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Vectorial perturbation theory for axisymmetric whispering gallery resonators
摘要： We propose a vectorial perturbation theory for axially symmetric, generally nonspherical whispering gallery resonators made of isotropic and anisotropic optical materials. It is based on analysis of the leading terms in the coupled equations for independent light-field components, as derived from Maxwell's equations, and true boundary conditions. Strong localization of the whispering gallery modes (WGMs) near the resonator rim, controlled by the azimuth modal number m, is the main prerequisite for our analysis. The theory gives high-precision expressions for the WGM frequencies and modal functions, including the evanescent effects. One of important applications of the theory is analysis of anticrossings of the WGM resonances in anisotropic resonators detected in experiments. Simple relations for the frequency avoidance gaps during the anticrossings are derived and compared with experimental data obtained in lithium-niobate-based WGM resonators. We show also that the vectorial effects substantially restrict the field of applicability of the scalar WGM models.
关键词： frequency avoidance gaps,whispering gallery resonators,anisotropic materials,vectorial perturbation theory,anticrossings
更新于2025-09-23 15:23:52
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Construction of Long Narrow Gaps in Ag Nanoplates
摘要： Hexagonal Ag nanoplates with long and ultra-narrow gaps (about 90 nm in length, 2 nm in width) are synthesized via seed-mediated growth method. By growing around the polymer shell on the seed, the Ag domain cannot merge at the meet-up point, leaving a long narrow gap in the resulting plate. These gapped nanoplates exhibit high sensitivity in SERS detection, with limitation of 10-9 M for 2-naphthalenethiol.
关键词： SERS,ultra-narrow gaps,Ag nanoplates,seed-mediated growth
更新于2025-09-23 15:21:01
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Defects modes in one-dimensional photonic filter star waveguide structure
摘要： In this work, by the formalism of the interface response theory, we study the existence of defects’ modes in the transmission spectrum of a one-dimensional photonic star waveguide (SWG) structure. The perfect structure presents large forbidden bands (gaps) that come from the periodicity of the system and the resonance states of the grafted lateral branches which play the role of the resonators. The defects modes result from the presence of the defects of the lateral branches level located in two different sites. The behavior of the defects’ modes is analyzed as a function of the defects’ lengths, the number of defectives resonators, the positions of the defects’ and the number of sites. This system can be considered as a selective ?lter with high performance.
关键词： Electromagnetic ?lter,Green Function,Star waveguide structure,Defects modes,Gaps
更新于2025-09-23 15:21:01
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Electroic and optical properties of germanene/MoS2 heterobilayers: first principles study
摘要： First principles calculations have been performed to investigate the structural, electronic, and optical properties of germanene/MoS2 heterostructures. The results show that a weak van der Waals coupling between germanene and MoS2 layers can lead to a considerable band-gap opening (53 meV) as well as the preserved Dirac cone with a linear band dispersion of germanene. The applied external electric filed can not only enhance the interaction strength between two layers, but also linearly control the charge transfer between germanene and MoS2 layers, and consequently lead to a tunable band gap. Furthermore, the reduction in the optical absorption intensity of the heterostructures with respect to the separated monolayers has been predicted. These findings suggest that the Ge/MoS2 hybrid can be designed as the device where both finite band gap and high carrier mobility are required.
