effective model with two exponential decay components, where both decay times are fit parameters. We find significant broadening of the pulse for ER due to delayed luminescence from the recombination process. In addition to the effective model, we fit a model describing the recombination luminescence for ER at zero field and obtain good agreement. We estimate the best performance of a combined S2/S1 and pulse shape ER/NR discrimination and show that even with 2 ns time resolution, the improvement over S2/S1 discrimination alone is marginal, so that pulse shape discrimination will likely not be useful for future dual-phase liquid xenon experiments looking for elastic dark matter recoil interactions.

Automated single-ion peak fitting as an efficient approach for analyzing complex chromatographic data

Chromatography provides important detail on the composition of environmental samples and their chemical processing. However, the complexity of these samples and their tendency to contain many structurally and chemically similar compounds frequently results in convoluted or poorly resolved data. Data reduction from raw chromatograms of complex environmental data into integrated peak areas consequently often requires substantial operator interaction. This difficulty has led to a bottleneck in analysis that increases analysis time, decreases data quality, and will worsen as advances in field-based instrumentation multiply the quantity and informational density of data produced. In this work, we develop and validate an automated approach to fitting chromatographic data within a target retention time window with a combination of multiple idealized peaks (Gaussian peaks either with or without an exponential decay component). We compare this single-ion peak fitting approach to drawn baseline integration methods of more than 70,000 peaks collected by field-based chromatographs spanning across a wide range of volatilities and functionalities. Accuracy of peak fitting under real-world conditions is found to be within 10%. The quantitative parameters describing the fit (e.g. coefficients, fit residuals, etc.) are found to provide valuable information to increase the efficiency of quality control and provide constraints to accurately integrate peaks that are significantly convoluted with neighboring peaks. Implementation of the peak fitting method is shown to yield accurate integration of peaks otherwise too poorly resolved to separate into individual compounds and improved quantitative metrics to determine the fidelity of the data reduction process, while substantially decreasing the time spent by operators on data reduction.

Opposite domination of cyclic and pseudocyclic electron flows in short-illuminated dark-adapted leaves of angiosperms and gymnosperms.

The present work was aimed to explain the recently reported higher O2-dependent electron flow capacity in gymnosperms than in angiosperms and to search for other differences in the electron transport processes by simultaneous characterization of the relative capacities of pseudocyclic (direct or Flavodiiron proteins (Flv)-mediated O2-reduction, Mehler(-like) reactions) and cyclic electron flows around photosystem I (CEF-PSI). To this end, a comparative multicomponent analysis was performed on the fluorescence decay curves of dark-adapted leaves after illumination with a 1-s saturating light pulse. In both gymnosperms and angiosperms, two or three exponential decay components were resolved: fast (t 1/21 ~ 170-260ms), middle (~1.0-2.3s), and slow (>4.2s). The sensitivity of the decay parameters (amplitudes A1-3, halftimes t 1/2 1-3) to the alternative electron flows was assessed using Arabidopsis pgr5 and ndhM mutants, defective in CEF-PSI, Synechocystis sp. PCC 6803 Δflv1 mutant, defective in Flv-mediated O2-photoreduction, different O2 concentrations, and methyl viologen treatment. A1 reflected the part of electrons involved in linear and O2-photoreduction pathways after PSI. The middle component appeared in pgr5 (but not in ndhM), in gymnosperms under low O2, and in Δflv1, and reflected limitations at the PSI acceptor side. The slow component was sensitive to CEF-PSI. The comparison of decay parameters provided evidence that Flv mediate O2-photoreduction in gymnosperms, which explains their higher O2-dependent electron flow capacity. The concomitant quantification of relative electrons branching in O2-photoreduction and CEF-PSI pathways under the applied non-steady-state photosynthetic conditions reveals that CEF-PSI capacity significantly exceeds that of O2-photoreduction in angiosperms while the opposite occurs in gymnosperms.

Estimation of white matter fiber parameters from compressed multiresolution diffusion MRI using sparse Bayesian learning

We present a sparse Bayesian unmixing algorithm BusineX: Bayesian Unmixing for Sparse Inference-based Estimation of Fiber Crossings (X), for estimation of white matter fiber parameters from compressed (under-sampled) diffusion MRI (dMRI) data. BusineX combines compressive sensing with linear unmixing and introduces sparsity to the previously proposed multiresolution data fusion algorithm RubiX, resulting in a method for improved reconstruction, especially from data with lower number of diffusion gradients. We formulate the estimation of fiber parameters as a sparse signal recovery problem and propose a linear unmixing framework with sparse Bayesian learning for the recovery of sparse signals, the fiber orientations and volume fractions. The data is modeled using a parametric spherical deconvolution approach and represented using a dictionary created with the exponential decay components along different possible diffusion directions. Volume fractions of fibers along these directions define the dictionary weights. The proposed sparse inference, which is based on the dictionary representation, considers the sparsity of fiber populations and exploits the spatial redundancy in data representation, thereby facilitating inference from under-sampled q-space. The algorithm improves parameter estimation from dMRI through data-dependent local learning of hyperparameters, at each voxel and for each possible fiber orientation, that moderate the strength of priors governing the parameter variances. Experimental results on synthetic and in-vivo data show improved accuracy with a lower uncertainty in fiber parameter estimates. BusineX resolves a higher number of second and third fiber crossings. For under-sampled data, the algorithm is also shown to produce more reliable estimates.

