Transition Band Research Articles

Wideband second‐order bandpass frequency selective surface with high selectivity

AbstractA wideband second‐order bandpass frequency selective surface (FSS), which is composed of three metallic arrays and two dielectric substrate layers, is proposed in this paper. The proposed design has high selectivity and wide passband properties simultaneously and is insensitive to incident angles and different polarizations. The equivalent circuit method and field distribution are analyzed to explain the mechanism of the proposed design. A prototype is fabricated and measured, demonstrating a well wideband second‐order passband ranging from 4.8 to 8.8 GHz with 58.8% bandwidth (−3 dB), and the insertion loss (IL) in‐band is less than 0.4 dB. Meanwhile, the transitional bands at both sides of the passband are 0.64 GHz (9.4%) and 0.6 GHz (8.8%), respectively. Moreover, the frequency responses of the proposed structure are stable up to 60° for both TE and TM polarizations.

Mechanisms of photoluminescence in cerium oxide pellets with different types of defects

We systematically investigated the photoluminescence (PL) spectra of sintered pellets of cerium oxide, CeO2. The crystal defects were controlled by commonly used techniques in the field of solid-state ionics. We observed the transitions between the O-2p and Ce-4f energy levels in intrinsic CeO2. By doping with trivalent lanthanide ions, which results in oxygen vacancies by charge compensation, a new transition band appears. It involves electronic levels that are similar to those of Ce2O3 clusters, rather than to those of oxygen vacancies. In reduced cerium oxide CeO2-δ, where the number of Ce3+ ions is larger, electrons partially fill the energy levels of defects similar to Ce2O3 clusters, and thus an optical response derived from the Ce-4f–Ce-5d transition appears.

Unraveling structural relaxation induced ductile-to-brittle transition from perspective of shear band nucleation kinetics in metallic glass

Structural relaxation induced ductile to brittle transition seriously hampers the practical applications in metallic glasses. Although there have been many researches focusing on the hidden physical mechanism of ductile to brittle transition, the correlation between ductile to brittle transition and shear band nucleation kinetics is still unclear. In this work, by applying for one new studying strategy based on the nanoindentation, the evolution of the thermodynamic property, the macroscopic mechanical behavior and the corresponding shear band nucleation kinetics including the nucleation site type, the nucleation site density and the nucleation rate with annealing time at one Sub-glass transition temperature, were systematically investigated. We found that there appears one critical annealing state corresponding to the ductile to brittle transition and then the ductile-to-brittle transition induced by structural relaxation is the comprehensive result of the transition of the shear band nucleation mode, the nucleation site density and the nucleation rate. These results provide new enlightenment for the structural relaxation effect on the shear band nucleation and the macroscopic deformation behaviors, and for developing new metallic glasses with improved mechanical properties.

A novel miniaturized sext‐band bandstop filter based on tree‐shaped branches loaded asymmetric multimode resonators

AbstractThe working mechanism of suggested tunable bandstop filtered with asymmetrical multiple‐mode resonator equipped with tree‐shape branches is described in this letter. Because of the established degree of freedom, the suggested bandstop filters’ (BSFs) centre frequencies may be modified based on the design approach. Meanwhile, the suggested BSFs have small dimensions and abrupt roll‐off skirts for transition bands. A technology demonstration filter with six stopbands placed at 1.91/2.38/3.54/5.19/5.79/6.85 GHz was constructed to verify the design and analyses. It is found that the measurement of a filter is consistent with the comparison of simulation data, which indicates that the proposed structure is an optional approach for BSF design. The filter proposed can be applied in the field of communication, information and coal mine automation.

Polarization Modulation on Charge Transfer and Band Structures of GaN/MoS2 Polar Heterojunctions

As important third-generation semiconductors, wurtzite III nitrides have strong spontaneous and piezoelectric polarization effects. They can be used to construct multifunctional polar heterojunctions or quantum structures with other emerging two-dimensional (2D) semiconductors. Here, we investigate the polarization effect of GaN on the interfacial charge transfer and electronic properties of GaN/MoS2 polar heterojunctions by first-principles calculations. From the binding energy, the N-polarity GaN/MoS2 heterojunctions show stronger structural stability than the Ga-polarity counterparts. Both the Ga-polarity and N-polarity GaN/MoS2 polar heterojunctions have type-II energy band alignments, but with opposite directions of both the built-in electric field and interfacial charge transfer. In addition, their heterostructure types can be effectively modulated by applying in-plane biaxial strains on GaN/MoS2 polar heterojunctions, which can undergo energy band transitions from type II to type I. As a result, it provides a feasible solution for the structural design and integrated applications of hybrid 3D/2D polar heterojunctions in advanced electronics and optoelectronics.

