Emission Anisotropy Research Articles

Determination of the mean energy of fast electron losses and anisotropies through thick-target emission on WEST

A new method to obtain the mean energy of fast electron losses in fusion plasmas using a versatile multi-energy hard x-ray (HXR) detector is presented. The method is based on measuring the thick-target emission of tungsten in the divertor region produced by fast electron losses interacting with the target and modeling the tungsten spectra by a Monte Carlo code which simulates the interaction between a beam of electrons and a solid target. The mean energy of the fast electron losses is determined through the comparison between the experimental and synthetic emission. The results show that fast electron losses during lower hybrid current drive discharges at WEST have a mean energy of 90–140 keV and represent only 2% of the total heat flux at the target. Additionally, anisotropic HXR emission has been detected for the first time at the WEST core and edge plasma, with opposite directions. It is due to the forward-peak emission of two distinctive populations of fast electrons: co-current fast electrons in the core and counter-current fast electron losses at the inner strike point. In view of future experiments like ITER where electron cyclotron current drive will generate a fast electron population, this technique could serve as a real-time monitor of fast electron losses and eventually feed an actuator on the current drive generation.

Photon and dilepton emission anisotropy for a magnetized quark-gluon plasma

Photon and dilepton emission anisotropy for a magnetized quark-gluon plasma

General model of nonradiative excitation energy migration on a spherical nanoparticle with attached chromophores

Theory of multistep excitation energy migration within the set of chemically identical chromophores distributed on the surface of a spherical nanoparticle is presented. The Green function solution to the master equation is expanded as a diagrammatic series. Topological reduction of the series leads to the expression for emission anisotropy decay. The solution obtained behaves very well over the whole time range and it remains accurate even for a high number of the attached chromophores. Emission anisotropy decay depends strongly not only on the number of fluorophores linked to the spherical nanoparticle but also on the ratio of critical radius to spherical nanoparticle radius, which may be crucial for optimal design of antenna-like fluorescent nanostructures. The results for mean squared excitation displacement are provided as well. Excellent quantitative agreement between the theoretical model and Monte–Carlo simulation results was found. The current model shows clear advantage over previously elaborated approach based on the Padé approximant.

Spectral and type I X-ray burst studies of 4U 1702−429 using AstroSat observations

ABSTRACT 4U 1702−429, an atoll-type neutron star low-mass X-ray binary, was observed twice by the AstroSat/Soft X-ray Telescope (SXT) and Large Area X-ray Proportional Counters (LAXPC-20) on 2018 April 27 and 2019 August 8. Persistent emission spectra of the source were well fitted with the model combination - constant × tbabs (thcomp × diskbb+powerlaw). The parameters obtained from the spectral analysis revealed the source to be in a hard spectral state during the observations. Time-resolved spectral analyses were performed on the three type I X-ray bursts detected from the source. Burst analysis showed that the source underwent a photospheric radius expansion. Consequently, the radius of the neutron star and distance to the source (with isotropic and anisotropic burst emission) were obtained as 12.65$\substack{+0.90\\-0.86}$ km and 6.92$\substack{+0.16\\-0.09}$ and 8.43$\substack{+0.20\\-0.10}$ kpc, respectively.

Investigation of the nonlinear optical frequency conversion in ultrathin franckeite heterostructures

Layered franckeite is a natural superlattice composed of two alternating layers of different compositions, SnS2- and PbS-like. This creates incommensurability between the two species along the planes of the layers, resulting in spontaneous symmetry-break periodic ripples in the a-axis orientation. Nevertheless, natural franckeite heterostructure has shown potential for optoelectronic applications mostly because it is a semiconductor with 0.7 eV bandgap, air-stable, and can be easily exfoliated down to ultrathin thicknesses. Here, we demonstrate that few-layer franckeite shows a highly anisotropic nonlinear optical response due to its lattice structure, which allows for the identification of the ripple axis. Moreover, we find that the highly anisotropic third-harmonic emission strongly varies with material thickness. These features are further corroborated by a theoretical nonlinear susceptibility model and the nonlinear transfer matrix method. Overall, our findings help to understand this material and propose a characterization method that could be used in other layered materials and heterostructures to assign their characteristic axes.

