Showing papers on "Photon energy published in 2016"

Splitting of Interlayer Shear Modes and Photon Energy Dependent Anisotropic Raman Response in N-Layer ReSe2 and ReS2

Abstract: We investigate the interlayer phonon modes in N-layer rhenium diselenide (ReSe2) and rhenium disulfide (ReS2) by means of ultralow-frequency micro-Raman spectroscopy. These transition metal dichalcogenides exhibit a stable distorted octahedral (1T') phase with significant in-plane anisotropy, leading to sizable splitting of the (in-plane) layer shear modes. The fan-diagrams associated with the measured frequencies of the interlayer shear modes and the (out-of-plane) interlayer breathing modes are perfectly described by a finite linear chain model and allow the determination of the interlayer force constants. Nearly identical values are found for ReSe2 and ReS2. The latter are appreciably smaller than but on the same order of magnitude as the interlayer force constants reported in graphite and in trigonal prismatic (2Hc) transition metal dichalcogenides (such as MoS2, MoSe2, MoTe2, WS2, WSe2), demonstrating the importance of van der Waals interactions in N-layer ReSe2 and ReS2. In-plane anisotropy results in a complex angular dependence of the intensity of all Raman modes, which can be empirically utilized to determine the crystal orientation. However, we also demonstrate that the angular dependence of the Raman response drastically depends on the incoming photon energy, shedding light on the importance of resonant exciton-phonon coupling in ReSe2 and ReS2.

Single microwave-photon detector using an artificial Λ-type three-level system

Abstract: Single-photon detection is a requisite technique in quantum-optics experiments in both the optical and the microwave domains. However, the energy of microwave quanta are four to five orders of magnitude less than their optical counterpart, making the efficient detection of single microwave photons extremely challenging. Here we demonstrate the detection of a single microwave photon propagating through a waveguide. The detector is implemented with an impedance-matched artificial Λ system comprising the dressed states of a driven superconducting qubit coupled to a microwave resonator. Each signal photon deterministically induces a Raman transition in the Λ system and excites the qubit. The subsequent dispersive readout of the qubit produces a discrete ‘click’. We attain a high single-photon-detection efficiency of 0.66±0.06 with a low dark-count probability of 0.014±0.001 and a reset time of ∼400 ns. This detector can be exploited for various applications in quantum sensing, quantum communication and quantum information processing. Single-photon detection is challenging in the microwave regime due to the small photon energy. Here, the authors demonstrate the deterministic detection of single microwave photons through an impedenance-matched artificial Λ system composed by a driven superconducting qubit and a microwave resonator.

Optical and transport properties in three-dimensional Dirac and Weyl semimetals

Abstract: Within a Kubo formalism, we study dc transport and ac optical properties of 3D Dirac and Weyl semimetals Emphasis is placed on the approach to charge neutrality and on the differences between Dirac and Weyl materials At charge neutrality, the zero-temperature limit of the dc conductivity is not universal and also depends on the residual scattering model employed However, the Lorenz number $L$ retains its usual value ${L}_{0}$ With increasing temperature, the Wiedemann-Franz law is violated At high temperatures, $L$ exhibits a new plateau at a value dependent on the details of the scattering rate Such details can also appear in the optical conductivity, both in the Drude response and interband background In the clean limit, the interband background is linear in photon energy and always extrapolates to the origin This background can be shifted to the right through the introduction of a massless gap In this case, the extrapolation can cut the axis at a finite photon energy as is observed in some experiments It is also of interest to differentiate between the two types of Weyl semimetals: those with broken time-reversal symmetry and those with broken spatial-inversion symmetry We show that, while the former will follow the same behavior as the 3D Dirac semimetals, for the zero magnetic field properties discussed here, the latter type will show a double step in the optical conductivity at finite doping and a single absorption edge at charge neutrality The Drude conductivity is always finite in this case, even at charge neutrality

Design and performance of BOREAS, the beamline for resonant X-ray absorption and scattering experiments at the ALBA synchrotron light source