Effects of High Pressure on the Bandgap and the d–d Crystal Field Transitions in Wolframite NiWO4

The pressure effects on the optical and structural properties of NiWO4 have been studied experimentally and theoretically. The fundamental bandgap decreases with a pressure coefficient of −12.0 ± 0.2 meV/GPa. Meanwhile, the Ni2+ d–d transition energies increase at a rate of 7.4–14.8 meV/GPa. Therefore, the energy differences between the fundamental band and the Ni2+ d–d transition bands gradually decrease under pressure, which is beneficial to improve its optical performance. These optical phenomena are associated with structural variations. The shrinkage of the WO6 octahedron enhances the hybridization between the W 5d and O 2p orbitals, resulting in bandgap reduction. The pressure-induced enhancement of the NiO6 octahedral symmetry increases the crystal field splitting, thereby yielding increases in the Ni2+ d–d intraband transition energies. Besides, a pressure-induced structural phase transition is also observed around 20.0 GPa by both angle-dispersive synchrotron X-ray diffraction (ADXRD) and Raman experiments. This study provides valuable insight into the electron–lattice coupling of NiWO4 under compression and an effective way to modulate the electronic structure and optical properties of isomorphic wolframite materials.

Cryogenic Ion Spectroscopy of Protonated and Sodiated Methyladenine Derivatives.

Ultraviolet photodissociation (UVPD) spectra of protonated 9-methyladenine (H+9MA), protonated 7-methyl adenine (H+7MA), protonated 3-methyladenine (H+3MA), and sodiated 7-methyladenine (Na+7MA) near the origin bands of the S0-S1 transition were obtained using cryogenic ion spectroscopy. The UV-UV hole burning, infrared (IR) ion-dip, and IR-UV double resonance spectra showed that all the ions were present as single isomers in a cryogenic ion trap. The UVPD spectrum of H+9MA exhibited only a broad absorption band, whereas the spectra of H+7MA, H+3MA, and Na+7MA displayed moderately or well-resolved vibronic bands. Potential energy profiles were computed to understand the reason for the different bandwidths of the vibronic bands in the spectra. The broadening of the bands was correlated with the slopes between the Franck-Condon point and the conical intersection between the S1 and S0 states in the potential energy profiles, thus reflecting the deactivation rates in the S1 state.

Spectroscopic study of the F[formula omitted] outer well state in H2, HD and D2

Two-photon UV-photolysis of hydrogen sulfide molecules is applied to produce hydrogen molecules in highly excited vibrational levels in the X1Σg+ electronic ground state, up to the dissociation energy and into the quasibound region. Photolysis precursors H2S, HDS and D2S are used to produce vibrationally hot H2, HD and D2. The wave function density at large internuclear separation is excited via two-photon transitions in the F1Σg+ - X1Σg+ system to probe ro-vibrational levels in the first F1Σg+ outer well state of gerade symmetry. Combining with accurate knowledge of the X1Σg+ (v,J) levels from advanced ab initio calculations, energies of rovibrational levels in the F1Σg+ state are determined. For the H2 isotopologue a three-laser scheme is employed yielding level energies at accuracies of 4×10−3 cm−1 for F(v=0,J) up to J=21 and for some low J values of F(v=1). A two-laser scheme was applied to determine level energies in H2 for F(v=0−4) levels as well as for various F levels in HD and D2, also up to large rotational quantum numbers. The latter measurements in the two-laser scheme are performed at lower resolution and the accuracy is strongly limited to 0.5 cm−1 by ac-Stark effects. For H2 a new quasibound resonance X1Σg+ (v=6, J=23) is detected through the Q(23) and O(23) transitions in the F0-X6 band. Also a quasi-bound resonance in D2 is assigned, for the first time in this species: X1Σg+ (v=17 , J=15). The experimental results on F(v,J) level energies are compared with previously reported theoretical results from multi-channel quantum-defect calculations as well as with results from newly performed non-adiabatic quantum calculations.