PH dependence of perturbed angular correlation in DOTA chelated 111\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$^{111}$$\\end{document}In measured with ring-shape gamma-ray detectors

The conventional nuclear medical imaging, such as PET and SPECT, visualizes the accumulation of radioisotopes in the human body and has been widely used for diagnosis of cancer and other diseases. Perturbed angular correlation is a powerful sensing tool detecting local electro-magnetic environment and has been used for chemical analysis or nuclear physics research. The use of perturbed angular correlation in nuclear medicine is recently proposed and pH sensing capability with liquid 111\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$^{111}$$\\end{document}In has been confirmed. In more practical applications, DOTA based radioisotope labeled drug delivery is common in nuclear medicine. This study aims to characterize the pH dependance of perturbed angular correlation in Psyche-DOTA chelated 111\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$^{111}$$\\end{document}In with the custom ring-shape gamma-ray detectors. The observed transition in anisotropy of cascade gamma-ray emission can contribute to the detection of a specific phenomenon changing pH inside the human body.

Hemispherical harmonic illumination and reflectance angular spectrum.

Light scattering plays an important role in physics, with wide applications in science and engineering. However, accurate and effective modeling of scattering remains a great challenge. In this study, we exploited the rendering equation using hemispherical harmonics to demonstrate an angular frequency representation that directly depicts scattering in a two-dimensional spectrum, free from any underlying assumptions. This representation offers a compact and intuitive characterization of mirror reflection, isotropic scattering, and anisotropic emission. The robust support of theoretical proofs and data-driven experimental results establishes the broad applicability of our computational model in conducting scattering analyses across diffuse, specular, and glossy materials. With the capability to characterize the scattering in angular frequency domain, we expect our proposed model to emerge as an essential tool in various domains, including surface feature recognition, reflectance data compression, and computer rendering.

Actively tuning anisotropic light–matter interaction in biaxial hyperbolic material α-MoO3 using phase change material VO2 and graphene

Anisotropic hyperbolic phonon polaritons (PhPs) in natural biaxial hyperbolic material α-MoO3 has opened up new avenues for mid-infrared nanophotonics, while active tunability of α-MoO3 PhPs is still an urgent problem necessarily to be solved. In this study, we present a theoretical demonstration of actively tuning α-MoO3 PhPs using phase change material VO2 and graphene. It is observed that α-MoO3 PhPs are greatly dependent on the propagation plane angle of PhPs. The insulator-to-metal phase transition of VO2 has a significant effect on the hybridization PhPs of the α-MoO3/VO2 structure and allows to obtain actively tunable α-MoO3 PhPs, which is especially obvious when the propagation plane angle of PhPs is 90°. Moreover, when graphene surface plasmon sources are placed at the top or bottom of α-MoO3 in α-MoO3/VO2 structure, tunable coupled hyperbolic plasmon–phonon polaritons inside its Reststrahlen bands (RBs) and surface plasmon–phonon polaritons outside its RBs can be achieved. In addition, the above-mentioned α-MoO3-based structures also lead to actively tunable anisotropic spontaneous emission (SE) enhancement. This study may be beneficial for realization of active tunability of both PhPs and SE of α-MoO3, and facilitate a deeper understanding of the mechanisms of anisotropic light–matter interaction in α-MoO3 using functional materials.

Shockwave velocimetry using wave-based image processing to measure anisotropic shock emission

Noninvasive optical measurements of the shockwave propagation velocity using multiple pulse illumination allow deducing the shockwave pressure amplitude through Hugoniot relations and an appropriate equation of state of the medium. This technique is particularly useful for spatially resolved measurements near the shockwave emission site. Due to diffraction, however, a shockwave front can significantly change its morphology, rendering precise velocity measurements non-trivial. As solution we propose a wave front evolution (WaFE) velocimetry technique, which applies Huygens principle. We take a shadowgraph of the wave front at subsequent times as initial condition for the acoustic Helmholtz equation and numerically propagate the fronts in time. From the instance of time, when two subsequently taken wave front shadows numerically interfere and form one sharp wave front, the local shock velocity is obtained and the local shock pressure amplitude measured. With artificial test images, it is shown that this technique has excellent sub-pixel accuracy, robustness to noise, and can work with low contrast images and even overlapping and interfering wave fronts. The software is made available freely and can be applied to general shock front velocity measurements. We apply WaFE to determine the anisotropic shockwave emission from an elongated laser-induced plasma in water from shadowgraphs of the shockwave front imaged four times onto the same camera frame using multiple pulse illumination at a repetition rate of 60 MHz. The direction dependence of attenuation of the shockwave pressure amplitude is measured at distances of 50–300 μm to the plasma.

