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Summary Perovskite solar cells (PSCs) and organic photovoltaics (OPVs) have both undergone rapid development recently. The composition and molecular tunability of perovskite and organic semiconductors enable a large material pool with different band gaps and various physical characters, giving feasibility to construct perovskite/organic tandem solar cells (TSCs). Here, we developed a semi-empirical model, rationally selected the best available material combination, and successfully demonstrated the efficient and reproducible TSCs benefiting from their complementary band gaps and orthogonal processing solvents. Featuring with all-thermally evaporated low-loss interconnecting layers (ICLs), our 2-termimal (2T) monolithic perovskite/organic TSCs deliver high reproducibility with power conversion efficiency (PCE) narrowly distributed between 20% and 20.6% (certified as 19.54%). In addition to the promising efficiency, the UV sensitivity of OPVs is eliminated in the tandem structure, demonstrating its advantage on device stability. Those results unfold significant potentials of perovskite/organic tandem devices as reproducible and cost-effective structures to achieve high-performance TSCs.

In this study, an epidemic model was developed to simulate and predict the disease variations of Guangdong province which was focused on the period from Jan 27 to Feb 20, 2020. To explore the impacts of the input population and quarantine strategies on the disease variations at different scenarios, four time points were assumed as Feb 6, Feb 16, Feb 24 and Mar 5 2020. The major results suggest that our model can well capture the disease variations with high accuracy. The simulated peak value of the confirmed cases is 1002 at Feb 10, 2020 which is mostly close to the reported number of 1007 at Feb 9, 2020. The disease will become extinction with peak value of 1397 at May 11, 2020. Moreover, the increased numbers of the input population can mainly shorten the disease extinction days and the increased percentages of the exposed individuals of the input population increase the number of cumulative confirmed cases at a small percentage. Increasing the input population and decreasing the quarantine strategy together around the time point of the peak value of the confirmed cases, may lead to the second outbreak.

Well-engineered light trapping designs will significantly improve power conversion efficiency of thin-film organic solar cells. Recently, metallic nanoparticles (NPs) have been widely used to concentrate sunlight in the active layer of OSCs. To critically examine the influence of geometry of metallic NPs on absorption of OSCs, the parallel finite-element method is applied to simulate the near-field multiple scattering effects in plasmonic NPs incorporated OSCs. The geometry-varied NPs including nano-cone, nano-inverted-cone, nano-cylinder, and nano-cuboid are systematically investigated. Furthermore, the absorption enhancement from the dielectric silicon NPs and perfectly reflecting NPs are also comprehensively offered. Compared to the off-plasmon resonance case, the absorption enhancement factor is higher for on-plasmon resonance case. However, the absorption of organic active material near plasmon resonance weakly contributes to exciton generation. Moreover, the height-dependent Fabry-Perot mode plays a key role in the light trapping off the plasmon resonance. Additionally, the tapered structure of the silver nano-cone, which leads to the best absorption enhancement of 3 at the wavelength of 680 nm, since it can reduce unwanted reflection loss and achieve broadband plasmonic resonance simultaneously.

Metafibers expand the functionalities of conventional optical fibers to unprecedented nanoscale light manipulations by integrating metasurfaces on the fiber tips, becoming an emerging light-coupling platform for both nanoscience and fiber optics communities. Mostly exploring the isolated bare fibers, current metafibers remain as proof-of-concept demonstrations due to a lack of standard interfaces with the universal fiber networks. Here, we develop new methodologies to fabricate well-defined plasmonic metasurfaces directly on the end facets of commercial single mode fiber jumpers using standard planar technologies and provide a first demonstration of their practical applications in the nonlinear optics regime. Featuring plug-play connections with fiber circuitry and arbitrary metasurfaces landscapes, the metafibers with tunable plasmonic resonances are implemented into fiber laser cavities, yielding all-fiber sub-picosecond (minimum 513 fs) soliton mode locked lasers at optical wavelengths of 1.5 micrometer and 2 micrometer, demonstrating their unusual polarimetric nonlinear transfer functions and superior saturation absorption responses. Novel insights into the physical mechanisms behind the saturable absorption of plasmonic metasurfaces are provided. The nanofabrication process flow is compatible with existing cleanroom technologies, offering metafibers an avenue to be a regular member of functionalized fiber components. The work paves the way towards next generation of ultrafast fiber lasers, optical frequency combs, optical neural networks and ultracompact "all-in-fibers" optical systems for sensing, imaging, communications, and many others., Comment: 27 pages, 15 figures