关键词： Heterobilayers,Optical properties,Band gaps,Electric field,Germanene/MoS2
更新于2025-09-23 15:21:01
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Structural Evolutions of Vertically-Aligned Two-Dimensional MoS <sub/>2</sub> Layers Revealed by <i>In Situ</i> Heating Transmission Electron Microscopy
摘要： Benefited from a large density of layer edges exposed on the surface, vertically-aligned two-dimensional (2D) molybdenum disulfide (MoS2) layers have recently harvested excellent performances in the field of electrochemical catalysis and chemical sensing. With their increasing versatility for high-temperature demanding applications, it is vital to identify their thermally-driven structural and chemical stability as well as clarify its underlying principle. Despite various ex situ and in situ characterizations on horizontally-aligned 2D MoS2 layers, the direct in situ heating of vertically-aligned 2D MoS2 layers and the real-time observation of their near-atomic scale dynamics have never been approached, leaving their thermal stability poorly understood. Moreover, the geometrical advantage of the surface-exposed vertically-aligned 2D MoS2 layers is anticipated to unveil the structural dynamics of interlayer van der Waals (vdW) gaps and its correlation with thermal energy, unattainable with 2D MoS2 layers in any other geometry. Herein, we report a comprehensive in situ heating TEM study on cleanly transferred vertically-aligned 2D MoS2 layers up to 1000 °C. Several striking phenomena were newly observed in the course of heating: (1) formation and propagation of voids between the domains of vertical 2D MoS2 layers with distinct grain orientations starting at ~875 °C, (2) subsequent decompositions of the 2D MoS2 layers accompanying a formation of Mo nanoparticles at ~950 °C, much lower than the melting temperature of their bulk counterpart, and (3) initiation of decomposition from the surface-exposed 2D layer vertical edge sites, congruently supported by molecular dynamics (MD) simulation. These new findings will offer critical insights into better understanding the thermodynamic principle that governs the structural stability of general vdW 2D crystals as well as providing useful technological guidance for materials design and optimization in their potential high-temperature applications.
关键词： van der Waals gaps,in situ heating,structural evolution,MoS2,two-dimensional,vertically-aligned,transmission electron microscopy,thermal stability,molecular dynamics simulation
更新于2025-09-23 15:19:57
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Inorganic halide double perovskites with optoelectronic properties modulated by sublattice mixing
摘要： All-inorganic halide double perovskites have emerged as a promising class of materials that are potentially more stable and less toxic than lead-containing hybrid organic-inorganic perovskite optoelectronic materials. In this work, 311 cesium chloride double perovskites (Cs2BB’Cl6) were selected from a set of 903 compounds as likely being stable based on a statistically learned tolerance factor (t) for perovskite stability. First-principles calculations on these 311 double perovskites were then performed to assess their stability and identify candidates with band gaps appropriate for optoelectronic applications. We predict that 261 of the 311 Cs2BB’Cl6 compounds are likely synthesizable based on a thermodynamic analysis of their decomposition to competing compounds (decomposition enthalpy < 0.05 eV/atom). Of these 261 likely synthesizable compounds, 47 contain no toxic elements and have direct or nearly direct (within 100 meV) band gaps between 1 and 3 eV as computed with hybrid density functional theory (HSE06). Within this set, we identify the triple alkali perovskites Cs2[Alk]+[TM]3+Cl6, where Alk is a group 1 alkali cation and TM is a transition metal cation, as a class of Cs2BB’Cl6 double perovskites with remarkable optical properties, including large and tunable exciton binding energies as computed by the GW-Bethe Salpeter Equation (GW-BSE) method. We attribute the unusual electronic structure of these compounds to the mixing of the Alk-Cl and TM-Cl sublattices, leading to materials with small band gaps, large exciton binding energies, and absorption spectra that are strongly influenced by the identity of the transition metal. The role of the double perovskite structure in enabling these unique properties is probed through analysis of the electronic structures and chemical bonding of these compounds as compared with other transition metal and alkali transition metal halides.
关键词： stability,sublattice mixing,band gaps,inorganic halide double perovskites,optoelectronic properties,excitons
更新于2025-09-23 15:19:57
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Efficient and accurate calculation of band gaps of halide perovskites with the Tran-Blaha modified Becke-Johnson potential
摘要： We report on a reoptimization of the Tran-Blaha modified Becke-Johnson (TB-mBJ) potential dedicated to the prediction of the band gaps of three-dimensional (3D) and layered hybrid organic-inorganic perovskites (HOPs) within pseudopotential-based density functional theory methods. These materials hold promise for future photovoltaic and optoelectronic applications. We begin by determining a set of parameters for 3D HOPs optimized over a large range of materials. Then we consider the case of layered HOPs. We design an empirical relationship that facilitates the prediction of band gaps of layered HOPs with arbitrary interlayer molecular spacers with a computational cost considerably lower than that of more advanced methods like hybrid functionals or GW. Our study also shows that substituting interlayer molecular chains of layered HOPs with Cs atoms is an appealing and cost-effective route to band gap calculations. Finally, we discuss the pitfalls and limitations of TB-mBJ for HOPs, notably its tendency to overestimate the effective masses due to the narrowing of the band dispersions. We expect our results to extend the use of TB-mBJ for other low-dimensional materials.
