High-energy Peak Research Articles

Origin of multiwavelength emission from flaring high redshift blazar PKS 0537−286

ABSTRACT The high redhsift blazars powered by supermassive black holes with masses exceeding 109 M⊙ have the highest jet power and luminosity and are important probes to test the physics of relativistic jets at the early epochs of the Universe. We present a multifrequency spectral and temporal study of high redshift blazar PKS 0537−286 by analysing data from Fermi-LAT, NuSTAR Swift XRT, and UVOT. Although the time averaged γ-ray spectrum of the source is relatively soft (indicating the high-energy emission peak is below the GeV range), several prominent flares were observed when the spectrum hardened and the luminosity increased above 1049 erg s−1. The X-ray emission of the source varies in different observations and is characterized by a hard spectrum ≤1.38 with a luminosity of >1047 erg s−1. The broad-band spectral energy distribution in the quiescent and flaring periods was modelled within a one-zone leptonic scenario assuming different locations of the emission region and considering both internal (synchrotron radiation) and external (from the disc, broad-line region, and dusty torus) photon fields for the inverse Compton scattering. The modelling shows that the most optimistic scenario, from the energy requirement point of view, is when the jet energy dissipation occurs within the broad-line region. The comparison of the model parameters obtained for the quiescent and flaring periods suggests that the flaring activities are most likely caused by the hardening of the emitting electron spectral index and shifting of the cut-off energy to higher values.

Electron and ion acceleration from femtosecond laser-plasma peeler scheme

Using three-dimensional particle-in-cell simulations, we further investigate the electron and ion acceleration from femtosecond laser–plasma peeler scheme which was proposed in our recent paper (Shen et al 2021 Phys. Rev. X 11 041002). In addition to the standard setup where a laser pulse impinges on an edge of a single tape target, two new variants of the target, i.e. a parallel tape and a cross tape target, were proposed, where strong surface plasma waves can also be efficiently excited at the front edges of the target. By using a tabletop 200 TW-class laser pulse, we observe generation of high-flux, well-collimated, superponderomotive electrons. More importantly, quasimonoenergetic proton beams can always be obtained in all the three setups, while with the single tape case, the obtained proton beam has the highest peak energy and narrowest spectrum.

Electron-Hole Asymmetry of Quantum Collective Excitations in High-Tc Copper Oxides

We carry out a systematic study of collective spin- and charge excitations for the canonical single-band Hubbard, $t$-$J$-$U$, and $t$-$J$ models of high-temperature copper-oxide superconductors, both on electron- and hole-doped side of the phase diagram. Recently developed variational wave function approach, combined with the expansion in inverse number of fermionic flavors, is employed. All three models exhibit a substantial electron-hole asymmetry of magnetic excitations, with a robust paramagnon emerging for hole-doping, in agreement with available resonant inelastic $x$-ray scattering data for the cuprates. The $t$-$J$ model yields additional high-energy peak in the magnetic spectrum that is not unambiguously identified in spectroscopy. For all considered Hamiltonians, the dynamical charge susceptibility contains a coherent mode for both hole- and electron doping, with overall bandwidth renormalization controlled by the on-site Coulomb repulsion. Away from the strong-coupling limit, the antiferromagnetic ordering tendency is more pronounced on electron-doped side of the phase diagram.

Investigation and Analysis of Thermoelectrically Cooled CZT Performance

Cadmium zinc telluride (CZT) spectrometers have been considered for objectives and missions with variable ambient temperatures. Spectrometer-grade crystals of various sizes have been studied under conditions as low as −40°C for 2 × 2 × 2 and 5 × 5 × 2-mm3 crystals, and −10°C for 5 × 5 × 5-mm3 crystals for resolution improvement spanning 5.9-, 59.5-, and 122-keV photo peak energies. It is unclear from previously published data if cooling the spectrometer-grade crystals beyond −10°C results in increased resolution improvement or if the effect occurs with higher-energy photo peaks and trends among multiple crystals from the same manufacturer. Therefore, we acquired two CZT crystals from Kromek and cooled them in an insulated box to −25°C. Our measurements were performed every 5°C, and tested with 241Am or 241Am/152Eu mixed sources. The 241Am peaks were compared for both crystals, and the higher-energy resolution changes were explored using the mixed source. Overall, at 59.5 keV, both crystals yielded 3% to 4% resolution improvement for the cooling cycle and 6% improvement during the warming cycle. Resolution performance varied between the two tested crystals, and each had a different temperature where we observed optimum resolution. The 121.8-keV peak resolution improved by 1.2% for the cooling cycle and 3.6% for the warming cycle. There were no discernable resolution increases or changes for the two higher-energy peaks, 224.7 and 334.3 keV, respectively. Slight cooling of the CZT crystals can increase resolution performance by 4% in the lower-energy region.