Abstract: The optical design of the BOREAS beamline operating at the ALBA synchrotron radiation facility is described. BOREAS is dedicated to resonant X-ray absorption and scattering experiments using soft X-rays, in an unusually extended photon energy range from 80 to above 4000 eV, and with full polarization control. Its optical scheme includes a fixed-included-angle, variable-line-spacing grating monochromator and a pair of refocusing mirrors, equipped with benders, in a Kirkpatrick–Baez arrangement. It is equipped with two end-stations, one for X-ray magnetic circular dichroism and the other for resonant magnetic scattering. The commissioning results show that the expected beamline performance is achieved both in terms of energy resolution and of photon flux at the sample position.

Multiwavelength Study Of Quiescent States Of Mrk 421 With Unprecedented Hard X-Ray Coverage Provided By Nustar In 2013

Abstract: We present coordinated multiwavelength observations of the bright, nearby BL Lacertae object Mrk 421 taken in 2013 January–March, involving GASP-WEBT, Swift, NuSTAR, Fermi-LAT, MAGIC, VERITAS, and other collaborations and instruments, providing data from radio to very high energy (VHE) γ-ray bands. NuSTAR yielded previously unattainable sensitivity in the 3–79 keV range, revealing that the spectrum softens when the source is dimmer until the X-ray spectral shape saturates into a steep Γ ≈ 3 power law, with no evidence for an exponential cutoff or additional hard components up to ~80 keV. For the first time, we observed both the synchrotron and the inverse-Compton peaks of the spectral energy distribution (SED) simultaneously shifted to frequencies below the typical quiescent state by an order of magnitude. The fractional variability as a function of photon energy shows a double-bump structure that relates to the two bumps of the broadband SED. In each bump, the variability increases with energy, which, in the framework of the synchrotron self-Compton model, implies that the electrons with higher energies are more variable. The measured multi band variability, the significant X-ray-to-VHE correlation down to some of the lowest fluxes ever observed in both bands, the lack of correlation between optical/UV and X-ray flux, the low degree of polarization and its significant (random) variations, the short estimated electron cooling time, and the significantly longer variability timescale observed in the NuSTAR light curves point toward in situ electron acceleration and suggest that there are multiple compact regions contributing to the broadband emission of Mrk 421 during low-activity states.

Investigations of gamma ray and fast neutron shielding properties of tellurite glasses with different oxide compositions

Abstract: In the present paper, the basic radiation parameters of tellurite glasses with different forming oxides (B2O3, BaO, K2O, V2O5, WO3, and ZnO) have been studied over a wide photon energy range from 1...

Photon energy storage materials with high energy densities based on diacetylene–azobenzene derivatives

Abstract: Photocontrolled self-assembly of molecules has been utilized to change the physical properties of organic materials for various applications, while photon energy storage materials that incorporate photochromic molecules such as azobenzenes have been recognized as another highly attractive class of materials that convert and store photon energy in the strained chemical bonds. Herein, we demonstrate the photocontrolled self-assembly and disassembly of photon energy storage materials based on new diacetylene derivatives with azobenzene moieties and with varied alkyl spacers and linkers. We developed a series of symmetric diacetylenes and polydiacetylenes and obtained high energy-density materials that can store up to 176.2 kJ mol−1 (or 200.2 kJ mol−1, if completely charged); more than double that of pristine azobenzene. The extra energy storage in the materials in addition to the isomerization enthalpy of azobenzene units is enabled by the different phase of materials in the ground state (crystalline solid) and in metastable state (amorphous solid/liquid). It is notable that the phase change characteristic of organic materials can be a parameter to consider in terms of designing high energy density photon energy storage materials.

