Magnitude In Intensity Research Articles

Multidimensional photoemission spectroscopy—the space-charge limit

Photoelectron spectroscopy, especially at pulsed sources, is ultimately limited by the Coulomb interaction in the electron cloud, changing energy and angular distribution of the photoelectrons. A detailed understanding of this phenomenon is crucial for future pump–probe photoemission studies at (x-ray) free electron lasers and high-harmonic photon sources. Measurements have been performed for Ir(111) at hν = 1000 eV with photon flux densities between ∼102 and 104 photons per pulse and μm2 (beamline P04/PETRA III, DESY Hamburg), revealing space-charge induced energy shifts of up to 10 eV. In order to correct the essential part of the energy shift and restore the electron distributions close to the Fermi energy, we developed a semi-analytical theory for the space-charge effect in cathode-lens instruments (momentum microscopes, photoemission electron microscopes). The theory predicts a Lorentzian profile of energy isosurfaces and allows us to quantify the charge cloud from measured energy profiles. The correction is essential for the determination of the Fermi surface, as we demonstrate by means of ‘k-space movies’ for the prototypical high-Z material tungsten. In an energy interval of about 1 eV below the Fermi edge, the bandstructure can be restored up to substantial shifts of ∼7 eV. Scattered photoelectrons strongly enhance the inelastic background in the region several eV below EF, proving that the majority of scattering events involves a slow electron. The correction yields a gain of two orders of magnitude in usable intensity compared with the uncorrected case (assuming a tolerable shift of 250 meV). The results are particularly important for future experiments at SASE-type free electron lasers, since the correction also works for strongly fluctuating (but known) pulse intensities.

Photophysical Model for Non-Exponential Relaxation Dynamics in Hybrid Perovskite Semiconductors

The photoluminescence (PL) decay of hybrid halide perovskite single crystals (MAPbX3, MA = CH3NH3+, Pb = Pb2+, X = Br–, and I–) is measured over 4 orders of magnitude in intensity over the time scales of 100s of nanoseconds to a few microseconds. This long PL decay is non-exponential, suggesting the presence of a distribution of carrier relaxation times. Spectro-temporal studies show that the emission peak red-shifts with increasing time. The physics of this problem is closely related to donor–acceptor pair recombination in crystalline semiconductors and recombination in a-Si:H. Based on these models, we present a simple model to account for the recombination dynamics in the perovskite systems. This model also accounts for the fluence dependence of the recombination kinetics. In this model, a fraction of the photogenerated electrons and holes are trapped in localized states. The electrons tunnel to the hole sites for recombination. The broad distribution of lifetimes is a consequence of the fact that the ...

Facile fabrication of a gadolinium-doped liquid scintillator with the mixed solvent

A gadolinium-doped liquid scintillator with the mixed solvent as the matrix was prepared for a good comprehensive performance. In the ternary liquid scintillator, the β-diketonate was used as the complexing ligand of gadolinium (Gd), the combination of 20vol% pseudocumene (PC) and 80vol% linear-alkyl benzene (LAB) was chosen as the mixed solvent, and 2,5-diphenyloxazole (PPO) and 1,4-bis(2-Methylstyryl) benzene (bis-MSB) were the primary fluor and wavelength shifter, respectively. The characterizations based on the optimal formulation were conducted. The light yield was characterized by adopting the home-made optical platform device. The decay time was tested by adopting the time-correlated single photon counting (TCSPC) technique featured in high dynamic range of several orders of magnitude in light intensity. The experimental test results showed that the ternary liquid scintillator had a fairly good comprehensive performance.