Simulation of the temperatures in the permanently shadowed region of the Moon's south pole and data validation

The permanently shadowed regions (PSRs) at the lunar poles without direct solar illumination maintain low temperatures throughout the year, making them optimal environments for the preservation of water ice. This paper investigates the diurnal and seasonal temperature variations of the PSRs within craters at the lunar south pole. The craters de Gerlache, Shoemaker, Amundsen, and Wiechert E are selected to explore how latitudes and shapes impact PSR temperature. Using the ray tracing approach, the shadow effect of terrain on the scattered sunlight and thermal radiation is calculated. PSR temperatures are simulated with the one-dimensional heat conduction equation. The thermal emissivity of the lunar surface is studied. Simulations with anisotropic emissivity are closer to the Diviner data in PSRs than the simulations with isotropic emissivity. In addition, by comparing the simulations to the Diviner data, it is found that the thermal conductivity at the extremely low temperatures in PSRs might be lower than that at high temperatures outside the PSRs. Simulations of the selected craters are used to study the influence of polar day phenomena occurring at high-latitude craters during the summer and the effect of direct solar illumination at the floors of low-latitude craters on the temperature distribution of PSRs. The influence of crater shape is analyzed as well by investigating the temperature distributions within PSRs of flat-floored and bowl-shaped craters. With the analysis of the heat flux propagation distances, emission angles, and incident angles, this study quantitatively elucidates the spatial and temporal variations in PSR temperatures. To explain why the Diviner data are lower than the simulation results at the solar-illuminated crater walls, an undulating rough surface is generated. The locations of the Sun and the observer on opposite sides of the rough surface may decrease the observed infrared brightness temperature according to the simulations. The PSR peak temperatures simulated with the anisotropic emissivity and thermal conductivity for PSRs are used to search for low-temperature areas suitable for preserving water ice within de Gerlache and Shoemaker.

Electron Emission Channeling for lattice location of radioactive isotopes in single crystals: Improvements from a Timepix3 quad detector and new PyFDD data analysis software

Electron Emission Channeling (EC) is a powerful technique for the investigation of the lattice location of radioactive isotopes implanted into single crystals. After implantation the isotopes occupy certain lattice locations in the crystal, which can in some cases be altered by annealing. Upon decay, the emission of a charged particle, typically a beta, may result in a channeling trajectory when its starting lattice location is aligned with major symmetry axes or planes of the crystal. By measuring the emission anisotropy in the direction of these axes for distinct annealing temperatures, the lattice location of the isotope can be determined with great precision and insightful information can be obtained on how annealing affects the occupied sites. This work reports on the installation of a Timepix3 quad detector and Katherine Gen2 readout in an experimental setup located at ISOLDE at CERN. The large increase in the number of pixels of the Timepix3, in comparison to previously used pad detectors, required more sophisticated tools for data treatment and fitting of channeling patterns. From this need, the PyFDD software was born . Its latest update features an intuitive graphical interface, with tools for noise masking, pattern visualization, simulations browsing, chi-square or maximum likelihood based fits and gamma background correction.

Anisotropic emission wavelength distribution of semipolar InGaN quantum wells on symmetric convex lens-shaped GaN microstructures

We fabricated InGaN quantum wells (QWs) on convex lens-shaped GaN microstructures formed on semipolar (1¯1¯22¯) GaN substrates. Despite the centrosymmetric lens-like shapes, the overgrown InGaN QWs exhibit an anisotropic emission wavelength distribution. This behavior is distinct from the concentric wavelength distribution found in microlens QWs on the polar (0001) plane. The anisotropic distribution is mainly attributed to the In composition variations, which are influenced by the asymmetry of the (1¯1¯22¯) crystallographic plane. The obtained results demonstrate that the (1¯1¯22¯) microlens QWs serve as multi-wavelength light emitters and allow us to investigate unstable planes around the (1¯1¯22¯) plane, which have remained unexplored.