Control of ion flux-energy distribution at dielectric wafer surfaces by low frequency tailored voltage waveforms in capacitively coupled plasmas

Capacitively coupled plasmas are routinely used in an increasing number of technological applications, where a precise control of the flux and energy distribution of ions impacting boundary surfaces is required. In the presence of dielectric wafers and targets the accumulation of charges on these surfaces can significantly alter the time evolution of the sheath electric field that is accountable for ion acceleration from the plasma bulk to the surfaces and, thus, lead to parasitic distortions of process relevant ion flux-energy distributions. We apply particle in cell with Monte Carlo collisions simulations to provide insights into the operation, ion acceleration mechanisms, and the formation of such distributions at dielectric wafers for discharges in argon gas. The discharges are driven by a combination of a single high frequency (HF) (27.1 MHz) voltage signal and a low frequency (LF) (100 kHz) customized pulsed voltage waveform. The LF waveform includes a base square signal to realize narrow and controllable high energy peaks of the ion distribution, and steady-slope ramp voltage components. We discuss the distorting effect of dielectric surface charging on the ion flux-energy distribution and provide details about how the voltage ramps can restore its narrow peaked shape. The dependence of the surface charging properties on the LF pulse duty cycle and amplitude, as well as the HF voltage amplitude is revealed. The radial homogeneity of the ion flux is found to be maintained within % around the mean value for all quantities investigated. The radial electric field developing at the edge of the dielectric wafer with finite width has only a small influence on the overall homogeneity of the plasma across the whole surface, its effect remains localized to the outermost few mm of the wafer.

Detection of hot electrons originating from an upper valley at ∼1.7eV above the Γ valley in wurtzite GaN using electron emission spectroscopy

Using electron emission spectroscopy, measurement and analysis were conducted on the energy distribution of vacuum emitted electrons from electrically driven InGaN/GaN green (peak wavelengths $\ensuremath{\lambda}\ensuremath{\approx}515\phantom{\rule{0.16em}{0ex}}\mathrm{nm}$) light-emitting diodes (LEDs) with and without a prewell superlattice (SL). We report on the detection of a high-energy upper valley at $\ensuremath{\sim}1.7\phantom{\rule{0.16em}{0ex}}\mathrm{eV}$ above the $\mathrm{\ensuremath{\Gamma}}$ valley from samples with no prewell SL. We propose that these upper valley electrons originate predominantly from trap-assisted Auger recombination (TAAR) in green LEDs, as the intensity of these peaks is found to have quadratic dependence on the carrier density $n$ [see Espenlaub et al., J. Appl. Phys. 126, 184502 (2019)]. The high-energy upper valley peak was not observed in the sample with a prewell SL which is attributed to gettering by the prewell SL of still unidentified impurities that act as TAAR centers.

Identificação dinâmica de uma passarela estaiada utilizando um sistema de aquisição de dados de baixo custo

Abstract This study describes the dynamic identification of a cable-stayed footbridge using an alternative data acquisition system and output-only modal identification methods. Data were collected during dynamic tests performed using an acquisition system based on the Arduino platform, consisting of low-cost devices with on-board Micro-Electro-Mechanical System (MEMS) accelerometers. The Peak-Picking (PP) method was used for the stay cables. The Frequency Domain Decomposition (FDD) and Stochastic Subspace Identification - Unweighted Principal Components (SSI-UPC) methods were used for the complete structure. The proposed data acquisition system efficiently recorded the time series required for Operational Modal Analysis and the acceleration acquisition process provided stable results. At least four mode shapes were identified in all tests. A minimum of four high energy peaks in the 0 - 24 Hz frequency range of the spectrum were obtained by the acquisition system in all of the cable tests and selected by the method. In the complete structure tests, the low-cost data acquisition system and the identification methods provided the first four flexural mode shapes, within the 0 - 9 Hz frequency range. Results for the frequency domain method showed a maximum difference of 2.37% in the first experimental frequency when compared to a 3D finite element numerical model, while in the other frequencies the difference was between 1 and 2%. For the time domain method, the maximum difference was 1.74% in the fourth frequency, with differences of between 0.1 and 0.7% in the other frequencies. The mode shapes were evaluated using the Modal Assurance Criterion (MAC) index, and the results varied between 0.8513 and 0.9990.