Synthesis, crystal structure and luminescent properties of Eu3+-activated halotungstates applied in W-LEDs

The self-activated and Eu3+ ions activated halotungstates Ca3WO5Cl2 phosphors have been prepared by the facile solid-state reaction. The crystal structure, photoluminescence excitation (PLE) and emission (PL) spectra, temperature-dependent PLE, and decay curves were investigated. When excited by ultraviolet (UV) light, the pure Ca3WO5Cl2 shows blue emission, which can be ascribed to the typical self-activated emission of charge transfer band (CTB) transitions in [WO5Cl]5- groups. As the temperature increases from 10 to 480 K, the PL intensity decreases, and the emission band appears blue shift. The concentration dependent PL spectra of Ca3WO5Cl2:Eu3+ show several characteristic emission peaks of Eu3+ ions in the red emission region around 615 nm together with the host emission band in the blue emission region at around 450 nm. Accordingly, the broadband originates from the CT behavior in [WO5Cl]5- group, while the sharp lines originate from the 5D0-7F2 transitions in Eu3+ ions. The energy transfer process between Eu3+ and WO5Cl was discussed. All results indicate the possibility for UV to blue LED applications with multi-color emissions in a single stable host.

Physical origins of optical anisotropy in quantum-confined semiconductors: The roles of valence band mixing, transition broadening, and state filling

We investigate experimentally and theoretically the impact of valence band mixing and spectrum of confined states on the polarization of light emitted from or absorbed by GaAs/AlGaAs semiconductor quantum dots and quantum wires with tailored heterostructure potential. In particular, such nanostructures with parabolic-profile confinement potentials, realized by organometallic vapor phase epitaxy inside pyramidal pits, served as model systems for the study. Different degrees of linear polarization (DOLP) of emitted light, depending on the confinement potential profile, the specific excitonic transition, and the level of excitation, are observed. A theoretical model shows that, besides the impact of valence band mixing, the overlap of conduction and valence band wavefunctions as well as state occupation probability and broadening of transitions determine the DOLP. The conclusions are useful for the design of quantum light emitters with controlled polarization properties.

Understanding photoluminescence of bismuth-doped ternary alkaline earth d10 metal oxides via first-principles calculations

Bismuth-activated light emissions have been much reported in ternary alkaline earth ${d}^{10}$ metal oxides, and the self-activation nature of these hosts makes it possible to achieve efficient luminescence via energy transfer between the host and the activator ions. A detailed analysis of the potential intrinsic defects accounting for the intrinsic emissions is still absent, and the identifications of bismuth-related transitions remain unclear or controversial. Given these facts, first-principles calculations using hybrid functionals are employed to systematically study the bismuth-doped $(\mathrm{Sr}/\mathrm{Ca}){\mathrm{Sb}}_{2}{\mathrm{O}}_{6}$, ${(\mathrm{Sr}/\mathrm{Ca})}_{2}{\mathrm{Sb}}_{2}{\mathrm{O}}_{7}$, $(\mathrm{Sr}/\mathrm{Ca}){\mathrm{SnO}}_{3}$, and $(\mathrm{Sr}/\mathrm{Ca}){\mathrm{Ga}}_{2}{\mathrm{O}}_{4}$, which contain ${d}^{10}$ metals belonging to VA, IVA, and IIIA groups, respectively. Primary intrinsic defects and self-trapped holes in these hosts are revealed and the roles played by them on intrinsic emissions are evaluated. Bismuth ions are identified as trivalent dopants and their $^{3}P_{0,1}\ensuremath{\rightarrow}\phantom{\rule{4pt}{0ex}}^{1}S_{0}$ (referred to as $A$-band) transition is assigned to account for the ultraviolet emission observed in bismuth-doped $(\mathrm{Sr}/\mathrm{Ca}){\mathrm{Sb}}_{2}{\mathrm{O}}_{6}$, while the visible emission observed in $(\mathrm{Sr}/\mathrm{Ca}){\mathrm{Sb}}_{2}{\mathrm{O}}_{6}$:Bi is attributed to ${\mathrm{Bi}}^{3+}$ dopants in the minute amount of ${(\mathrm{Sr}/\mathrm{Ca})}_{2}{\mathrm{Sb}}_{2}{\mathrm{O}}_{7}$ impurities. In ${\mathrm{Sr}}_{2}{\mathrm{Sb}}_{2}{\mathrm{O}}_{7}$ and ${\mathrm{Ca}}_{2}{\mathrm{Sb}}_{2}{\mathrm{O}}_{7}$ with bismuth, the excitation bands are identified as $^{1}S_{0}\ensuremath{\rightarrow}^{3}P_{0,1}$ transition of ${\mathrm{Bi}}^{3+}$, and the observed emission bands are attributed to charge transfer transition (${\mathrm{Bi}}^{2+}+{h}_{\mathrm{VBM}}^{+}\ensuremath{\rightarrow}{\mathrm{Bi}}^{3+}$) and $A$-band transition ($^{3}P_{0,1}\ensuremath{\rightarrow}\phantom{\rule{4pt}{0ex}}^{1}S_{0}$), respectively. Furthermore, the bismuth-related transitions are also well identified in $(\mathrm{Sr}/\mathrm{Ca}){\mathrm{SnO}}_{3}$ and $(\mathrm{Sr}/\mathrm{Ca}){\mathrm{Ga}}_{2}{\mathrm{O}}_{4}$ and the large Stokes shifts of the $A$-band transition in $(\mathrm{Sr}/\mathrm{Ca}){\mathrm{Ga}}_{2}{\mathrm{O}}_{4}$ are revealed and analyzed. The insights gained in this work extend our understanding of the properties of intrinsic defects and the luminescence mechanisms of insulating bismuth-doped ternary oxides.