The multi-user Holographic Multiple-Input and Multiple-Output Surface (MU-HMIMOS) paradigm, which is capable of realizing large continuous apertures with minimal power consumption, has been recently considered as an energyefficient solution for future wireless networks, offering increased flexibility in impacting electromagnetic (EM) wave propagation according to the desired communication, localization, and sensing objectives. The tractable channel modeling in MU-HMIMOS wireless systems is one of the most critical research challenges, mainly due to the coupling effect induced by the excessively large number of closely spaced patch antennas. In this paper, we focus on this challenge for the downlink of multi-user MIMO communications and extend an EM-compliant channel model to multiuser case, which is expressed in the wavenumber domain using the Fourier plane wave approximation. Based on the presented channel model, we investigate the spectral efficiency of maximumratio transmission and Zero-Forcing (ZF) precoding schemes. We also introduce a novel hardware efficient ZF precoder, leveraging Neumann series (NS) expansion to replace the required matrix inversion operation, which is very hard to be computed in the conventional way due to the extremely large number of patch antennas in the envisioned MU-HMIMOS communication systems. In comparison with the conventional independent and identical Rayleigh fading channels that ignore antenna coupling effects, the proposed EM-compliant channel model captures the mutual couplings induced by the very small antenna spacing. Our extensive performance evaluation results demonstrate that our theoretical performance expressions approximate sufficiently well ...

The effects of gain and loss on the band structures of a bulk topological dielectric photonic crystal (PC) with $C_{6v}$ symmetry and the PC-air-PC interface are studied based on first-principle calculation. To illustrate the importance of parity-time (PT) symmetry, three systems are considered, namely the PT-symmetric, PT-asymmetric, and lossy systems. We find that the system with gain and loss distributed in a PT symmetric manner exhibits a phase transition from a PT exact phase to a PT broken phase as the strength of the gain and loss increases, while for the PT-asymmetric and lossy systems, no such phase transition occurs. Furthermore, based on the Wilson loop calculation, the topology of the PT-symmetric system in the PT exact phase is demonstrated to keep unchanged as the Hermitian system. At last, different kinds of edge states in Hermitian systems under the influences of gain and loss are studied and we find that while the eigenfrequencies of nontrivial edge states become complex conjugate pairs, they keep real for the trivial defect states., 6 pages, 6 figures

Compared with a single-input-single-output (SISO) wireless communication system, the benefit of multiple-input-multiple-output (MIMO) technology originates from its extra degree of freedom (DOF), also referred to as scattering channels or spatial electromagnetic (EM) modes, brought by spatial multiplexing. When the physical sizes of transmitting and receiving arrays are fixed and there are sufficient antennas (typically with half-wavelength spacings), the DOF limit is only dependent on the propagating environment. Analytical methods can be used to estimate this limit in free space, and some approximate models are adopted in stochastic environments, such as Clarke’s model and Ray-tracing methods. However, this DOF limit in a certain inhomogeneous environment has not been well discussed with rigorous full-wave numerical methods. In this work, volume integral equation (VIE) is implemented for investigating the limit of MIMO effective degree of freedom (EDOF) in three representative two-dimensional (2-D) inhomogeneous environments. Moreover, we clarify the relation between the performance of a MIMO system and the scattering characteristics of its propagating environment.

A comprehensive equivalent circuit model is developed to investigate the surface recombination, bulk recombination, shunt resistance, series resistance and the abnormal hysteresis effect for perovskite solar cells. By comparing the introduced model and the ionic drift-diffusion model, two typical capacitance expressions are summarized to characterize the hysteresis. The proposed equivalent circuit model together with typical capacitance expressions reveals the hidden device physics of solar cells, being a valuable synthesis tool for device simulation and analysis.

Edge states in topological photonic systems have the robust unidirectional propagation feature which originates from the topological nature of the bulk band dispersion. In this work, we propose a topological photonic crystal (PC) waveguide that is formed by introducing an air gap in an all-dielectric PC with $C_{6v}$ symmetry. The dispersion of the line defect states is tunable by varying the width of the air gap. Based on their field distributions, the states can be classified into even- and odd-symmetric states. The even-symmetric states are found to be the nontrivial topological edge states and exhibit pseudospin-locking unidirectional propagation property. By introducing disorders to the waveguide, these states are proven to be robust with well suppressed backscattering.