Scintillation properties of Ce-doped (Gd0.32Y0.68)3Al5O12 transparent ceramics

In this work, we have investigated optical and scintillation properties of 0.35–0.65% Ce:(Gd0.32Y0.68)3Al5O12 (Ce:GYAG) transparent ceramics prepared by the vacuum sintering method. Obtained samples showed high transmittance in the spectral region longer than 500 nm, and two strong absorption bands were clearly observed below 380 nm and between 400 and 500 nm due to the 4f-5d transitions of Ce3+. Under UV and X-rays, we have also observed emission due to the 5d-4f transitions of Ce3+ which appeared around 500–700 nm. The emission decay profile of PL consisted of a single exponential decay component with the decay time of 62.7–64.1 ns while the scintillation decay profile was approximated by a second-order exponential decay function with the decay times of 87.3–100 ns and 1.14–1.32 μs. In addition, it has been revealed that 0.65% Ce:GYAG transparent ceramic showed a notable light yield of 18,000 ph/MeV and low afterglow (13 ppm).

X-ray induced luminescence properties of (Y,Eu)AlO3 single crystals

We investigated photoluminescence, scintillation and dosimeter properties of (Y1-x Eux)AlO3 (x = 0.001, 0.5 and 1) single crystals (hereafter denoted as Eu:YAP for x = 0.001, EYAP for x = 0.5 and EAP for x = 1). The samples were prepared by the Floating Zone method. In photoluminescence (PL), we observed a broad emission around 300–400 nm due to host under excitation of 280 nm, and emissions due to the 4f state transitions of Eu3+ appeared around 590 nm and 615 nm. Scintillation spectra also show emission peaks around 590 and 615 nm due to the 4f state transitions of Eu3+ in all the samples. In addition, emissions around 300–400 nm due to YAP host and around 550–700 nm due to 5d-4f transitions of Eu2+ appeared in Eu:YAP. The PL and scintillation decay time profiles consisted of several exponential decay components. The fast (ns) component group was possibly due to host emission, and especially Eu:YAP demonstrated a very fast PL decay time of 16 ns. The intermediate (μs) component group was due to the 5d-4f transitions of Eu2+. The slow (ms) component group was ascribed to the 4f state transitions of Eu3+ ion. The Eu:YAP sample showed intense thermally-stimulated luminescence (TSL) with peaks at 46, 155, 255 and 443 °C. The intensity was much higher than those of EAP and EYAP. In particular, the peak at 254 °C, which showed the highest intensity, was due to doping with Eu. The TSL dose response function showed a good linearity (R2 > 0.99) over a wide dose range from 0.1 mGy to 100 mGy for Eu:YAP, which showed the highest sensitivity among the present samples.

Scintillation Properties of Ga2O3 Crystal

Ionizing radiation detectors are used in many kinds of non-destructive investigations. Many of such applications use a scintillation detector which consists of scintillator and photodetector. A scintillator is a type of the luminescent material that converts a high energy ionizing radiation photon/particle to hundreds of visible photons via energy migration from the host matrix to emission centers. Scintillators have been playing an important role in many scientific and industrial fields, including astrophysics, particle physics, medical imaging, security, nuclear plants, and well-logging. Although insulator materials doped with rare-earth or transition metal ions are commonly used in practical applications, semiconductor crystals have also attracted attentions due to their intrinsically high light yield and fast decay time without doping of any emission centers. In this work, optical and scintillation properties of well-known Ga2O3 semiconductor material are investigated. Under UV excitation, photoluminescence (PL) emission peak appeared around 380 nm with the quantum yield of 6%, and fast decay of 8 and 793 ns were observed. In contrast, X-ray induced scintillation spectrum also showed an intense emission band around 380 nm, which of decay curve was reproduced by two exponential decay components with the time constants of 8 and 977 ns. The pulse height spectrum of 137Cs g-rays measured using measured Ga2O3 showed a clear photoabsorption peak with a light yield of 15000 ± 1500 ph/MeV. This high light yield was the highest value among the semiconductor scintillators reported.

Scintillation Characterizations of Tin Doped Lithium Iodide Crystals at Room and Low Temperature

The crystals of 0.02%, 0.05%, 0.1%, and 0.5% Sn doped LiI are grown by the vertical Bridgman technique. The luminescence and scintillation properties of the grown crystals are investigated. X-ray excited emission spectrum of LiI (Sn) showed a broad emission band between 400–650 nm wavelength range at room temperature. Such emission is attributed to $\hbox {Sn}^{2 +}$ ion. Scintillation properties such as energy resolution, decay time profiles, and light yield are measured under 662 keV ( $^{137}\hbox {Cs}$ ) $\gamma $ -ray excitation at room temperature. A maximum light yield of $6000 \pm 600 \hbox{ph/MeV}$ is measured at room temperature. LiI (Sn) single crystals showed two exponential decay components under $\gamma $ -ray excitation. The dependence of scintillation properties on the temperature is also presented. Changes in the decay time and light yield are measured from 295 K down to 10 K. The light yield of LiI (Sn) at 50 K is found to be nine times higher than that at room temperature.