Radical increase of the parametric X-ray intensity under condition of extremely asymmetric diffraction

Parametric X-ray radiation (PXR) from relativistic electrons moving in a crystal along the crystal-vacuum interface is considered. In this geometry the emission of photons is happening in the regime of extremely asymmetric diffraction (EAD). In the EAD case the whole crystal length contributes to the formation of X-ray radiation opposed to Laue and Bragg geometries, where the emission intensity is defined by the X-ray absorption length. We demonstrate that this phenomenon should be described within the dynamical theory of diffraction and predict a radical increase of the PXR intensity. In particular, under realistic electron-beam parameters, an increase of two orders of magnitude in PXR-EAD intensity can be obtained in comparison with conventional experimental geometries of PXR. In addition we discuss in details the experimental feasibility of the detection of PXR-EAD.

Preparation and performance study of a novel liquid scintillator with mixed solvent as the matrix

A novel liquid scintillator with the mixed solvent as the matrix was prepared for obtaining a good comprehensive performance. In this ternary liquid scintillator, the combination of 20% pseudocumene (PC) and 80% linear-alkyl benzene (LAB) by volume was chosen as the mixed solvent, and 2,5-diphenyloxazole (PPO) and 1,4-bis(2-Methylstyryl) benzene (bis-MSB) were as the primary fluor and wavelength shifter, respectively. The optimum prescription was obtained with regard to the light yield. Some characterizations based on the optimal formulation were conducted. The fluorescence emission spectra and wavelength-dependent optical attenuation length of the sample were measured by the fluorescence spectrophotometer and an UV–Vis spectrometer, respectively. The light yield was characterized by adopting the home-made optical platform device. The decay time was tested by adopting the time-correlated single photon counting (TCSPC) technique featured in high dynamic range of several orders of magnitude in light intensity. The experimental test results showed that the sample had a fairly good comprehensive performance.

Effect of Three-Stage Growth Modification on a CIGS Microstructure

Cu(In,Ga)Se2 (CIGS) thin-film solar cells from a Global Solar Energy (GSE) production line with 17.7% cell efficiency have been analyzed by a variety of imaging techniques, which include high-resolution electroluminescence and 3-D optical profilometry. The microstructure of supergrown CIGS nodules has been observed by an optical microscope and a scanning electron microscope (SEM). Statistical distribution of the nodule microstructure matched well with the bright spots in electroluminescence. A variation of several orders of magnitude in electroluminescence intensity indicates a wide margin of the quasi-Fermi level splitting potential and a possibility of efficiency improvement. Substrate temperatures of (In,Ga)2Se3 precursor in the three-stage growth process have been modified to eliminate the nodule growth, which resulted in $V_{{\rm OC}}$ increase from the baseline value of 653 to 677 mV for a CIGS optical bandgap of 1.15 eV. Additional measurements, such as conductive-tip atomic force microscopy, energy-dispersive X-ray spectroscopy, and glow-discharge optical emission spectroscopy, are presented, and the results are discussed.

Engineering biodegradable micelles of polyethylenimine-based amphiphilic block copolymers for efficient DNA and siRNA delivery

Polycationic micelles have shown advantageous properties as nucleic acid delivery vectors both in vitro and in vivo. In contrast to polycationic micelles reported so far, we designed particles integrating a sufficient nucleic acid condensation capability by polycationic polyethylenimine (PEI) segments as well as only a mild cytotoxic behavior. The micelles composed of a hydrophobic oligoester core with glycolide units resulting in fast degradation after cellular internalization in combination with PEG moieties acting as shielding agents. By grafting branched 25kDa polyethylenimine (PEI25) and poly(ethylene glycol) (PEG) on poly[(ε-caprolactone)-co-glycolide] (CG), amphiphilic PEI-CG-PEI and PEG-CG block copolymers were used to form a series of micelles via self-assembly of PEI-CG-PEI or co-assembly of both copolymers for DNA and siRNA delivery. This modular system enabled a systematic investigation of different parameters and their synergetic effects as different functions were introduced. The polyplex formation and serum stability, cytotoxicity, and transfection activity could be tailored by changing the CG chain length in PEI-based copolymer, incorporating PEG-CG, and varying the N/P ratio. All micelle-based polyplex compositions showed high DNA transfection activity according to reporter gene-expression and an exceptionally high knockdown in siRNA delivery experiments. Remarkably, the GFP expression of >99% cells was successfully knocked down by micelle-mediated siRNA interference, resulting in a decrease of two orders of magnitude in fluorescence intensity. Incorporation of PEG-CG in the micelles reduced the PEI-related cytotoxicity, and markedly enhanced the serum stability of both DNA and siRNA polyplexes. Compared with homo-PEI25, these micelles showed several advantages including the lower toxicity, higher siRNA transfection efficiency and higher polyplex stability in the presence of serum. This study therefore provides an effective approach to tune the structure, property and function of polycationic micelles for efficient DNA and siRNA delivery, which could contribute to the design and development of novel non-viral transfection vectors with superb functionality.