Ultra-wideband hybrid polarization conversion-absorption metasurface with a transmission window and narrow transition bands

This paper proposes an ultra-wideband hybrid polarization conversion-absorption (PCA) metasurface with a transmission band for radar cross section (RCS) reduction application. To achieve the in-band transmission of the incident wave and the out-of-band suppression of the scattering wave, the proposed metasurface is composed of a low-frequency PCA array, a high-frequency polarization conversion (PC) array, and a second-order frequency selective surface (FSS). The lumped resistors are used in the PCA array to generate the absorption characteristic, thus giving rise to the reduction of the co-polarization component of the scattering field and meanwhile the elimination of the high-frequency resonances. An interdigital structure is introduced into the PCA array to not only achieve a good transmission for a co-polarization component of the incoming wave but also result in an extra PC band for narrowing the transition band between the transmission window and the high-frequency PCA band. The orthogonal layout of the PCA array and the PC array is used to decrease the interaction between them, thus resulting in an ultra-wideband PCA band. The second-order FSS consisting of a stacked structure is designed to generate a good passband with the steep roll-off and the good out-of-band suppression. The proposed metasurface has an ultra-wideband PCA band, a good transmission band, and narrow transition bands. When the proposed metasurface is arranged into an array in a rotation way, the ultra-wideband RCS reduction performance can be obtained. Simulated and measured results are given to demonstrate a 10 dB RCS reduction band of 3.4–24.6 GHz with a −1 dB transmission band of 8.45–10 GHz.

Micelle Sensitized Physicochemical Studies on Erbium(III) Complexes with Hypoglycemic Sulfonylurea Drugs

Hypoglycemic sulfonylurea derivatives ligands viz. glimepiride, glibenclamide, gliclazide and glipizide show interaction with lanthanide metal Er (III) in non-micellar and micellar medium. Anionic, cationic and non-ionic surfactant micellar media show great variation in oscillator strength of Er (III) with hypoglycemic sulfonylurea drugs as reflected in UVVisible absorption spectra. Interpretation of UVVisible absorption spectra produced that 4G11/2 ← 4I15/2 and 2H11/2 ← 4I15/2 electronic transition bands show different behavior on account of oscillator strength (P) compared to other electronic f-f transitions, depicted as hypersensitive peaks which are characterized as ratio of Pmicellar / PDMF. Exploitation of different parameters viz: oscillator strength (P), Judd-Ofelt parameter Tλ, nephelauxetic effect 𝛽 and bonding parameter b1/2 from absorption spectra supports the covalency characters in metal ligand Er (III)-drugs doped systems to a greater extent. Thermodynamic exploits of hypersensitive transition correlate the spectroscopic data via Work function A, Thermodynamic efficiency of transition TET and Partition function Q. Thermodynamic parameters notify the microscopic behaviour of Er (III)- sulfonylurea drugs.