A star-shaped polymer enables inverted perovskite solar cells with efficiency over 22% and a very high fill factor of 0.862., Stabilizing high-efficiency perovskite solar cells (PSCs) at operating conditions remains an unresolved issue hampering its large-scale commercial deployment. Here, we report a star-shaped polymer to improve charge transport and inhibit ion migration at the perovskite interface. The incorporation of multiple chemical anchor sites in the star-shaped polymer branches strongly controls the crystallization of perovskite film with lower trap density and higher carrier mobility and thus inhibits the nonradiative recombination and reduces the charge-transport loss. Consequently, the modified inverted PSCs show an optimal power conversion efficiency of 22.1% and a very high fill factor (FF) of 0.862, corresponding to 95.4% of the Shockley-Queisser limited FF (0.904) of PSCs with a 1.59-eV bandgap. The modified devices exhibit excellent long-term operational and thermal stability at the maximum power point for 1000 hours at 45°C under continuous one-sun illumination without any significant loss of efficiency.

In this letter, a new deep learning (DL) approach is proposed to solve the electromagnetic inverse scattering (EMIS) problems. The conventional methods for solving inverse problems face various challenges including strong ill-conditions, high contrast, expensive computation cost, and unavoidable intrinsic nonlinearity. To overcome these issues, we propose a new two-step machine learning based approach. In the first step, a complex-valued deep convolutional neural network is employed to retrieve initial contrasts (permittivities) of dielectric scatterers from measured scattering data. In the second step, the previously obtained contrasts are input into a complex-valued deep residual convolutional neural network to refine the reconstruction of images. Consequently, the EMIS problem can be solved with much higher accuracy even for high-contrast objects. Numerical examples have demonstrated the capability of the newly proposed method with the improved accuracy. The proposed DL approach for EMIS problem serves a new path for realizing real-time quantitative microwave imaging for high-contrast objects.

The power conversion efficiency (PCE) of low-bandgap mixed Pb–Sn perovskite solar cells (PSCs) has been significantly hindered by large open-circuit voltage (Voc) loss and poor fill factor (FF). He...

Thermal cooling mechanisms of human exposed to electromagnetic (EM) radiation are studied in detail. The electromagnetic and thermal co-simulation method is utilized to calculate the electromagnetic and temperature distributions. Moreover, Pennes' bioheat equation is solved to understand different thermal cooling mechanisms including blood flow, convective cooling and radiative cooling separately or jointly. Numerical results demonstrate the characteristics and functions for each cooling mechanism. Different from the traditional view that the cooling effect of blood is usually reflected by its influence on sweat secretion and evaporation, our study indicates that the blood flow itself is an important factor of thermal cooling especially for high-intensity EM radiation. This work contributes to fundamental understanding of thermal cooling mechanisms of human., 7 pages, 5 figures

The spectral functions are studied in conjunction with the dyadic Green’s functions for various media. The dyadic Green’s functions are found using the eigenfunction expansion method for homogeneous, inhomogeneous, periodic, lossless, lossy, and anisotropic media, guided by the BlochFloquet theorem. For the lossless media cases, the spectral functions can be directly related to the photon local density of states, and hence, to the electromagnetic energy density. For the lossy case, the spectral function can be related to the field correlation function. Because of these properties, one can derive properties for field correlations and the Langevin-source correlations without resorting to the fluctuation dissipation theorem. The results are corroborated by the fluctuation dissipation theorem. An expression for the local density of states for lossy, inhomogeneous, and dispersive media has also been suggested. PACS numbers: Valid PACS appear here