Electromagnetic spectral properties and Debye screening of a strongly magnetized hot medium

Abstract: We evaluate the electromagnetic spectral function and its spectral properties by computing the one-loop photon polarization tensor involving quarks in the loop, particularly in a strong-field approximation compared to the thermal scale. When the magnetic scale is higher than the thermal scale the lowest Landau level (LLL) becomes an effectively ($1+1$)-dimensional strongly correlated system that provides a kinematical threshold based on the quark mass scale. Beyond this threshold the photon strikes the LLL and the spectral strength starts with a high value due to the dimensional reduction and then falls off with the increase of the photon energy due to LLL dynamics in a strong-field approximation. We obtain analytically the dilepton production rates from the LLL considering the lepton pair remains unaffected by the magnetic field when produced at the edge of a hot magnetized medium or it is affected by the magnetic field if produced inside a hot magnetized medium. For the latter case the production rate is of $\mathcal{O}[|eB{|}^{2}]$ along with an additional kinematical threshold due to the lepton mass. We also investigate the electromagnetic screening by computing the Debye screening mass and it depends distinctively on three different scales (mass of the quasiquark, temperature and the magnetic field strength) of a hot magnetized system. The mass dependence of the Debye screening supports the occurrence of a magnetic catalysis effect in the strong-field approximation.

Two-photon double ionization of neon using an intense attosecond pulse train

Abstract: We present a demonstration of two-photon double ionization of neon using an intense extreme ultraviolet (XUV) attosecond pulse train (APT) in a photon energy regime where both direct and sequential mechanisms are allowed. For an APT generated through high-order harmonic generation (HHG) in argon we achieve a total pulse energy close to 1μJ, a central energy of 35 eV, and a total bandwidth of ∼30 eV. The APT is focused by broadband optics in a neon gas target to an intensity of 3×1012Wcm−2. By tuning the photon energy across the threshold for the sequential process the double ionization signal can be turned on and off, indicating that the two-photon double ionization predominantly occurs through a sequential process. The demonstrated performance opens up possibilities for future XUV-XUV pump-probe experiments with attosecond temporal resolution in a photon energy range where it is possible to unravel the dynamics behind direct versus sequential double ionization and the associated electron correlation effects.

Photon Upconversion and Molecular Solar Energy Storage by Maximizing the Potential of Molecular Self-Assembly

Abstract: The self-assembly of functional molecules into ordered molecular assemblies and the fulfillment of potentials unique to their nanotomesoscopic structures have been one of the central challenges in chemistry. This Feature Article provides an overview of recent progress in the field of molecular self-assembly with the focus on the triplet–triplet annihilation-based photon upconversion (TTA-UC) and supramolecular storage of photon energy. On the basis of the integration of molecular self-assembly and photon energy harvesting, triplet energy migration-based TTA-UC has been achieved in varied molecular systems. Interestingly, some molecular self-assemblies dispersed in solution or organogels revealed oxygen barrier properties, which allowed TTA-UC even under aerated conditions. The elements of molecular self-assembly were also introduced to the field of molecular solar thermal fuel, where reversible photoliquefaction of ionic crystals to ionic liquids was found to double the molecular storage capacity with the...

Time-resolved ARPES with sub-15 fs temporal and near Fourier-limited spectral resolution.

Abstract: An experimental setup for time- and angle-resolved photoelectron spectroscopy with sub-15 fs temporal resolution is presented. A hollow-fiber compressor is used for the generation of 6.5 fs white light pump pulses, and a high-harmonic-generation source delivers 11 fs probe pulses at a photon energy of 22.1 eV. A value of 13 fs full width at half-maximum of the pump-probe cross correlation signal is determined by analyzing a photoemission intensity transient probing a near-infrared interband transition in 1T-TiSe2. Notably, the energy resolution of the setup conforms to typical values reported in conventional time-resolved photoemission studies using high harmonics, and an ultimate resolution of 170 meV is feasible.

Circular photon drag effect in bulk tellurium

Abstract: The circular photon drag effect is observed in a bulk semiconductor. The photocurrent caused by a transfer of both translational and angular momenta of light to charge carriers is detected in tellurium in the midinfrared frequency range. Dependencies of the photocurrent on the light polarization and on the incidence angle agree with the symmetry analysis of the circular photon drag effect. Microscopic models of the effect are developed for both intra- and intersubband optical absorption in the valence band of tellurium. The shift contribution to the circular photon drag current is calculated. An observed decrease of the circular photon drag current with the increase of the photon energy is explained by the theory for intersubband optical transitions. Theoretical estimates of the circular photon drag current agree with the experimental data.

