Nitrogen-doped carbon spheres: an X-ray absorption near-edge structure spectroscopy study

Carbon spheres (CSs) and low content nitrogen (2.5 and 3.5 at%) doped carbon spheres (NCSs) were synthesized by a pyrolysis process at 900 °C using $\mathrm{C}_{2}\mathrm{H}_{2}$ and CH₃CN. The electronic structure and chemical bonding of CSs and NCSs were studied using Raman spectroscopy, valence-band photoemission spectroscopy (VBPES) and X-ray absorption near-edge structure spectroscopy (XANES). XANES spectroscopy showed the effect of carbon and nitrogen bonds and graphitic as well as pyridinic structures on the NCS structure. For the spheres VBPES showed the effect of nitrogen on the spectral shape of the density of states and confirmed the role played by the $\pi$ bonds in controlling the electronic and structural properties of the NCSs. VBPES also showed that both $\pi$ and $\sigma$ bands shift towards higher binding energies after N-doping which is consistent with the XANES data. The consistency of the spherical geometry, chemical purity, suitable electronic/bonding structure and the availability of macroscopic quantities of the N–CSs makes these spheres promising new materials for different applications.     

Structural evolution of nanopores and cracks as fundamental constituents of ultrashort pulse-induced nanogratings

We present an extensive study of the underlying structure of femtosecond laser-induced nanogratings in fused silica. To explore the evolution of the three-dimensional structure of the nanopores and cracks, of which the nanogratings consist, we performed small angle X-ray scattering measurements as well as focused ion beam milling and scanning electron microscopy. Our results show that cracks with dimensions of (280  $\times $  25  $\times $  380) nm $^{3}$ and nanopores with typical diameters of (30  $\times $  25  $\times $  75) nm $^{3}$ are formed independent of various illumination parameters. With increasing number of laser pulses the smaller pores fuse to larger structures. Furthermore, the data suggest a cross-sectional change of the pores from cuboidal to ellipsoidal.     

Electro-Production of Light Lambda Hypernuclei

Through the study of light hypernuclei, we can learn about hyperon nucleon interaction. The hypernuclear spectroscopy with electron beams is one of most powerful methods to study detailed structure of light hypernuclei thanks to its high energy resolution. With a decade of efforts at Jefferson Lab, the spectroscopy of Λ hypernuclei with an electron beam is now established. Observation of ${_{\Lambda}^{7}}$ He which gave the last missing binding energy of the A = 7, T = 1 iso-triplet hypernuclei provides an important experimental input for the charge symmetry breaking (CSB) effect of the ΛN interaction. Further study about A = 4 hypernuclear iso-doublet, ${_{\Lambda}^{4}}$ H and ${_{\Lambda}^{4}}$ He, is necessary and such experiments are now planned.     

Laser assisted decay spectroscopy at the CRIS beam line at ISOLDE

The new collinear resonant ionization spectroscopy (Cris) experiment at Isolde, Cern uses laser radiation to stepwise excite and ionize an atomic beam for the purpose of ultra-sensitive detection of rare isotopes and hyperfine structure measurements. The technique also offers the ability to purify an ion beam that is contaminated with radioactive isobars, including the ground state of an isotope from its isomer. A new program using the Cris technique to select only nuclear isomeric states for decay spectroscopy commenced last year. The isomeric ion beam is selected using a resonance within its hyperfine structure and subsequently deflected to a decay spectroscopy station. This consists of a rotating wheel implantation system for alpha and beta decay spectroscopy, and up to three high purity germanium detectors for gamma-ray detection. This paper gives an introduction to the Cris technique, the current status of the laser assisted decay spectroscopy set-up and recent results from the experiment in November 2011.     