Flat band and Lifshitz transition in long-range-ordered supergraphene obtained by Erbium intercalation

We report the observation of graphene doped up to the Lifshitz transition obtained solely by the intercalation of Erbium atoms. ARPES measurements show that a wide flat band is generated around the $M$ point of the Brillouin zone. We propose that this is the effect of an induced spin-orbit coupling. Scanning Tunneling Microscopy (STM) experiments reveal a longrange ordered hexagonal superstructure: we argue that this reflects the ordering of the intercalated Er atoms acting as impurities. This system provides a playground to study the interaction between a magnetic order and a divergent density of states at the Fermi level.

Thorium‐Doped Lithium Fluoride Single Crystal: A Potential Promising Candidate for Solid State Nuclear Optical Clock Materials

AbstractThe solid‐state nuclear optical clock based on 229Th‐doped crystal has great advantages in frequency stability, excitation and detection of nuclear transition, miniaturization, commercialization, and spaceborne ability of nuclear optical clock. Here, the potential of LiF crystal as a host for Th ions is predicted by theoretical calculation, and on this basis, the Th:LiF crystal is successfully grown experimentally for the first time. The theoretical results demonstrate that although the challenges of charge nonconservation and ion radius mismatch between Li+ and Th4+ ions exist, effective doping of thorium can still be achieved via energetically favorable charge compensation mechanisms in an F‐rich environment, and the transmittance of Th:LiF crystal in nuclear transition band will only have a slight decrease, smaller than that of pure LiF crystal. The experimental results prove that the doping concentration of Th:LiF crystal can reach about 8.3 × 1018 cm−3 and the transmittances of 1 mm thick Th:LiF and pure LiF crystal samples grown in similar conditions are ≈68% and 72% at 152.7 nm, respectively. The Th:LiF crystal with high doping concentration, high transmittance, and low background luminescence is expected to be a promising candidate for solid‐state nuclear optical clock materials.

Synthesis and characterization of luminescent Cu2+-doped fluorapatite nanocrystals as potential broad-spectrum antimicrobial agents.

Nanomaterials based on metal-doped fluorapatite (FAP) have attracted considerable interest as potential next-generation antimicrobial agents. In this study, Cu2+-doped FAP nanocrystals have been successfully synthesized by a neutralization method at room temperature. Their structural, optical, antimicrobial, and hemcompatible properties have been investigated. XRD, FTIR, FESEM, and N2 adsorption-desorption studies indicate the formation of single-phase FAP mesoporous nanopowders, composed of rod-like particles. TEM images confirmed the formation of nanorodes with a length of 60nm and a width of about 18nm. Rietveld analysis shows that the Cu2+ ions preferentially substitute Ca2 (6h) sites in the hexagonal fluorapatite crystal structure. Fluorescence spectroscopy accompanied by MCR-ALS method confirms substitution of Cu2+ ions in FAP crystal lattice with extracting additional d-d band transition at green color from FAP broadband self-activated luminescence in violet-blue color. Antimicrobial studies conducted on Staphylococcus aureus, Escherichia coli and Micrococcus lysodeikticus showed that FAP nanopowder with the highest Cu2+ content have strong bacteriostatic action on Staphylococcus aureus bacterial strain in mediums containing nutrition matters. In addition, this sample in comparison to pure FAP achieved a high percentage of relative reduction of bacterial population for all three species, being >90% in most cases. Fungistatic action is noticed too, throwgh the slowing down mycelium growth of fungus Aspergillus niger, Aspergillus flavus and Penicillium roqueforti and reduction of sporulation of Aspergillus niger species. Cu2+-doped FAP nanocrystals shows a synergistic antimicrobial effect with Cu2+ and F- ions. Concerning the potential biomedical applications, the hemolysis ratios of the Cu2+-doped FAP samples were below 5%. The obtained results pointed out the possible use of the synthesized nanocrystals as broad-spectrum antimicrobial agents for various biomedical and health care preparations.