Elimination of X-Ray Diffraction through Stimulated X-Ray Transmission.

X-ray diffractive imaging with laterally coherent x-ray free-electron laser (XFEL) pulses is increasingly utilized to obtain ultrafast snapshots of matter. Here we report the amazing disappearance of single-shot charge and magnetic diffraction patterns recorded with resonantly tuned, narrow bandwidth XFEL pulses. Our experimental results reveal the exquisite sensitivity of single-shot charge and magnetic diffraction patterns of a magnetic film to the onset of field-induced stimulated elastic x-ray forward scattering. The loss in diffraction contrast, measured over 3 orders of magnitude in intensity, is in remarkable quantitative agreement with a recent theory that is extended to include diffraction.

Experimental Bounds on Classical Random Field Theories

Alternative theories to quantum mechanics motivate important fundamental tests of our understanding and descriptions of the smallest physical systems. Here, using spontaneous parametric downconversion as a heralded single-photon source, we place experimental limits on a class of alternative theories, consisting of classical field theories which result in power-dependent normalized correlation functions. In addition, we compare our results with standard quantum mechanical interpretations of our spontaneous parametric downconversion source over an order of magnitude in intensity. Our data match the quantum mechanical expectations, and do not show a statistically significant dependence on power, limiting quantum mechanics alternatives which require power-dependent autocorrelation functions.

Gap enhancement and transmittance spectra of a periodic bowtie nanoantenna array buried in a silica substrate

The gap enhancement and transmittance spectra properties of a periodic outline bowtie nanoantenna array (POBNA) which is embedded in a substrate with different depths are numerically investigated by using the finite element method. The surface plasmon resonance (SPR) modes obtained from the embedding cases are quite different from that of the free standing case. Near-field interactions across the bowtie-gaps and POBNA hollows at the localized SPR wavelengths resulted in an increase of magnitude in local field intensity and a redshift of transmittance resonance dip wavelength. The POBNA embedded in a silica substrate can be used as critical elements for nanoscale optical devices.

Cosmic rays around1018eV: Implications of contemporary measurements on the origin of the ankle feature

The impressive power-law decay of the energy spectrum of cosmic rays over more than thirty orders of magnitude in intensity and for energies ranging over eleven decades between $\simeq 10^9 $eV and $\simeq 10^{20} $eV is actually dotted with small irregularities. These irregularities are highly valuable for uncovering and understanding the modes of production and propagation of cosmic rays. They manifest themselves through changes in the spectral index characterising the observed power laws. One of these irregularities, known as the \textit{ankle}, is subject to conflicting interpretations for many years. If contemporary observations characterising it have shed new lights, they are still far from being able to deliver all the story. The purpose of this contribution is to give an overview of the physics of cosmic rays in the energy range where the transition between Galactic and extragalactic cosmic rays is expected to occur, and to deliver several lines of thought about the origin of the ankle.