Extremely large bandwidth and ultralow-dispersion slow light in photonic crystal waveguides with magnetically controllability

A line-defect waveguide within a two-dimensional magnetic-fluid-based photonic crystal with 45^o-rotated square lattice is presented to have excellent slow light properties. The bandwidth centered at $ \lambda_{0} $  = 1,550 nm of our designed W1 waveguide is around 66 nm, which is very large than that of the conventional W1 waveguide as well as the corresponding optimized structures based on photonic crystal with triangular lattice. The obtained group velocity dispersion $ \beta_{2} $ within the bandwidth is ultralow and varies from ?1,191 $ a/(2\pi c^{2} ) $ to 855 $ a/(2\pi c^{2} ) $ (a and c are the period of the lattice and the light speed in vacuum, respectively). Simultaneously, the normalized delay-bandwidth product is relatively large and almost invariant with magnetic field strength. It is indicated that using magnetic fluid as one of the constitutive materials of the photonic crystal structures can enable the magnetically fine tunability of the slow light in online mode. The concept and results of this work may give a guideline for studying and realizing tunable slow light based on the external-stimulus-responsive materials.     

Discovering Echinococcus granulosus thioredoxin glutathione reductase inhibitors through site-specific dynamic combinatorial chemistry

In this study, we report a strategy using dynamic combinatorial chemistry for targeting the thioredoxin (Trx)-reductase catalytic site on Trx glutathione reductase (TGR), a pyridine nucleotide thiol-disulfide oxido-reductase. We chose Echinococcus granulosus TGR since it is a bottleneck enzyme of platyhelminth parasites and a validated pharmacological target. A dynamic combinatorial library (DCL) was constructed based on thiol-disulfide reversible exchange. We demonstrate the use of 5-thio-2-nitrobenzoic acid (TNB) as a non-covalent anchor fragment in a DCL templated by E. granulosus TGR. The heterodimer of TNB and bisthiazolidine (2af) was identified, upon library analysis by HPLC (IC $_{50}$  = 24  $\upmu $ M). Furthermore, 14 analogs were synthetically prepared and evaluated against TGR. This allowed the study of a structure–activity relationship and the identification of a disulfide TNB-tricyclic bisthiazolidine (2aj) as the best enzyme inhibitor in these series, with an IC $_{50}$  = 14  $\upmu $ M. Thus, our results validate the use of DCL for targeting thiol-disulfide oxido-reductases.     

Alteration of the Axial Met Ligand to Electron Acceptor A0 in Photosystem I: Effect on the Generation of P 700 ·+ A 1 ·− Radical Pairs as Studied by W-band Transient EPR

Photosystem I (PS I) mutants from the cyanobacterium Synechocystis sp. PCC 6803 bearing point mutations to the axial ligands of A_0A (M688N_PsaA) and A_0B (M668N_PsaB) were studied by high-field W-band electron paramagnetic resonance (EPR) spectroscopy. It was found that the EPR observables of PS I from the M668N_PsaB mutant were virtual identical to that of the wild type (WT), and are clearly distinct from the M688N_PsaA mutant. In particular, the P ₇₀₀ ^·+ decay kinetics in the M688N_PsaA mutant is significantly slower than in the WT or the M668N_PsaB mutant. The analysis of the out-of-phase electron–electron dipolar electron spin echo envelope modulation shows that in the M668N_PsaB mutant, the estimated distance of 26.0 ± 0.3 Å agrees well with the 25.8 Å distance for the P ₇₀₀ ^·+ A _1A ^·? radical pair measured in the X-ray crystal structure. In the M688N_PsaA mutant, two populations are found with estimated distances of 26.0 ± 0.3 and 25.0 ± 0.3 Å in a ratio of 0.7–0.3, which agree well with the 25.8 Å distance for the P ₇₀₀ ^·+ A _1A ^·? radical pair and the 24.6 Å distance for the P ₇₀₀ ^·+ A _1B ^·? radical pair measured in the X-ray crystal structure. The data confirm that under the experimental conditions employed in this work, which involve dark-adapted samples without the pre-reduction of the iron–sulfur clusters, electron transport in cyanobacterial PS I is asymmetrical at 100 K, with the majority of electron transfer taking place through the A-branch of cofactors.     