关键词： Tran-Blaha modified Becke-Johnson potential,halide perovskites,computational methods,band gaps,density functional theory,hybrid organic-inorganic perovskites,pseudopotential
更新于2025-09-19 17:15:36
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Accelerated Discovery of Two-Dimensional Optoelectronic Octahedral Oxyhalides via High-Throughput <i>Ab Initio</i> Calculations and Machine Learning
摘要： Traditional trial-and-error methods are obstacles for large-scale searching of new optoelectronic materials. Here, we introduce a method combining high-throughput ab initio calculations and machine-learning approaches to predict two-dimensional octahedral oxyhalides with improved optoelectronic properties. We develop an effective machine-learning model based on an expansive data set generated from density functional calculations including the geometric and electronic properties of 300 two-dimensional octahedral oxyhalides. Our model accelerates the screening of potential optoelectronic materials of 5000 two-dimensional octahedral oxyhalides. The distorted stacked octahedral factors proposed in our model play essential roles in the machine-learning prediction. Several potential two-dimensional optoelectronic octahedral oxyhalides with moderate band gaps, high electron mobilities, and ultrahigh absorbance coefficients are successfully hypothesized.
关键词： band gaps,optoelectronic materials,two-dimensional octahedral oxyhalides,absorbance coefficients,electron mobilities,high-throughput ab initio calculations,machine learning
更新于2025-09-19 17:13:59
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Asymptotics of Eigenvalues in Spectral Gaps of Periodic Waveguides with Small Singular Perturbations
摘要： The asymptotics of eigenvalues appearing near the lower edge of a spectral gap of the Dirichlet problem is studied for the Laplace operator in a d-dimensional periodic waveguide with a singular perturbation of the boundary by creating a hole with a small diameter ε. Several versions of the structure of the gap edge are considered. As usual, the asymptotic formulas are di?erent in the cases d ≥ 3 and d = 2, where the eigenvalues occur at distances O(ε2(d?2)) or O(ε2d) and O(| ln ε|?2) or O(ε4), respectively, from the gap edge. Other types of singular perturbation of the waveguide surface and other types of boundary conditions are discussed, which provide the appearance of eigenvalues near both edges of one or several gaps. Bibliography: 51 titles.
关键词： eigenvalues,spectral gaps,periodic waveguides,asymptotics,singular perturbations
更新于2025-09-16 10:30:52
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Mechanical Deformation–Assisted Fabrication of Plasmonic Nanobowties with Broken Symmetry and Tunable Gaps
摘要： A symmetry broken and gap tunable gold nanobowtie array is demonstrated by nanosphere lithography performed on a mechanically uniaxially pre-stretched elastomeric substrate. Due to symmetry breaking of each nanotriangle with its three neighboring nanotriangles once the uniaxial prestretch is released, the structure exhibits polarization-dependent optical properties. In addition, the decrease in the gap between adjacent triangles provided by mechanical relaxation of the substrate enhances the electric field enhancement between adjacent triangles, which in turn results in enhanced Raman scattering from molecules present in the gap. Triangle apex-to-apex gaps as small as 20 nm are generated using a colloidal crystal formed from 500 nm colloids on at 30% prestretched substrate (gaps formed when an unstretched substrate is used are ≈115 nm).
关键词： mechanical deformation-assisted,tunable gaps,broken symmetry,plasmonic nanobowties
更新于2025-09-16 10:30:52