An optimal solvent of liquid scintillator for fast-neutron imaging applications

For fast neutron detection and imaging applications in the inertial confinement fusion (ICF) experiments, a scintillator with high reflective index, good transparency in the wavelength range of the fluorescence emission spectra, fast decay component with a few nanoseconds decay time is required. In this work, when 2,5-diphenyloxazol and p-bis(o-methylatyryl)-benzene were used as the fluors of a liquid scintillator, three kinds of solvents including PX (p-xylene), PXE (l-phenyl-l-xylylethane) and DIN (di-isopropylnaphthalene) were selected to prepare a liquid scintillator. The wavelength-dependent attenuation length and the fluorescence emission spectra of liquid scintillator mixtures were measured by an UV–vis spectrometer and a FL spectrofluorimeter, respectively. The decay time of liquid scintillator was tested employing the time-correlated single photon counting (TCSPC) technique featured in high dynamic range of several orders of magnitude in light intensity. The experimental results indicated that the optimal solvent was PX from the point of energy transfer efficiency and optical properties.

Convective and shear-induced turbulence in the deep Kane Gap

[1] The boundary layer above a 4569 m deep slope in the near-equatorial N-Atlantic Ocean Kane Gap, a throughflow for Antarctic Bottom Water (AABW), is characterized by two distinct turbulent regimes that differ by an order of magnitude in intensity depending on the direction of throughflow. During southward and downward flow, vertical mixing is vigorous. This is inferred from high-resolution temperature observations between 6 and 132 m above the bottom. For a representative case study of 2 days, average values are found for dissipation rate of e?=?2.1?±?1 × 10-9 W kg-1 and eddy diffusivity of Kz?=?7?±?4 × 10-4 m2 s-1. The mixing is across relatively large vertical overturns. During northward and upward flow, smaller overturns are more horizontal as in stratified shear flow (with representative 2 day mean e?=?6?±?3 × 10-11 W kg-1, Kz?=?4?±?2 × 10-5 m2 s-1). Stratification is approximately the same during both flow directions. Although the different turbulence regimes are partially associated with frictional boundary layers of large-scale flows above sloping topography, but not with those over flat bottoms, and partially with flow across a hill-promontory, internal waves are a dominant process in promoting turbulence. In addition, internal waves are observed to push stratification toward the bottom thereby importantly contributing to the mixing of AABW.

Poisson-spot intensity reduction with a partially transparent petal-shaped optical mask

The presence of a Poisson spot inside a shadow region can best be described as the consequence of constructive interference of light waves diffracted on the edge of the obstruction where its central position can be determined by the symmetry of the object. More recently, the elimination of this spot has received attention in the fields of particle physics, high-energy lasers, astronomy, and lithography. The desired level of intensity suppression is dependent on the type of light source and the field of application. In this paper, we introduce a novel, partially transparent petaled mask shape that suppresses the bright spot by up to ten orders of magnitude in intensity at optical ranges, with potential powerful applications in many of the above fields. The optimization technique formulated in this design can identify mask shapes having partial transparency only near the petal tips.

Digital optical phase conjugation of fluorescence in turbid tissue

We demonstrate a method for phase conjugating fluorescence. Our method, called reference free digital optical phase conjugation, can conjugate extremely weak, incoherent optical signals. It was used to phase conjugate fluorescent light originating from a bead covered with 0.5 mm of light-scattering tissue. The phase conjugated beam refocuses onto the bead and causes a local increase of over two orders of magnitude in the light intensity. Potential applications are in imaging, optical trapping, and targeted photochemical activation inside turbid tissue.