Wavelength-tunable split-and-delay optical system for hard X-ray free-electron lasers.

Abstract: We developed a hard X-ray split-and-delay optical (SDO) system based on Bragg diffraction in crystal optics for generating two split pulses with a variable temporal separation. To achieve both high stability and operational flexibility, the SDO system was designed to include variable-delay and fixed-delay branches. As key optical elements, we fabricated high quality thin crystals and channel-cut crystals by applying the plasma chemical vaporization machining technique. The SDO system using Si(220) crystals covered a photon energy range of 6.5-11.5keV and a delay time range from a negative value to > 45 ps over the photon energy range (up to 220 ps at 6.5 keV). A simple alignment method for realizing a spatial overlap between the split pulses was developed. The SDO system was tested at a SPring-8 beamline in combination with a focusing system. We achieved an excellent overlap with an accuracy of 30 nm for ∼ 200 nm focused beams in both the horizontal and vertical directions. This achievement is an important progress towards the realization of time-resolved studies using multiple X-ray pulses with a time range from femtosecond to subnanosecond scales at X-ray free-electron laser facilities.

Wavelength Dependence of UV Photoemission from Solvated Electrons in Bulk Water, Methanol, and Ethanol

Abstract: We have measured the wavelength dependence (340-215 nm) of one-photon photoemission from the ground electronic state of solvated electrons in bulk water, methanol, and ethanol. In every case, the vertical electron binding energy (VBE) gradually increased with photon energy, indicating that the photoelectron kinetic energy diminishes as a result of electron-vibration inelastic scattering prior to emission from the liquid surface. In contrast, the VBE of the Rydberg electron in DABCO (1,4-diazabicyclo[2,2,2]octane), which has a surface-excess density, revealed no clear wavelength dependence. These results suggest that the solvated electrons are created predominantly in the bulk and that VBEs measured using UV photoemission spectroscopy of liquids generally require energy corrections to account for inelastic scattering effects. From the wavelength dependence, we have re-estimated the VBEs of solvated electrons in bulk water, methanol, and ethanol to be 3.3, 3.1, and 3.1 eV, respectively. Hydrated electrons were also identified by photoemission spectroscopy using 90 nm radiation.

Pump laser-induced space-charge effects in HHG-driven time- and angle-resolved photoelectron spectroscopy

Abstract: With the advent of ultrashort-pulsed extreme ultraviolet sources, such as free-electron lasers or high-harmonic-generation (HHG) sources, a new research field for photoelectron spectroscopy has opened up in terms of femtosecond time-resolved pump-probe experiments. The impact of the high peak brilliance of these novel sources on photoemission spectra, so-called vacuum space-charge effects caused by the Coulomb interaction among the photoemitted probe electrons, has been studied extensively. However, possible distortions of the energy and momentum distributions of the probe photoelectrons caused by the low photon energy pump pulse due to the nonlinear emission of electrons have not been studied in detail yet. Here, we systematically investigate these pump laser-induced space-charge effects in a HHG-based experiment for the test case of highly oriented pyrolytic graphite. Specifically, we determine how the key parameters of the pump pulse—the excitation density, wavelength, spot size, and emitted electron energy distribution—affect the measured time-dependent energy and momentum distributions of the probe photoelectrons. The results are well reproduced by a simple mean-field model, which could open a path for the correction of pump laser-induced space-charge effects and thus toward probing ultrafast electron dynamics in strongly excited materials.