Event shape selection method in search of the chiral magnetic effect in heavy-ion collisions

The search for the chiral magnetic effect (CME) in heavy-ion collisions has been impeded by the significant background arising from the anisotropic particle emission pattern, particularly elliptic flow. To alleviate this background, the event shape selection (ESS) technique categorizes collision events according to their shapes and projects the CME observables to a class of events with minimal flow. In this study, we explore two event shape variables to classify events and two elliptic flow variables to regulate the background. Each type of variable can be calculated from either single particles or particle pairs, resulting in four combinations of event shape and elliptic flow variables. By employing a toy model and the realistic event generator, event-by-event anomalous-viscous fluid dynamics (EBE-AVFD), we discover that the elliptic flow of resonances exhibits correlations with both the background and the potential CME signal, making the resonance flow unsuitable for background control. Through the EBE-AVFD simulations of Au+Au collisions at sNN=200 GeV with various input scenarios, we ascertain that the optimal ESS strategy for background control entails utilizing the single-particle elliptic flow in conjunction with the event shape variable based on particle pairs.

Investigation of multi-keV silver x-ray sources at OMEGA

We report on the results of experiments carried out at the OMEGA laser to address the performances of silver x-ray sources across several target and laser configurations. Every target consisted of a thin silver foil and a polystyrene shield mounted on the top of the foil in order to filter the x-ray emission in some spatial directions. Seven targets were fired with ∼4 to ∼7.7 kJ of 3ω energy and various incident powers, including pre-pulsed laser pulses. The radiant energies measured by the broadband spectrometer DMX were found to range in 350–710 J/sr and 80–120 J/sr for the 0–2 keV and >2 keV spectral bands, respectively. These experimental results are in good agreement with the predictions of 2D simulations performed with the radiation-hydrodynamics code TROLL. The radiant energies measured by the broadband spectrometer miniDMX in the >2 keV band range in 40–70 J/sr and give a measurement of the emission anisotropy of the sources. The TROLL simulated electron temperatures are within the error bars of the temperatures inferred from Thomson scattering during the laser pulses for most of the shots but deviate at later times when the laser is turned off.

Anisotropic electronic structure of NaAlSi studied by angle-resolved soft x-ray emission spectroscopy

The characteristic x-ray emission direction of a material indicates the direction of the bonding orbitals and spatial symmetry of the electron orbitals. Accordingly, the intensity of x-ray emission, which varies with the direction of emission and crystal orientation, provides crucial information regarding anisotropic electronic structures. This study utilized angle-resolved soft x-ray emission spectroscopy (SXES) on a layered material, NaAlSi, to ascertain the spatial distribution of the valence electrons. Distinct alterations in the spectral intensity distributions were observed in the Al–L2,3 and Si–L2,3 spectra with respect to the emission angle. To interpret the anisotropic SXES spectra, the spatial distribution of each valence electronic state was simulated using first-principle calculations. Although the anisotropic emission intensity could not explain the symmetry of the spatial distributions of the isolated s and d atomic orbitals, the anisotropy of the SXES spectra could be interpreted as the spatial distribution of these orbitals when hybridized with p orbitals. Furthermore, the spectral structure corresponding to the electronic states near the Fermi level reflected the characteristics of the d orbitals. Therefore, angle-resolved SXES measurements can effectively discern the spatial distribution of hybridized electron orbitals with specific energy levels, which could enhance techniques related to electron distribution analysis, with potential applications in material science and electronic structure characterization.

Tunable Anisotropic Extrinsic Self‐Trapped Exciton Emission in Van Der Waals Layered In4/3P2S6

AbstractSelf‐trapped exciton (STE) induced broad‐band emission (BE) has sparked considerable interest due to its potential applications in white‐light emitters and optoelectronics. This phenomenon is widely observed in organic–inorganic hybrid perovskites with soft lattice structures, and its physical origin is still under debate. Herein, strong sub‐bandgap STE emission with a large Stokes shift and a photoluminescence quantum yield of up to 9.2% in van der Waals (vdW) layered In4/3P2S6 is reported. Combining comprehensive optical characterizations and theoretical calculations, this concludes that defect‐assisted extrinsic STE is responsible for the BE. The excitonic state can be further localized by hydrostatic pressure, resulting in a threefold PL intensity enhancement. In addition, angle‐resolved polarized Raman demonstrates the anisotropic lattice dynamics in IPS, which may underpin the highly linear anisotropy of the STE emission. This work clarifies the defect, STE, and anisotropy coupling effect in vdW crystal, and provides innovative avenues to modulate the STE luminescence.