Perovskite solar cells (PSCs) have attracted tremendous interest because of their rapid improvement in power conversion efficiency (PCE) from the initial PCE of 3.8% for the first prototype to the certified PCE of 25.2% in 2019. However, the inherent chemical instability of organic–inorganic hybrid perovskite halides influenced by moisture, heat and ultraviolet light is still a critical issue for them to meet application-specific requirements owing to the weak-bonded organic components in the hybrid crystal structure. The use of all-inorganic perovskites CsPbI3−xBrx (x = 0, 1, 2, and 3) as light-harvesters by completely substituting organic species with inorganic Cs+ ions has been recently regarded as a promising solar conversion technology. Since the initial efficiency of 2.9% achieved in 2015, the highest PCE record for inorganic PSCs has risen to 18.4% through structure optimization, compositional engineering, interfacial engineering, solvent control and surface passivation, etc. This article is dedicated to providing an up-to-date review on the development of inorganic PSCs tailored by various inorganic perovskite materials with gradually changed optical properties and stability, as well as the film-making methods and interfacial engineering technologies. Their limited efficiencies in theory and recombination mechanisms are also predicted with a detailed balance model. Finally, we focused on the state-of-the-art strategies for enhancing the photovoltaic performance and identified new challenges and outlooks for future studies in this field.

Terahertz photoconductive antenna (PCA) is an important device for generating ultrabroadband terahertz radiations, being applicable in various scenarios. However, the metallic electrodes in PCAs, a pair of coplanar strip lines (CSL), always produce horizontal electrode modes in a broad THz band, thus resulting in low directivity in the vertical direction. Here, we introduce spoof surface plasmon polariton (SSPP) structures to suppress horizontal electrode modes in a broad band. The suppression principles are accounted to both the forbidden band of the fundamental SSPP mode and the orthogonality between source and higher-order SSPP modes. In the SSPP-modified PCA, we achieve around 2 dBi higher directivity in the vertical direction compared to a typical CSL PCA. Unlike the narrow bands inheriting from conventional metamaterial resonators, the relative operational band of the SSPP-modified PCA is as broad as 48%. This planar SSPP structure is compatible with the well-developed micro fabrication technologies. Thus, our scheme can be combined with the semiconductor material engineering and plasmonic nanoscale structures for further increasing THz output power.

It is well known that Green’s function can be expressed by multipole expansion, plane-wave expansion, and exponential expansion (spectral representation). These three expansions constitute of the foundations of the fast multipole algorithm (FMA). The plane-wave expansion has the low-frequency breakdown issue due to its failure in capturing the evanescent spectra, while the multipole expansion is inefficient at high frequencies. The spectral representation usually involves in direction-dependent issue. In this communication, the 2-D FMA is interpreted as Parseval’s theorem in Fourier transform. To achieve a stable and accurate transition between the multipole expansion and the plane-wave expansion, a novel diagonalization in the 2-D FMA is proposed with scaled special functions based on a discrete Fourier transform. A wideband fast algorithm with high accuracies can be achieved efficiently.

The book chapter will aim at introducing the background knowledge, basic theories, supporting techniques, numerical results, and future research for the high-order symplectic finite-difference time-domain scheme. The theories of symplectic geometry and Hamiltonian are reviewed in Section 2 followed by the symplectiness of Maxwell’s equations presented in Section 3. Next, the numerical stability and dispersion analyses are given in Section 4. Then, in Section 5, we will make a tour of the supporting techniques but do not discuss them in detail. These techniques involve source excitation, perfectly matched layer, near-to-farfield transformation, inhomogeneous boundary treatments, and parameter extractions. The numerical results on propagation, scattering, and guided-wave problems are shown in Section 6. The high-order symplectic finite-difference time-domain scheme demonstrates the powerful advantages and potentials for the time-domain solution of Maxwell’s equations, especially for electrically-large objects and for long-term simulation. Finally, the conclusion and future research are summarized in Section 7.

A communication prototype of plane spiral OAM (PSOAM) mode-group (MG) based multiple-in-multiple-out (MIMO) system is built in a line-of-sight (LoS) scenario. Then, a long-range wireless transmission experiment is conducted at a distance of 30 m (beyond Rayleigh distance). The bit-error-rate (BER) performance proves the feasibility of the proposed system. The obtainable single-way spectrum efficiency is 3.7 bits/s/Hz/stream. MG's vorticity can provide another controllable degree of free (DoF) to reduce the spatial correlation, it's more suitable for the long-range wireless transmission relative to conventional OAM multiplexing technique.