Establishing nonlinearity thresholds with ultraintense X-ray pulses

Abstract: X-ray techniques have evolved over decades to become highly refined tools for a broad range of investigations. Importantly, these approaches rely on X-ray measurements that depend linearly on the number of incident X-ray photons. The advent of X-ray free electron lasers (XFELs) is opening the ability to reach extremely high photon numbers within ultrashort X-ray pulse durations and is leading to a paradigm shift in our ability to explore nonlinear X-ray signals. However, the enormous increase in X-ray peak power is a double-edged sword with new and exciting methods being developed but at the same time well-established techniques proving unreliable. Consequently, accurate knowledge about the threshold for nonlinear X-ray signals is essential. Herein we report an X-ray spectroscopic study that reveals important details on the thresholds for nonlinear X-ray interactions. By varying both the incident X-ray intensity and photon energy, we establish the regimes at which the simplest nonlinear process, two-photon X-ray absorption (TPA), can be observed. From these measurements we can extract the probability of this process as a function of photon energy and confirm both the nature and sub-femtosecond lifetime of the virtual intermediate electronic state.

High-harmonic generation in α -quartz by electron-hole recombination

Abstract: A calculation of the high-harmonic generation (HHG) in $\ensuremath{\alpha}$-quartz using the time-dependent density functional theory is reported. The interband process is attributed to the dominant in HHG above the band gap. The photon energy is set to 1.55 eV, and the cutoff energy of the plateau region is found to be limited at the 19th harmonic (30 eV). The dependence of the HHG efficiency at the cutoff energy region on laser intensity is consistent with that of the hole density in the lowest-lying valence band. Numerical results indicate that electron-hole recombination plays a crucial role in HHG in $\ensuremath{\alpha}$-quartz. It is found that a 200 attosecond pulse train is produced using HHG around the plateau cutoff energy.

Spatio-energetic cross talk in photon counting detectors: Detector model and correlated Poisson data generator.

Abstract: Purpose: An x-ray photon interacts with photon counting detectors (PCDs) and generates an electron charge cloud or multiple clouds. The clouds (thus, the photon energy) may be split between two adjacent PCD pixels when the interaction occurs near pixel boundaries, producing a count at both of the pixels. This is called double-counting with charge sharing. (A photoelectric effect with K-shell fluorescence x-ray emission would result in double-counting as well). As a result, PCD data are spatially and energetically correlated, although the output of individual PCD pixels is Poisson distributed. Major problems include the lack of a detector noise model for the spatio-energetic cross talk and lack of a computationally efficient simulation tool for generating correlated Poisson data. A Monte Carlo (MC) simulation can accurately simulate these phenomena and produce noisy data; however, it is not computationally efficient. Methods: In this study, the authors developed a new detector model and implemented it in an efficient software simulator that uses a Poisson random number generator to produce correlated noisy integer counts. The detector model takes the following effects into account: (1) detection efficiency; (2) incomplete charge collection and ballistic effect; (3) interaction with PCDs via photoelectric effect (with or without K-shell fluorescence x-ray emission, which may escape from the PCDs or be reabsorbed); and (4) electronic noise. The correlation was modeled by using these two simplifying assumptions: energy conservation and mutual exclusiveness. The mutual exclusiveness is that no more than two pixels measure energy from one photon. The effect of model parameters has been studied and results were compared with MC simulations. The agreement, with respect to the spectrum, was evaluated using the reduced χ2 statistics or a weighted sum of squared errors, χred2(≥1), where χred2=1 indicates a perfect fit. Results: The model produced spectra with flat field irradiation that qualitatively agree with previous studies. The spectra generated with different model and geometry parameters allowed for understanding the effect of the parameters on the spectrum and the correlation of data. The agreement between the model and MC data was very strong. The mean spectra with 90 keV and 140 kVp agreed exceptionally well: χred2 values were 1.049 with 90 keV data and 1.007 with 140 kVp data. The degrees of cross talk (in terms of the relative increase from single pixel irradiation to flat field irradiation) were 22% with 90 keV and 19% with 140 kVp for MC simulations, while they were 21% and 17%, respectively, for the model. The covariance was in strong agreement qualitatively, although it was overestimated. The noisy data generation was very efficient, taking less than a CPU minute as opposed to CPU hours for MC simulators. Conclusions: The authors have developed a novel, computationally efficient PCD model that takes into account double-counting and resulting spatio-energetic correlation between PCD pixels. The MC simulation validated the accuracy.

Phonon-assisted upconversion charging in Zn3Ga2GeO8:Cr(3+) near-infrared persistent phosphor.