A multifunctional optoelectronic device based on 2D material with wide bandgap

Low-dimensional materials exhibit unique quantum confinement effects and morphologies as a result of their nanoscale size in one or more dimensions, making them exhibit distinctive physical properties compared to bulk counterparts. Among all low-dimensional materials, due to their atomic level thickness, two-dimensional materials possess extremely large shape anisotropy and consequently are speculated to have large optically anisotropic absorption. In this work, we demonstrate an optoelectronic device based on the combination of two-dimensional material and carbon dot with wide bandgap. High-efficient luminescence of carbon dot and extremely large shape anisotropy (>1500) of two-dimensional material with the wide bandgap of >4 eV cooperatively endow the optoelectronic device with multi-functions of optically anisotropic blue-light emission, visible light modulation, wavelength-dependent ultraviolet-light detection as well as blue fluorescent film assemble. This research opens new avenues for constructing multi-function-integrated optoelectronic devices via the combination of nanomaterials with different dimensions.

Coherent perfect absorber and laser induced by directional emissions in the non-Hermitian photonic crystals

In this study, we propose the application of non-Hermitian photonic crystals (PCs) with anisotropic emissions. Unlike the ring of exceptional points (EPs) found in isotropic non-Hermitian PCs, the EPs of anisotropic non-Hermitian PCs appear as symmetrical lines about the Γ point. The formation of EPs is related to the non-Hermitian strength and the real spectrum appears in the ΓY direction. The PCs have been validated as the complex conjugate medium (CCM) by effective medium theory (EMT). Conversely, EMT indicates that the effective refractive index has a large imaginary part along the ΓX direction, which forms an evanescent wave inside the PCs. Consequently, coherent perfect absorption (CPA) and laser can be achieved in the directional emission of the ΓY. The outgoing wave in the ΓX direction is weak, which can significantly reduce the losses and electromagnetic interference. The non-Hermitian PCs enable many fascinating applications such as signal amplification, collimation, and angle sensors.

Improving optical coherence of light-emitting diodes by surface plasmons via shallow-etched conic pit array.

We propose the coupling of multiple quantum wells and surface plasmons can improve coherence of light emitted from LED wafers, as evidenced herein by a shallow-etched conic pit array with evaporated Ag (V-Ag) on a GaN-based LED wafer. The improvement in spatial coherence is critically verified by angle-resolved spectra. The temporal coherence length of the V-Ag wafer is 1.4 times larger than that of the plain wafer. The coherence-enhanced wafer achieves anisotropic and deflective emission in micro area and at far field by diffraction. This research provides a novel perspective on research of plasmonic LEDs and a new straightforward architecture to acquire partially coherent light from LEDs.

Insight-HXMT observations on thermonuclear X-ray bursts from 4U 1608–52 in 2022: The accretion rate dependent anisotropy of burst emission

Thermonuclear X-ray bursts occur on the surface of an accreting neutron star (NS), and their characteristics and interplay with the surrounding circumstance could be a clue to understand the nature of the NS and accretion process. For this purpose, Insight-HXMT has performed high cadence observations on the bright thermonuclear X-ray burster–4U 1608–52 during its outburst in July and August 2022; nine bursts were detected, including seven bursts with the photospheric radius expansion (PRE). Time-resolved spectroscopy of the bright PRE bursts reveals that an enhancement of accretion rate or the Comptonization of the burst emission by the corona could reduce the residuals when fitting their spectra with the conventional model–blackbody. The inferred energy increment rate of the burst photon gained from the corona is up to ∼40%, even though the bursts have different peak fluxes and locate at different accretion rates. Moreover, the flux shortage of the rising PRE is observed in the bursts at a high mass accretion rate, but not for the burst with a faint persistent emission, which has been predicted theoretically but first observed in this work. If the flux shortage is due to the disk obscuration, i.e., the burst emission is anisotropic, the phenomenon above could indicate that the anisotropy of the burst emission is accretion rate dependent, which could also be evidence of the truncated disk in the low/hard state.