Growth and optical characterization of diamond-shaped zinc oxide nanostructures deposited by pulsed laser ablation

We report on the growth of zinc oxide diamond-shaped nanostructured thin films by pulsed laser deposition technique on silicon substrate at different substrate temperatures (room temperature to 600  $^{\circ }$ C) and at fixed background pressure of oxygen using Nd:YAG laser at a wavelength of 532 nm. The influence of substrate temperature on the grain size, surface morphology and optical properties is characterized by x-ray diffraction, field-emission scanning electron microscope, photoluminescence and Raman spectroscopy. x-ray diffraction results show that the grain size increases with increasing substrate temperature during the growth of ZnO thin films due to improved crystallinity but at 450  $^{\circ }$ C the crystallinity degrades. It is attributed to the formation of diamond-shaped ZnO nanostructures as supported by the field emission scanning electron microscope images. Consequently, increase in photoluminescence and Raman intensities is also attributed to the formation of diamond-shaped structures. The growth of diamond-shaped structure is discussed in the light of growth of various planes in the hexagonal structure of ZnO.     

A DFG-based cavity ring-down spectrometer for trace gas sensing in the mid-infrared

Continuing studies into an all-diode laser-based 3.3 μm difference frequency generation cavity ring-down spectroscopy system are presented. Light from a 1,560 nm diode laser, amplified by an erbium-doped fibre amplifier, was mixed with 1,064 nm diode laser radiation in a bulk periodically poled lithium niobate crystal to generate 16 μW of mid-IR light at 3,346 nm with a conversion efficiency of $0.05\,\%\,{\text{W}}^{-1}\,{\text{cm}}^{-1}$ . This radiation was coupled into a 77 cm long linear cavity with average mirror reflectivities of 0.9996, and a measured baseline ring-down time of $6.07\pm 0.03\,\upmu{\rm s}$ . The potential of such a spectrometer was illustrated by investigating the $P(3)$ transition in the fundamental $\nu_{3}(F_{2})$ band of ${\text{CH}}_4$ both in a 7.5 ppmv calibrated mixture of ${\text{CH}}_4$ in air and in breath samples from methane and non-methane producers under conditions where the minimum detectable absorption coefficient ( $\alpha_{\rm min}$ ) was $2.8 \times 10^{-8}\,{\rm cm}^{-1}$ over 6 s using a ring-down time acquisition rate of 20 Hz. Allan variance measurements indicated an optimum $\alpha_{\rm min}$ of $2.9\times 10^{-9}\,{\rm cm}^{-1}$ over 44 s.     

Piezoelectric properties of tetragonal single-domain Mn-doped NBT-6 %BT single crystals

We report a study of properties of Mn-doped NBT-6 %BT single crystals. We show that tetragonal single-domain states can be stabilized by poling along a [001] direction. For carefully prepared crystals, the piezoelectric coefficient \(d_{33}\) can reach 570 pC/N. When poled along non-polar directions, the crystals exhibit ferroelectric domain structures consistent with tetragonal micron-sized domains, as revealed by optical observation and Raman spectroscopy. The multidomain crystals have lower \(d_{33}\) values, 225 and 130 pC/N for [011] and [111]-oriented crystals, respectively. This trend is commented on from a domain-engineering perspective.     

Concurrent implementation of all-optical half-adder and AND & XOR logic gates based on nonlinear photonic crystal

A new design for concurrent implementation of all-optical half-adder and AND & XOR logic gates based on nonlinear photonic crystal ring resonator has been proposed. The finite different time domain and plane wave expansion methods are used to analyze the behavior of the structure. The ring resonator has a low switching time of about 0.85 ps and low switching power equal to $277\,\text{ mW}/\upmu \text{m}^{2}$ 277 mW / μ m 2 . The simulation results show that the contrast ratio is 12.78 dB for AND gate and 5.67 dB for XOR gate. Moreover, the operational wavelength of the input ports is $1.55\,\upmu \text{m}$ 1 . 55 μ m . Since the structure has a simple geometric shape with clear operating principle, it is potentially applicable for photonic integrated circuits.     