Abstract: We report a new phonon-assisted upconversion excitation design that enables the excitation energy to be lower than the persistent luminescence emission energy in persistent phosphors. We demonstrate this upconversion excitation concept in Zn3Ga2GeO8:Cr(3+) near-infrared persistent phosphor by achieving Cr(3+) 700 nm persistent emission using 800 or 980 nm laser diode excitation. Depending on the sample temperature, the excitation photon energy can be tuned, and the persistent luminescence intensity can be adjusted. Depending on the excitation power, the upconversion trap filling process involves either one photon (for filling low-energy traps) or two photons (for filling high-energy traps). Our research provides a major step toward manipulating the electronic excitation in persistent luminescence, which has implication for many applications.

Self-modulated laser wakefield accelerators as x-ray sources

Abstract: The development of a directional, small-divergence, and short-duration picosecond x-ray probe beam with an energy greater than 50 keV is desirable for high energy density science experiments. We therefore explore through particle-in-cell (PIC) computer simulations the possibility of using x-rays radiated by betatron-like motion of electrons from a self-modulated laser wakefield accelerator as a possible candidate to meet this need. Two OSIRIS 2D PIC simulations with mobile ions are presented, one with a normalized vector potential a 0 = 1.5 and the other with an a 0 = 3. We find that in both cases direct laser acceleration (DLA) is an important additional acceleration mechanism in addition to the longitudinal electric field of the plasma wave. Together these mechanisms produce electrons with a continuous energy spectrum with a maximum energy of 300 MeV for a 0 = 3 case and 180 MeV in the a 0 = 1.5 case. Forward-directed x-ray radiation with a photon energy up to 100 keV was calculated for the a 0 = 3 case and up to 12 keV for the a 0 = 1.5 case. The x-ray spectrum can be fitted with a sum of two synchrotron spectra with critical photon energies of 13 and 45 keV for the a 0 of 3 and critical photon energies of 0.3 and 1.4 keV for a 0 of 1.5 in the plane of polarization of the laser. The full width at half maximum divergence angle of the x-rays was 62 × 1.9 mrad for a 0 = 3 and 77 × 3.8 mrad for a 0 = 1.5.

Role of Intrinsic Photogeneration in Single Layer and Bilayer Solar Cells with C60 and PCBM

Abstract: In an endeavor to examine how optical excitation of C60 and PCBM contribute to the photogeneration of charge carriers in organic solar cells, we investigated stationary photogeneration in single-layer C60 and PCBM films over a broad spectrum as a function of the electric field. We find that intrinsic photogeneration starts at a photon energy of about 2.25 eV, i.e., about 0.4 eV above the first singlet excited state. It originates from charge transfer type states that can autoionize before relaxing to the lower-energy singlet S1 state, in the spirit of Onsager’s 1938 theory. We analyze the internal quantum efficiency as a function of electric field and photon energy to determine (1) the Coulombic binding and separation of the electron–hole pairs, (2) the value of the electrical gap, and (3) which fraction of photoexcitations can fully separate at a given photon energy. The latter depends on the coupling between the photogenerated charge transfer states and the eventual charge transporting states. It is by ...

Spin-resolved photoelectron spectroscopy using femtosecond extreme ultraviolet light pulses from high-order harmonic generation

Abstract: The fundamental mechanism responsible for optically induced magnetization dynamics in ferromagnetic thin films has been under intense debate since almost two decades. Currently, numerous competing theoretical models are in strong need for a decisive experimental confirmation such as monitoring the triggered changes in the spin-dependent band structure on ultrashort time scales. Our approach explores the possibility of observing femtosecond band structure dynamics by giving access to extended parts of the Brillouin zone in a simultaneously time-, energy- and spin-resolved photoemission experiment. For this purpose, our setup uses a state-of-the-art, highly efficient spin detector and ultrashort, extreme ultraviolet light pulses created by laser-based high-order harmonic generation. In this paper, we present the setup and first spin-resolved spectra obtained with our experiment within an acquisition time short enough to allow pump-probe studies. Further, we characterize the influence of the excitation with femtosecond extreme ultraviolet pulses by comparing the results with data acquired using a continuous wave light source with similar photon energy. In addition, changes in the spectra induced by vacuum space-charge effects due to both the extreme ultraviolet probe- and near-infrared pump-pulses are studied by analyzing the resulting spectral distortions. The combination of energy resolution and electron count rate achieved in our setup confirms its suitability for spin-resolved studies of the band structure on ultrashort time scales.