Dirac Bands in the Topological Insulator Bi2Se3 Mapped by Time‐Resolved Momentum Microscopy

AbstractThe energy dispersion of the unoccupied Dirac bands of the topological insulator Bi2Se3 has been studied up to large parallel momenta and intermediate state energies using a setup for laser‐based time‐resolved momentum microscopy with 6 eV probe‐photons. A strongly momentum‐dependent evolution of the topologically protected Dirac states into a conduction band resonance is observed, highlighting the anisotropy dictated by the symmetry of the surface. The results are in remarkable agreement with the theoretical surface spectrum obtained from a GW‐corrected tight‐binding model, suggesting the validity of the approach in the prediction of the quasiparticle excitation spectrum of large systems with non‐trivial topology. After photoexcitation with 0.97 eV photons, assigned to a bulk valence band‐conduction band transition, the out‐of‐equilibrium population of the surface state evolves on a multi‐picosecond time scale, in agreement with a simple thermodynamical model with a fixed number of particles, suggesting a significant decoupling between bulk and surface states.

Realization of exceptional points along a synthetic orbital angular momentum dimension.

Exceptional points (EPs), at which more than one eigenvalue and eigenvector coalesce, are unique spectral features of non-Hermiticity (NH) systems. They exist widely in open systems with complex energy spectra. We experimentally demonstrate the appearance of paired EPs in a periodical-driven degenerate optical cavity along the synthetic orbital angular momentum dimension with a tunable parameter. The complex-energy band structures and the key features of EPs, i.e., their bulk Fermi arcs, parity-time symmetry breaking transition, energy swapping, and half-integer band windings, are directly observed by detecting the wavefront angle-resolved transmission spectrum. Our results demonstrate the flexibility of using the photonic synthetic dimensions to implement NH systems beyond their geometric dimension and EP-based sensing.

Pivotal role of oxygen during the synthesis of Cu(OH)2/TiO2 and its effect on photocatalytic hydrogen production activity

Herein, we have demonstrated that the structural bonding of TiO2/Cu(OH)2 significantly changes in the presence of dissolved oxygen in water, and the photocatalytic activity directly depends on the interaction of surface species in TiO2. The Cu(OH)2/TiO2 nanostructured photocatalysts were prepared in the presence and absence of molecular oxygen. A remarkable finding of the work is that the sample prepared in the presence of oxygen produced 1169 µmol/g/h higher hydrogen than the sample prepared with nitrogen. The Cu(OH)2/TiO2 photocatalysts generated 27592 μmol/g and 22916 μmol/g of hydrogen in 4 h of reaction time under solar light irradiation for samples prepared in the presence and absence of molecular oxygen, respectively. Compared to the bare TiO2, a 60 folds enhancement in hydrogen production was observed for Cu(OH)2/TiO2, and it has been attributed to the formation of interface bonding between Cu(OH)2 and TiO2, the semi-amorphous nature of Cu(OH)2, the existence of terminal hydroxyl species on the TiO2 surface, and extended visible light absorption due to the d-d transition bands of Cu. This cost-effective and room temperature synthesis strategy provides new insight into manipulating the surface chemistry of TiO2-based photocatalysts for improved visible light absorption and charge transport dynamics.

Characterization of CuO doped CdO nanomaterials synthesized by sol gel spin coating and hydrothermal method

CuO-CdO nanomaterials were synthesized by sol gel spin coating and hydrothermal methods. Undoped CdO and 0.2%, 0.5%, 1%, and 2% CuO doped CdO were produced as thin films and nanopowders. The structural, optical, and electrical properties of the thin films and nanopowders produced were investigated. It was determined by X-ray diffraction (XRD) analysis that thin films and nanopowders were polycrystalline and face-centered cubic structures. From Atomic Force Microscope (AFM) images, it was observed that the structure of the thin films was homogeneous and dense rod-shaped structures, and the nanopowders were urchin-like structures consisting of spherical nanoparticles from the Field Emission Scanning Electron Microscopy (FESEM) images. It was determined that nanopowder and thin films have direct band transition from Ultraviolet–visible spectroscopy (UV–Vis) analysis. From all the results obtained, it was concluded that structural, optical, and electrical properties of thin films and nanopowders vary depending on the doping rate and method differences.