Impact of the organic cation on the band-edge emission of two-dimensional lead-bromide perovskites.

Organic-inorganic low-dimensional layered metal-halide perovskites are semiconductors in which the optoelectronic properties can be tuned by the material composition and the design of the layered architecture. While the electronic band structure is mainly determined by the inorganic octahedra lattice, the binding and conformation of the organic cations induces related lattice distortions that can break the symmetry and lead to the splitting of the exciton energy levels, and influence the dielectric confinement. Furthermore, organic-induced lattice deformations lead to offsets in k-space (where k is the wavevector) that go along with the exciton energy level splitting. Hence, the electronic transitions between these levels require the momentum contribution of phonons, and contributions of phonons in the exciton recombination dynamics result in thermal broadening of the emission linewidth. In this work, we investigate the band-edge emission of two-dimensional Ruddlesden-Popper lead-bromide perovskites synthesized with different organic cations that vary in their binding head group and their alkyl chain length. We find several peaks in the low-temperature photoluminescence spectra, and the number of peaks in the band-edge emission and their decay dynamics depend strongly on the type of organic cation in the material, which we relate to the difference in the inorganic lattice distortions that the cations induce. For two-dimensional layered perovskites with mainly in-plane distortions, induced by short primary ammonium molecules, we find a two-fold splitting of the band edge emission at low temperatures. If also out-of-plane distortions are present, as for the long-chain primary ammoniums, a three-fold splitting is observed. Interestingly, the low-energy peaks of the split series merge into the highest energy peak with increasing temperature. Thermal broadening analysis of the temperature-dependent photoluminescence linewidth in the structures with out-of-plane distortions yields energies that are larger than those reported for the inorganic lattice phonons. This indicates the involvement of either high-frequency oscillations involving the organic cations, or the broadening might be related to higher order phonon scattering processes in the excitonic recombination process. The strong directionality of the phonon modes in the octahedral lattice could promote the involvement of multiple electron-phonon scattering processes in the exciton relaxation dynamics, for example involving modes with orthogonal directionality.

High-energy and high-peak-power GHz burst-mode all-fiber laser with a uniform envelope and tunable intra-burst pulses

AbstractWe report a Yb-doped all-fiber laser system generating burst-mode pulses with high energy and high peak power at a GHz intra-burst repetition rate. To acquire the uniform burst envelope, a double-pre-compensation structure with an arbitrary waveform laser diode driver and an acoustic optical modulator is utilized for the first time. The synchronous pumping is utilized for the system to reduce the burst repetition rate to 100 Hz and suppress the amplified spontaneous emission effect. By adjusting the gain of every stage, uniform envelopes with different output energies can be easily obtained. The intra-burst repetition rate can be tuned from 0.5 to 10 GHz actively modulated by an electro-optic modulator. Optimized by timing control of eight channels of analog signal and amplified by seven stages of Yb-doped fiber amplifier, the pulse energy achieves 13.3 mJ at 0.5 ns intra-burst pulse duration, and the maximum peak power reaches approximately 3.6 MW at 48 ps intra-burst pulse duration. To the best of our knowledge, for reported burst-mode all-fiber lasers, this is a record for output energy and peak power with nanosecond-level burst duration, and the widest tuning range of the intra-burst repetition rate. In particular, this flexibly tunable burst-mode laser system can be directly applied to generate high-power frequency-tunable microwaves.