Nonlinear topological photonics, which explores topics common to the fields of topological phases and nonlinear optics, is expected to open up a new paradigm in topological photonics. Here, we demonstrate second-harmonic generation (SHG) via nonlinear interaction of double topological valley-Hall kink modes in all-dielectric photonic crystals (PhCs). We first show that two topological frequency bandgaps can be created around a pair of frequencies, $\omega_0$ and $2\omega_0$, by gapping out the corresponding Dirac points in two-dimensional honeycomb PhCs. Valley-Hall kink modes along a kink-type domain wall interface between two PhCs placed together in a mirror-symmetric manner are generated within the two frequency bandgaps. Importantly, through full-wave simulations and mode dispersion analysis, we demonstrate that tunable, bi-directional phase-matched SHG via nonlinear interaction of the valley-Hall kink modes inside the two bandgaps can be achieved. In particular, by using Stokes parameters associated to the magnetic part of the valley-Hall kink modes, we introduce a new concept, SHG directional dichroism, which is employed to characterize optical probes for sensing chiral molecules. Our work opens up new avenues towards topologically protected nonlinear frequency mixing and active photonic devices implemented in all-dielectric material platforms., Comment: 6 pages, 5 figures

The topological corner state (TCS) and topological edge state (TES) have created new approaches to manipulate the propagation of light. The construction of a topological coupled cavity-waveguide (TCCW) based on the TCS and TES is worth looking forward to, due to its research prospects in realizing high-performance micro-nano integrated photonic devices. In this Letter, the TCCW is proposed in two-dimensional (2D) photonic crystal (PC), which possesses strong optical localization, high quality factor, and excellent robustness compared with the conventional coupled cavity-waveguide (CCCW). This work will pave the way toward designing high-performance logic gates, lasers, filters, and other micro-nano integrated photonics devices and expanding their applications.

Light harvesting is crucial for thin-film solar cells. To substantially reduce optical loss in perovskite solar cells (PSCs), hierarchical light-trapping nano-architectures enable absorption enhancement to exceed the conventional upper limit and have great potential for achieving state-of-the art optoelectronic performances. However, it remains a great challenge to design and fabricate a superior hierarchical light-trapping nano-architecture, which exhibits extraordinary light-harvesting ability and simultaneously avoids deteriorating the electrical performance of PSCs. Herein, colorful efficient moire-PSCs are designed and fabricated incorporating moire interference structures by the imprinting method with the aid of a commercial DVD disc. It is experimentally and theoretically demonstrated that the light harvesting ability of the moire interference structure can be well manipulated through changing the rotation angle (0°-90°). The boosted short-circuit current is credited to augment light diffraction channels, leading to elongated optical paths, and fold sunlight into the perovskite layer. Moreover, the imprinting process suppresses the trap sites and voids at the active-layer interfaces with eliminated hysteresis. The moire-PSC with an optimized 30° rotation angle achieves the best enhancement of light harvesting (28.5% higher than the pristine), resulting in efficiencies over 20.17% (MAPbI3 ) and 21.76% ((FAPbI3 )1-x (MAPbBr3 )x ).

Reducing carrier recombination and facilitating charge extraction at the interface is of great significance to improve the device performance of perovskite solar cells (PSCs) towards commercial use. However, there has been little work done concerning transportation and recombination mechanism at the interface of the metal electrode and the electron transport layer in inverted PSCs. Herein, a new strategy of interface modification is reported that leverages the unique metal-to-insulator transition (MIT) characteristics of vanadium dioxide which is inserted as the electron extraction layer (EEL) in p-i-n planar PSCs. Benefiting from the suitable intermediate energy level of VO2, the optimized device shows a power conversion efficiency (PCE) up to 22.11% with negligible hysteresis, as compared to the 20.96% benchmark at room temperature. Interestingly, the PCE of VO2-based PSC increases to over 23% at 85 °C, which can be attributed to the dramatic change in the electrical properties and better electron extraction caused by the MIT of VO2 beyond its critical phase-change temperature. In addition, the encapsulated VO2-PSC shows superior thermal stability for 1000 h at 85 °C under 1 Sun illumination, maintaining over 90% of initial PCE. This work initiates the state-of-art concept of inserting thermally-induced phase-transition material as an EEL to achieve efficient and durable perovskite photovoltaics. Ministry of Education (MOE) National Research Foundation (NRF) X.L. and Y.M. contributed equally to this work. This work was co-financed by National Natural Science Foundation of China (Grant No. 51572098 and 51632006), National Basic Research Program of China (Grant No. 2013CB632500), Natural Science Foundation of Hubei Province (Grant No. 2015CFB432), Open Fund of State Key Laboratory of Advanced Technology (No. 2016-KF-5). In addition, Y.L. acknowledges the National Research Foundation, Prime Minister’s Office, Singapore under its Campus for Research Excellence and Technologies Enterprise (CREATE) program and Singapore Ministry of Education (MOE) Academic Research Fund Tier 1 RG103/19 and RG86/20 for funding support. The technical assistance from the Analytical and Testing Center of HUST and NTU is likewise gratefully acknowledged.