Effects of cerium dopant concentration on structural properties and photocatalytic activity of electrospun Ce-doped TiO2 nanofibers

Electrospun \(\hbox {TiO}_2\) and Ce-doped \(\hbox {TiO}_2\) nanofibers were prepared with 0.5, 2.0 and 8.0 % weight Ce. The structural properties and phase composition were characterized using high-resolution transmission electron microscopy (HR-TEM), X-ray diffraction and X-ray absorption near edge spectroscopy (XANES) at the Ti K-edge. The undoped nanofibers are composed of an assembly of \(\hbox {TiO}_2\) nanoparticles and their crystal structure is a mixture of anatase and rutile phases with an anatase:rutile volume ratio close to 3:1. As Ce is introduced, the nanoparticles decrease in size and the rutile phase volume decreases. Ce \(\hbox {L}_3\) -edge XANES probed the local structure of Ce dopants. At 0.5 % Ce, most Ce ions are incorporated in the \(\hbox {Ce}^{3+}\) charge state but, at 2 % Ce, the majority are \(\hbox {Ce}^{4+}\) . Visible light absorption indicated that \(\hbox {Ce}^{3+}\) act as shallow acceptors that only participate in absorption of wavelengths below 420 nm but \(\hbox {Ce}^{4+}\) impurity states are associated with absorption of wavelengths up to 550 nm. Photocatalytic performance of the nanofibers was assessed by measuring the degradation of adsorbed Rhodamine B in aqueous solution under visible and ultraviolet light. The 0.5 % Ce-doped \(\hbox {TiO}_2\) nanofiber showed the best visible-light photocatalytic activity, which is probably due to the majority presence of \(\hbox {Ce}^{3+}\) . At higher Ce concentration, the photocatalytic reaction rate was lower than undoped nanofibers, indicating that recombination at the \(\hbox {Ce}^{4+}\) sites is rate limiting.     

All-carbon detector with buried graphite pillars in CVD diamond

A diamond detector of 3D architecture without any metallization is developed for spectroscopy of ionizing radiation and single particles detection. The carbon electrode system was fabricated using a femtosecond infrared laser ( $\lambda $ = 1,030 nm) to induce graphitization on the surface and inside 4.0  $\times $  4.0  $\times $  0.4 mm $^{3}$ single-crystal chemical vapor deposition diamond slab, resulting in an array of 84 buried graphite pillars of 30  $\upmu $ m diameter formed orthogonally to the surface and connected by surface graphite strips. Sensitivity to ionizing radiation with $^{90}$ Sr $\upbeta $ -source has been measured for the 3D detector and high charge collection efficiency is demonstrated.     

Color tunable ZnO nanorods by Eu^{3+} and Tb^{3+} co-doping for optoelectronic applications

Eu \(^{3+}\) /Tb \(^{3+}\) co-doped ZnO nanorods were prepared by co-precipitation method and the effect of Eu–Tb co-doping was studied by X-ray diffraction (XRD), transmission electron microscopy (TEM), high resolution transmission electron microscopy, Fourier transform infrared spectroscopy (FTIR), UV-Vis-NIR diffuse reflectance (DR) and photoluminescence (PL) spectroscopy. The XRD pattern shows typical peak pattern for pure hexagonal wurtzite structure to match with the JCPDS data. The samples are found to be consisting of nanorods of diameter 20–30 nm as revealed by the TEM image. The FTIR pattern confirms the formation of the compounds. The DR study was carried to show the variation of absorption edge and the variation in band gap values, which showed the crystal size effect in the co-doped sample of different rare-earth ratios. The room temperature PL study shows bright emission spectra for the samples with different rare-earth ratios. It shows a very good energy transfer from Tb \(^{3+}\) to Eu \(^{3+}\) ions. The energy transfer mechanism and color tunability were discussed thoroughly.     

Detailed non-destructive evaluation of UHMWPE composites in the terahertz range

We report on the terahertz analysis of an internal structure of an ultra-high molecular weight polyethylene (UHMWPE) composite material, which is based on the HB10-tape from Dyneema $^{\circledR }$ . This type of composite is very hard and resistant and therefore it is often used to manufacture personal armors such as bulletproof vests and helmets. The multilayer structure of the UHMWPE composite was investigated by means of a raster scanning time domain spectroscopy technique in a reflection configuration. The mechanism of the formation of many shifted in time THz pulses (reflected from the internal layers of the sample) originates from the periodic modulation of the refractive index along the propagation of the radiation. This modulation is connected with alternate layers of fibers, each having different direction (perpendicular to each other). As a result we obtained the detailed three dimensional profile of the 3.3-mm thick sample with all 74 layers clearly visible. Thicknesses of all layers, having around 45 $\upmu $ m each, were determined. Moreover, it is also possible to identify internal defects i.e. delaminations in the internal structure of this composite material.     