Effect of nonresonant states in near-resonant two-photon ionization of hydrogen

By numerically solving the three-dimensional time-dependent Schr\"odinger equation, two-photon ionization of hydrogen is investigated at the near-resonant frequencies of the $1s\text{\ensuremath{-}}2p$ transition. Due to the Rabi oscillations between the $1s$ and $2p$ states, the photoelectron energy spectra exhibit the Autler-Townes (AT) doublets and we focus on the energy spacing and the asymmetry of the doublets. Our results show that the laser frequency for the minimum energy spacing of AT doublets locates at the resonant frequency of the ac-stark-shifted states, while the symmetry of the AT doublets is affected both by the ac-stark shift and the nonresonant ionization pathway. Developing the minimal three-state model including all of the nonresonant (nonessential) states, the effects of the ac-stark shift and the nonresonant ionization pathway on the AT doublets due to the nonresonant states are identified. Furthermore, due to the nonresonant ionization pathway, the photoelectron angular distributions are distinctly different for the lower- and higher-energy peaks in the AT doublets and these angular distributions sensitively depend on the laser intensity and pulse duration. These results are well reproduced by our minimal three-state model.

Crossover from Reverse Saturable to Saturable Absorptions in Two-Dimensional Tungsten Disulfide

The Tungsten disulfide atomic layer has enormous scientific merits that play a key role in the nonlinear absorption process. Four distinct excitonic peaks in the absorption spectra are crucial for identifying the polarity of nonlinear absorption by virtue of the resonant or non-resonant excitation process. The excitation source of 532 nm is enough for the one-photon optical transition at both low-energy excitonic peaks and two-photon transition at both higher-energy excitonic peaks. The Tungsten disulfide atomic layer exhibited the reverse saturable (or positive nonlinear) absorption through the two-step excitation process which could be possible due to the optical transition at the higher energy excitonic peaks. In addition, the polarity of the nonlinear absorption coefficient and its interchange have been analytically studied with the aid of a magnitude of excited-state absorption cross-section and Ground-state absorption cross-section. Besides, the various possible factors affecting the Z-scan results are discussed.

Gradient boosting decision trees classification of blazars of uncertain type in the fourth Fermi-LAT catalogue

ABSTRACTThe deepest all-sky survey available in the γ-ray band – the last release of the Fermi-LAT catalogue (4FGL-DR3) based on the data accumulated in 12 years contains more than 6600 sources. The largest population among the sources is blazar subclass – 3743, 60.1 per cent of which are classified as BL Lacertae objects (BL Lacs) or Flat Spectrum Radio Quasars (FSRQs), while the rest are listed as blazar candidates of uncertain type (BCU) as their firm optical classification is lacking. The goal of this study is to classify BCUs using different machine learning algorithms, which are trained on the spectral and temporal properties of already classified BL Lacs and FSRQs. Artificial Neural Networks, XGBoost, and LightGBM algorithms are employed to construct predictive models for BCU classification. Using 18 input parameters of 2219 BL Lacs and FSRQs, we train (80 per cent of the sample) and test (20 per cent) these algorithms and find that LightGBM model, state-of-the-art classification algorithm based on gradient boosting decision trees, provides the highest performance. Based on our best model, we classify 825 BCUs as BL Lac candidates and 405 as FSRQ candidates, however, 190 remain without a clear prediction, but the percentage of BCUs in 4FGL is reduced to 5.1 per cent. The γ-ray photon index, synchrotron peak frequency, and high-energy peak frequency of a large sample are used to investigate the relationship between FSRQs and BL Lacs (LBLs, IBLs, and HBLs).

Laboratory Observations of Swash Signatures Using Video Imagery

Swash motions are important for the design and assessment of beach protection projects, while the limitation of insightful observations in the swash zone still exists. An automatic identification method based on video imagery is developed to monitor and extract instantaneous swash motions in the laboratory. The method increases video sampling frequency and improves image enhancement in identification. The improved method fits well with the results of the manual method. The incident band roll-off of the swash spectrum in the experiment is consistent with those observed on natural beaches. Results have shown that on the barred beach, water depths influence the peak frequency in the incident band and cause a multi-peak swash spectrum. A large incident wave period causes a low peak frequency but high peak energy in the swash spectrum. Large incident wave height results in wider energy coupling, making swash to be saturated and more energy in swash energy. The existence of a barred beach promotes swash energy transferring from a high frequency to a low frequency.