Electronic structure and optic absorption of phosphorene under strain

Abstract: We studied the electronic structure and optic absorption of phosphorene (monolayer of black phosphorus) under strain. Strain was found to be a powerful tool for the band structure engineering. The in-plane strain in armchair or zigzag direction changes the effective mass components along both directions, while the vertical strain only has significant effect on the effective mass in the armchair direction. The band gap is narrowed by compressive in-plane strain and tensile vertical strain. Under certain strain configurations, the gap is closed and the energy band evolves to the semi-Dirac type: the dispersion is linear in the armchair direction and is gapless quadratic in the zigzag direction. The band-edge optic absorption is completely polarized along the armchair direction, and the polarization rate is reduced when the photon energy increases. Strain not only changes the absorption edge (the smallest photon energy for electron transition), but also the absorption polarization.

Dielectric Antireflection Fiber Arrays for Absorption Enhancement in Thin-Film Organic Tandem Solar Cells

Abstract: Effort to improve the bulk heterojunction organic solar cells’ absorption efficiency utilizes tandem structures combining low- and high-bandgap absorbing polymers so that a broader spectrum of sunlight's energy can be used for the photon-to-electron conversion and the thermalization loss of photon energy can be reduced. However, at the intersection of the upper energy level of the high-bandgap polymer and the lower energy level of the low-bandgap polymer, photon absorption efficiency remains low due to the destructive interference of these energy states at the band edges. In order to circumvent this issue, we theoretically propose using dielectric fiber arrays to generate lensing effects atop of a thin-film polymer/organic tandem solar cell (OTSC), aiming to enhance its optical absorption efficiency. Dielectric fibers have been calibrated to generate the optimal antireflection coating resulting in an integrated absorption enhancement up to 11% compared to its flat-OTSC counterpart. The overall integrated AM1.5 G (air mass 1.5 global simulated solar spectra) absorption yields absorption efficiencies of 83% in an optimal configuration, with a cell size of 600 nm and fiber radius of 150 nm. By using fibers as focusing lenses/antireflectors, we demonstrate that there exists zero reflection at multiple visible frequencies, leading to a relative broadband absorption of an ultrathin substrate.

First NuSTAR observations of the BL Lac -- type blazar PKS 2155-304: constraints on the jet content and distribution of radiating particles

Abstract: We report the first hard X-ray observations with NuSTAR of the BL Lac-type blazar PKS 2155-304, augmented with soft X-ray data from XMM-Newton and γ-ray data from the Fermi Large Area Telescope, obtained in 2013 April when the source was in a very low flux state. A joint NuSTAR and XMM spectrum, covering the energy range 0.5–60 keV, is best described by a model consisting of a log-parabola component with curvature β = 0.3_(-0.1)^(+0.2) and a (local) photon index 3.04 ± 0.15 at photon energy of 2 keV, and a hard power-law tail with photon index 2.2 ± 0.4. The hard X-ray tail can be smoothly joined to the quasi-simultaneous γ-ray spectrum by a synchrotron self-Compton component produced by an electron distribution with index p = 2.2. Assuming that the power-law electron distribution extends down to γ min = 1 and that there is one proton per electron, an unrealistically high total jet power of L_p ~ 10^(47) erg s^(−1) is inferred. This can be reduced by two orders of magnitude either by considering a significant presence of electron–positron pairs with lepton-to-proton ratio n_(e + e-)/n_p ~ 30, or by introducing an additional, low-energy break in the electron energy distribution at the electron Lorentz factor γ_(br1) ~ 100. In either case, the jet composition is expected to be strongly matter-dominated.