Investigation of conductivity of adhesive layer including indium tin oxide particles for multi-junction solar cells

We report connection conductivity ( \(C_{\rm c}\) ) of adhesive which including \(\hbox {In}_2\hbox {O}_3\) – \(\hbox {SnO}_2\) (ITO) particles developed for fabrication of stacked-type-multi-junction solar cells. The commercial 20- \(\upmu \) m sized ITO particles were heated in vacuum at temperature ranging from 800 to 1,300  \(^{\circ }{\rm C}\) for 10 min to increase \(C_{\rm c}\) . 6.2 wt% ITO particles were dispersed in commercial Cemedine adhesive gel to form 100 samples structured with n-type Si/adhesive/n-type Si (n-Si sample) and p-type Si/adhesive/p-type Si (p-Si sample). Current density as a function of voltage (J–V) characteristics gave \(C_{\rm c}\) . It ranged from 4.3 to 1.0 S/cm \(^2\) for the n-Si sample with 800 \(^{\circ }{\rm C}\) heat-treated ITO particles. Its standard deviation was 0.59 S/cm \(^2\) . On the other hand, it ranged from 2.0 to 0.6 S/cm \(^2\) for the p-Si sample with 800  \(^{\circ }{\rm C}\) heat-treated ITO particles. Its standard deviation was 0.22 S/cm \(^2\) . The distribution of \(C_{\rm c}\) mainly resulted from contact efficiency of ITO particles to substrate. We theoretically estimated that present \(C_{\rm c}\) achieved a low loss of the power conversion efficiency ( \(E_{\rm ff}\) ) lower than 0.3 % in the application of fabrication of multi-junction solar cell with an intrinsic \(E_{\rm ff}\) of 30 % and an open circuit voltage above 1.9 V.     

On Gravitational Effects in the Schrödinger Equation

The Schrödinger  equation for a particle of rest mass $m$ and electrical charge $ne$ interacting with a four-vector potential $A_i$ can be derived as the non-relativistic limit of the Klein–Gordon  equation $\left( \Box '+m^2\right) \varPsi =0$ for the wave function $\varPsi $ , where $\Box '=\eta ^{jk}\partial '_j\partial '_k$ and $\partial '_j=\partial _j -\mathrm {i}n e A_j$ , or equivalently from the one-dimensional  action $S_1=-\int m ds +\int neA_i dx^i$ for the corresponding point particle in the semi-classical approximation $\varPsi \sim \exp {(\mathrm {i}S_1)}$ , both methods yielding the equation $\mathrm {i}\partial _0\varPsi \approx \left( \frac{1}{2m}\eta ^{\alpha \beta }\partial '_{\alpha }\partial '_{\beta } + m + n e\phi \right) \varPsi $ in Minkowski  space–time  , where $\alpha ,\beta =1,2,3$ and $\phi =-A_0$ . We show that these two methods generally yield equations  that differ in a curved background  space–time   $g_{ij}$ , although they coincide when $g_{0\alpha }=0$ if $m$ is replaced by the effective mass $\mathcal{M}\equiv \sqrt{m^2-\xi R}$ in both the Klein–Gordon  action $S$ and $S_1$ , allowing for non-minimal coupling to the gravitational  field, where $R$ is the Ricci scalar and $\xi $ is a constant. In this case $\mathrm {i}\partial _0\varPsi \approx \left( \frac{1}{2\mathcal{M}'} g^{\alpha \beta }\partial '_{\alpha }\partial '_{\beta } + \mathcal{M}\phi ^{(\mathrm g)} + n e\phi \right) \varPsi $ , where $\phi ^{(\mathrm g)} =\sqrt{g_{00}}$ and $\mathcal{M}'=\mathcal{M}/\phi ^{(\mathrm g)} $ , the correctness of the gravitational  contribution to the potential having been verified to linear order $m\phi ^{(\mathrm g)} $ in the thermal-neutron beam interferometry experiment due to Colella et al. Setting $n=2$ and regarding $\varPsi $ as the quasi-particle wave function, or order parameter, we obtain the generalization of the fundamental macroscopic Ginzburg-Landau equation of superconductivity to curved space–time. Conservation of probability and electrical current requires both electromagnetic gauge and space–time  coordinate conditions to be imposed, which exemplifies the gravito-electromagnetic analogy, particularly in the stationary case, when div ${{\varvec{A}}}=\hbox {div}{{\varvec{A}}}^{(\mathrm g)}=0$ , where ${{\varvec{A}}}^{\alpha }=-A^{\alpha }$ and ${{\varvec{A}}}^{(\mathrm g)\alpha }=-\phi ^{(\mathrm g)}g^{0\alpha }$ . The quantum-cosmological Schrödinger  (Wheeler–DeWitt) equation is also discussed in the $\mathcal{D}$ -dimensional  mini-superspace idealization, with particular regard to the vacuum potential $\mathcal V$ and the characteristics of the ground state, assuming a gravitational  Lagrangian   $L_\mathcal{D}$ which contains higher-derivative  terms up to order $\mathcal{R}^4$ . For the heterotic superstring theory  , $L_\mathcal{D}$ consists of an infinite series in $\alpha '\mathcal{R}$ , where $\alpha '$ is the Regge slope parameter, and in the perturbative approximation $\alpha '|\mathcal{R}| \ll 1$ , $\mathcal V$ is positive semi-definite for $\mathcal{D} \ge 4$ . The maximally symmetric ground state satisfying the field equations is Minkowski  space for $3\le {\mathcal {D}}\le 7$ and anti-de Sitter  space for $8 \le \mathcal {D} \le 10$ .     