Initiation and Propagation of Multiple Fractures in the Horizontal Well of Shale Reservoirs Under Pulse Injection Hydraulic Fracturing

Pulse injection hydraulic fracturing (HF) can release high-energy peak pressure in a split-second, forming a complex hydraulic fracture network in shale reservoirs. However, the initiation and propagation of multiple fractures in the horizontal well of shale reservoirs under pulse injection HF have been inadequately studied. Hence, in this study, a HF model coupling the fracturing fluid flow along the fracture and the rock deformation was adopted, and the effect of the pulsed injection method on multistage HF of horizontal wells was determined. The results show that the pulsed injection HF can improve the fracturing effect of horizontal wells. However, no matter how the injection displacement and pulse frequency change, the propagation length of cracks at both ends seriously disturbed by stress almost remains unchanged. When the dominant fracture is temporarily shielded, the fluid pressure of the non-dominant fractures at both ends disturbed by stress compression increases, and the non-dominant fractures can be further expanded. Thus, a new composite multistaged HF scheme suitable for horizontal wells with pulse-temporary shielding in shale reservoirs was proposed. The method presented here can be adopted to optimize the fracture growth regime and provide a scientific basis for the fracturing operation of the horizontal well in shale reservoirs.

Temperature and fluence dependence of the luminescence properties of Ce:YAG single crystals with ion beam-induced luminescence

The emissions of Ce3+ in Ce:YAG are at approximately 550 nm and are generated by the overlapping emission of two transitions 5d →2F5/2 (2F7/2), and their luminescence characteristics are markedly influenced by temperature and ion fluence. In this study, the temperature and ion fluence dependence of Ce3+ emission was investigated using ion beam-induced luminescence (IBIL) at four different temperatures (25, 150, 300, and 450 °C), and potential theoretical explanations for the variation were discussed. Experimental results show that (1) as the temperature increases, the intensity of Ce3+ decreases, and the central peak position continues to redshift; (2) as the temperature rises, the proportion of the high-energy peak at approximately 2.3 eV continuously increases; (3) a double exponential model can describe the evolution of the Ce3+ luminescence intensity with fluence; and (4) the influence of 2 MeV H+ on the luminescence characteristics of Ce:YAG is limited. The luminescence characteristics of the Ce:YAG single crystal deteriorate when exposed to high temperatures. Combined with the observed experimental phenomena, we recommend that the practical operating temperature of the Ce:YAG single crystal in a high ion fluence environment should be as low as 150 °C when possible to ensure a high scintillation yield and good luminescence performance.

Sub-nanosecond diode-pumped passively Q-switched Nd:LuAG ceramic microchip lasers

Diode-pumped continuous-wave and passively Q-switched Nd:LuAG ceramic microchip lasers at 1064 nm and 1053 nm have been demonstrated, for the first time to our knowledge. For the continuous-wave operation, the achieved maximum output powers of the two lasers reaches 1.62 W and 0.75 W, respectively. Using Cr:YAG crystal as saturable absorber, the two lasers can also been operated in passively Q-switched regimes with sub-nanosecond pulse durations of 489 ps and 556 ps, respectively. The maximum single pulse energy and pulse peak power for the 1064 nm and 1053 nm lasers are calculated to be about 22.25 μJ and 45.49 kW, as well as 12.74 μJ and 22.91 kW. All solid-state microchip lasers give the tremendous potential for high performance laser generation with sub-nanosecond short pulse duration, high single pulse energy and high pulse peak power.

Distinctive g-Factor of Moiré-Confined Excitons in van der Waals Heterostructures.

We investigated the valley Zeeman splitting of excitonic peaks in the microphotoluminescence (μPL) spectra of high-quality hBN/WS2/MoSe2/hBN heterostructures under perpendicular magnetic fields up to 20 T. We identify two neutral exciton peaks in the μPL spectra; the lower-energy peak exhibits a reduced g-factor relative to that of the higher energy peak and much lower than the recently reported values for interlayer excitons in other van der Waals (vdW) heterostructures. We provide evidence that such a discernible g-factor stems from the spatial confinement of the exciton in the potential landscape created by the moiré pattern due to lattice mismatch or interlayer twist in heterobilayers. This renders magneto-μPL an important tool to reach a deeper understanding of the effect of moiré patterns on excitonic confinement in vdW heterostructures.