With the increase of frequency and power, thermal management becomes more and more important in the design and application of RF and microwave circuits. In this paper, the microwave heating problem is studied. The discontinuous Galerkin time-domain (DGTD) method is adopted for EM simulation, while the finite-element time-domain (FETD) method is utilized for thermal simulation. The large-scale MPI parallel programming technique is used to accelerate both the DGTD and FETD method. Numerical examples are given to demonstrate the accuracy and efficiency of the proposed method.

Structured light characterized by spatially inhomogeneous optical fields found rich applications in optical communication, sensing, microscopy, manipulation, and quantum information. While generation of structured light has been extensively studied in linear optics, the nonlinear optical process, particularly in two-dimensional (2D) materials, is an emerging alternative for generating structured light at shorter wavelengths. In this work, we theoretically demonstrate that radially and azimuthally polarized beams and vortex beams carrying orbital angular momentum could be generated at second-harmonic frequencies by using 2D material-based metasurfaces comprising the same transition-metal dichalcogenide meta-atoms. Manipulation of translations and orientations of anisotropically nonlinear meta-atoms exhibiting a threefold rotation-symmetrical crystalline structure induces strong nonlinear spin-orbital coupling, which enables simultaneous control of spatial phase and polarization in second-harmonic generation. The nonlinear transition-metal dichalcogenide metasurface proposed is promising for on-chip integration of nonlinear generation of structured light.

Rapid progress in nonlinear plasmonic metasurfaces enabled many novel optical characteristics for metasurfaces, with potential applications in frequency metrology [Zimmermann et al. Opt. Lett. 29:310 (2004)], timing characterization [Singh et al. Laser Photonics Rev. 14:1 (2020)] and quantum information [Kues et al. Nature. 546:622 (2017)]. However, the spectrum of nonlinear optical response was typically determined from the linear optical resonance. In this work, a wavelength-multiplexed nonlinear plasmon-MoS2 hybrid metasurface with suppression phenomenon was proposed, where multiple nonlinear signals could to be simultaneously processed and optionally tuned. A clear physical picture to depict the nonlinear plasmonic bound states in the continuum (BICs) was presented, from the perspective of both classical and quantum approaches. Particularly, beyond the ordinary plasmon-polariton effect, we numerically demonstrated a giant BIC-inspired second-order nonlinear susceptibility 10−5 m/V of MoS2 in the infrared band. The novelty in our study lies in the presence of a quantum oscillator that can be adopted to both suppress and enhance the nonlinear quasi BICs. This selectable nonlinear BIC-based suppression and enhancement effect can optionally block undesired modes, resulting in narrower linewidth as well as smaller quantum decay rates, which is also promising in slow-light-associated technologies.

The purpose of this study is to generate a broadband high-performance vortex wave. By using the waveguide theory and the grating vectors model, a broadband quasi-continuous metasurface is proposed to generate the $l=\pm 2$ vortex beams with a measuring mode purity of 0.68 operating from 9 GHz to 19 GHz. In compare with previous designs, this metasurface provide a significant bandwidth improvement with continuous upper surface feature. The measured far-field patterns have a good agreement with those obtained from the simulation. This work offers a useful route of producing high-purity orbital angular momentum waves.