Investigation of a nanostrip patch antenna in optical frequencies

This is the first report and investigation of a patch antenna in optical frequency range. Variety of plasmonic nanoantenna reported so far is good at enhancing the local field intensity of light by orders of magnitude. However, their far-field radiation efficiency is very poor. The proposed patch antenna emits a directional beam with high efficacy in addition to enhancing the intensity of near field. The nano-patch antenna (NPA) consists of a square patch of gold film of dimension 480 nm², placed on a substrate of dielectric constant \( \varepsilon_{\text{r}} \)  = 3.9 and thickness 150 nm with a ground plane of gold film of dimension 1,080 nm². The NPA resonates at 210 THz and has gain nearly 2 dB and radiation efficiency 45.18 %. The NPA might be useful in variety of applications such as optical communication, nano-photonics, biosensing, and spectroscopy.     

Fused fiber components for “eye-safe” spectral region around 2 \upmu m

Thulium-doped fiber lasers operating at wavelengths around 2  $\upmu $ m are rapidly developing a new class of coherent light sources with a high slope efficiency reaching 70 %. The 2- $\upmu $ m radiation sources have many advantages over the 1- $\upmu $ m sources, e.g., better eye-safety, relaxed non-linear limits and often more efficient material processing. Particularly important application of 2- $\upmu $ m fiber lasers is in a highly-efficient generation of wideband mid-infrared radiation through third order nonlinear effects in soft-glass fibers. In this paper we report on the development of passive components intended for fiber laser operation around 2  $\upmu $ m, namely fiber couplers and wavelength division multiplexers for combination of 1.6- and 2- $\upmu $ m radiation. Three commercially available fibers were used for the preparation of these components. The measured characteristics of the components are compared and the limitations are discussed, particularly the two-mode operation and high bend loss. Specific fiber designs are proposed in order to optimize the performance of the wavelength division multiplexer.     

Cosmic Censorship of Smooth Structures

It is observed that on many 4-manifolds there is a unique smooth structure underlying a globally hyperbolic Lorentz metric. For instance, every contractible smooth 4-manifold admitting a globally hyperbolic Lorentz metric is diffeomorphic to the standard ${\mathbb{R}^4}$ . Similarly, a smooth 4-manifold homeomorphic to the product of a closed oriented 3-manifold N and ${\mathbb{R}}$ and admitting a globally hyperbolic Lorentz metric is in fact diffeomorphic to ${N\times \mathbb{R}}$ . Thus one may speak of a censorship imposed by the global hyperbolicty assumption on the possible smooth structures on (3 + 1)-dimensional spacetimes.