A novel bioinspired architectured materials with interlocking designs based on tessellation

Nature has achieved intricately designed materials with properties and functions that go far beyond the existing engineering materials. The remarkable efficiency of these biological materials is the result of billions of years of evolution. In this study, bioinspired design of architectured materials based on principles of edge-to-edge tessellation is additively manufactured. The tessellations are inspired by the atomic arrangements in cubic metallic crystal structures; simple cubic (SC), body-centered cubic (BCC), and face-centered cubic (FCC). The unit cells in these tessellations were evolved from the basic spherical morphology along with necessary surfaces at the joining area to achieve edge-to-edge tessellations. The biomimetic architectured materials called ‘advanced functional architectured materials (AFAMs)’ are designed and printed with HP-MJF 4200 powder bed fusion technology. The structural and functional properties of each AFAM were evaluated experimentally and numerically under uniaxial compression testing. The result showed FCC AFAM has high peak strength, elastic modulus, and energy absorption compared to other benchmarked surface and truss-based lattice structures. In contrast, SC AFAM demonstrated high deformation (ductility) before densification. The study also reveals that each AFAM behaves uniquely, almost as different material, in terms of its load-bearing and deformation abilities. All the AFAMs can be interlocked without any interface issues to achieve multi-material properties which otherwise cannot be achieved with functionally graded structure. The combination of these AFAMs can be utilized as an efficient solution for achieving a complex set of multi-functional requirements in additively manufactured products. • The novel architectured materials were designed based on the principles observed in nature. • All the architectured materials were edge-to-edge tessellated based on cubic metallic crystal structures: SC, BCC, and FCC. • Each architectured material demonstrates unique strength and ductility/flexibility. • FCC Tessellation shows the highest load-bearing ability and SC tessellation shows the highest deformation. • The designed architectured materials are interlocked into each other for multi-functional properties.

Tuning Optical and Electrical Properties of Ultra-Fast Prepared Nanoflower Mg:ZnO Films by MWCNTs Coating

Mg doped ZnO films were coated multi-walled carbon nanotubes (MWCNTs) via fast chemical bath onto ZnO seed layers. XRD analysis showed preferential orientation shift from high-energy (002) peak to low-energy (101) peak with MWCNTs coating. Average crystalline size of Mg doped ZnO samples are 15 nm. diameter and 50% percent reduction has been observed with MWCNTs coating. SEM images reveals the presence of high accumulative nanoflower forms on Mg:ZnO surfaces and homogenous net-shaped coating has been achieved by MWCNTs inclusion so active surface area may increase. No major difference of optical absorption edge is detected in both films however MWCNTs coating cause an increase direct band gap. Due to Burstein-Moss effect, Mg:ZnO and Mg:ZnO/MWCNTs films with using Tauc plot calculated band gap values are 3.04 eV and 3.34 eV, respectively. From FTIR spectra, no obvious change is not detected the functional groups of the samples. Electrical studies show that MWCNTs decrease the resistance and the resistance of films at room temperature were calculated 29.85 and 8.53 k for Mg:ZnO and Mg:ZnO/MWCNTs films , respectively.

Improvement of peak power of dissipative soliton resonance pulse in a thulium-doped fiber laser

Single pulse energy and pulse width can be theoretically increased indefinitely without pulse-breaking in a dissipative soliton resonance pulse (DSR) generated from a mode-locked fiber laser when the peak power is keeping in a certain value. But in some applications, both high pulse energy and high peak power are needed in one single pulse. To improve the peak power of the DSR pulse, the characteristics of DSR pulses were investigated and found that the position of the gain fiber is a factor to influence the peak power and the pulse width. Here, we demonstrate a thulium-doped mode-locked fiber laser operating in the DSR regime with a nonlinear amplifying loop mirror (NALM). When changing the position of the gain fiber, the peak power of DSR pulse could be improved from 0.38 W to 1.42 W with the same pump power. This approach provides a method for increasing the pulse energy and efficiency in mode locked fiber lasers, which may meet some special application.