A comparison of paraxial and extended paraxial approach for laser beam propagation in plasma

The propagation of laser beam in the plasma has been investigated for two cases, first with imposing the restriction of paraxial approximation on the beam profile, and second relaxing the restriction on the profile up to a certain extent. Besides the standard ponderomotive nonlinearity the work also includes the nonlinearity that arises from the relativistic electron velocities. The expression for the laser beam width parameter has been derived by expanding the eikonal, dielectric constant and other relevant quantities up to the fourth power of r, therefore, the contribution of both r2 and r4 terms is incorporated in the present analysis. The dynamical equation governing the beam width parameter has been numerically solved for typical laser plasma parameters. The result shows that the focusing of the laser beams has been affected and becomes fast due to the contribution of the off-axial rays in the extended-paraxial region. The intensity of the laser beam has also been examined in the light of these two cases and a notable change is found in the magnitude in the laser beam intensity with modified-paraxial case as compared to the paraxial case.

Effect of the external electric field on the kinetics of recombination of photoexcited carriers in a ZnSe/BeTe type II heterostructure

The kinetics of the radiative recombination of photoexcited electrons and holes for a spatially direct transition in a ZnSe/BeTe type II heterostructure in an external electric field has been analyzed. A strong decrease (more than two orders of magnitude) in the photoluminescence intensity, as well as a decrease in the duration of the relaxation of the direct transition, is observed when the electric field is applied. The energy levels and wavefunctions of electrons and holes in the ZnSe/BeTe heterostructure subjected to the electric field have been numerically calculated. It has been shown that the observed decrease in the photoluminescence intensity and duration of the relaxation of the direct transition is due to both an increase in the radiative recombination time and an increase in the rate of escape of photoexcited holes from the above-barrier level in the ZnSe layer to the BeTe layer.

Existence of two-solar-mass neutron star constrains gravitational constantGNat strong field

In General Relativity there is a maximum mass allowed for neutron stars that, if exceeded, entails their collapse into a black hole. Its precise value depends on details of the nuclear matter equation of state about which we are much more certain thanks to recent progress in low-energy effective theories. The discovery of a two-solar mass neutron star, near that maximum mass, when analyzed with modern equations of state, implies that Newton's gravitational constant in the star cannot exceed its value on Earth by more than 8% at 95% confidence level. This is a remarkable leap of ten orders of magnitude in the gravitational field intensity at which the constant has been constrained.

A simple 3D plasma instrument with an electrically adjustable geometric factor for space research

We report on the design and experimental verification of a novel charged particle detector and an energy spectrometer with variable geometric factor functionality. Charged particle populations in the inner heliosphere create fluxes that can vary over many orders of magnitude in flux intensity. Space missions that plan to observe plasma fluxes, for example when travelling close to the Sun or to a planetary magnetosphere, require rapid particle measurements over the full three-dimensional velocity distribution. Traditionally, such measurements are carried out with plasma instrumentation with a fixed geometrical factor, which can only operate in a limited range of flux intensity. Here we report on the design and testing of a prototype sensor, which is capable of measuring particle flux with high angular and energy resolution, yet has a variable geometric factor that is controlled without moving parts. This prototype was designed in support of a proposal to make fast electron measurements on the Solar Probe Plus (SP+) mission planned by NASA. We simulated the ion optics inside the instrument and optimized the performance to design and build our prototype. This prototype was then tested in the MEFISTO facility at the University of Bern and its performance was verified over the full range of azimuth, elevation, energy and intensity.

Extraordinary Nonlinear Absorption in 3D Bowtie Nanoantennas

This paper reports that arrays of three-dimensional (3D), bowtie-shaped Au nanoparticle dimers can exhibit extremely high nonlinear absorption. Near-field interactions across the gap of the 3D bowties at the localized surface plasmon resonance wavelengths resulted in an increase of more than 4 orders of magnitude in local field intensity. The imaginary part of the third-order nonlinear susceptibility (Im χ((3))) for the 3D bowtie arrays embedded in a dielectric material was measured to be 10(-4) esu, more than 2 orders of magnitude higher than reported for other metal nanoparticle-dielectric composites. Moreover, 3D dimers with increased nanoscale structure (such as folding) exhibited increased optical nonlinearity. These 3D nanoantennas can be used as critical elements for nanoscale nonlinear optical devices.