Chiral surface states along the zigzag edge of a valley photonic crystal in the honeycomb lattice are demonstrated. By decomposing the local fields into orbital angular momentum (OAM) modes, we find that the chiral surface states present OAM-dependent unidirectional propagation characteristics. Particularly, the propagation directivities of the surface states are quantified by the local OAM decomposition and are found to depend on the chiralities of both the source and surface states. These findings allow for the engineering control of the unidirectional propagation of electromagnetic energy without requiring an ancillary cladding layer. Furthermore, we examine the propagation of the chiral surface states against sharp bends. It turns out that although only certain states successfully pass through the bend, the unidirectional propagation is well maintained due to the topology of the structure., 9 pages, 6 figures

As an important figure of merit for characterizing the quantized collective behaviors of the wavefunction, Chern number is the topological invariant of quantum Hall insulators. Chern number also identifies the topological properties of the photonic topological insulators (PTIs), thus it is of crucial importance in PTI design. In this paper, we develop a first principle computatioal method for the Chern number of 2D gyrotropic photonic crystals (PCs), starting from the Maxwell's equations. Firstly, we solve the Hermitian generalized eigenvalue equation reformulated from the Maxwell's equations by using the full-wave finite-difference frequency-domain (FDFD) method. Then the Chern number is obtained by calculating the integral of Berry curvature over the first Brillouin zone. Numerical examples of both transverse-electric (TE) and transverse-magnetic (TM) modes are demonstrated, where convergent Chern numbers can be obtained using rather coarse grids, thus validating the efficiency and accuracy of the proposed method., Comment: 13 pages, 6 figures

Electromagnetic topological insulators have been explored extensively due to the robust edge states they support. In this work, we propose a topological electromagnetic system based on a line defect in topologically nontrivial photonic crystals (PCs). With a finite-difference supercell approach, modal analysis of the PCs structure is investigated in detail. The topological line-defect states are pseudospin polarized and their energy flow directions are determined by the corresponding pseudospin helicities. These states can be excited by using two spatially-symmetric line-source arrays carrying orbital angular momenta. The feature of the unidirectional propagation is demonstrated and it is stable when disorders are introduced to the PCs structure., 6 pages, 8 figures

We demonstrate the coexistence of pseudospin- and valley-Hall-like edge states in a photonic crystal with $C_{3v}$ symmetry, which is composed of three interlacing triangular sublattices with the same lattice constants. By tuning the geometry of the sublattices, three complete photonic band gaps with nontrivial topology can be created, one of which is due to the band inversion associated with the pseudospin degree of freedom at the $\Gamma$ point and the other two due to the gapping out of Dirac cones associated with the valley degree of freedom at the $K, K'$ points. The system can support tri-band pseudospin- and valley-momentum locking edge states at properly designed domain-wall interfaces. Furthermore, to demonstrate the novel interplay of the two kinds of edge states in a single configuration, we design a four-channel system, where the unidirectional routing of electromagnetic waves against sharp bends between two routes can be selectively controlled by the pseudospin and valley degrees of freedom. Our work combines the pseudospin and valley degrees of freedom in a single configuration and may provide more flexibility in manipulating electromagnetic waves with promising potential for multiband and multifunctional applications., Comment: 6 pages, 5 figures

The influences of electromagnetic radiation from mobile phones on human health have aroused significant public concern. One of the most serious consequences is the increment of temperature of tissues and organs. Due to the specificity of the research object, numerical methods for electromagnetic and thermal analysis are usually adopted instead of trials. Most of the existing studies adopt a finite-difference time-domain method for electromagnetic and thermal simulation. Moreover, the radiative cooling phenomenon is neglected during the thermal simulation. In this letter, we utilized the finite-element method with tetrahedral elements to get a better modeling of the human head. Furthermore, the radiative cooling is also taken into account by using the radiation boundary condition. Numerical results validate the accuracy and capability of the proposed method.

Ternary blend organic solar cells (OSCs) composed of three components in the active layer shows the potential to achieve higher power conversion efficiency (PCE) as compared to the binary counterpart due to the wider absorption spectrum, higher generation rate, and better morphology. However, the physical understanding of carrier generation and transport processes in the ternary blend OSCs has been limited explored. In the work, together with experimental studies of the two donors, one acceptor ternary blend OSCs with PCE > 12%, we will theoretically and experimentally describe the roles of the carrier generation (including exciton transfer, delocalization and dissociation), and carrier transport (particularly the hole transport) on the performance of ternary blend OSCs. Through theoretical and experimental investigations, critical design rules for improving the device performance are concluded: (1) improving the exciton delocalization ratio via donor ratio optimization with physical understanding, (2) selecting the donors with well overlap of emission and absorption spectra to promote a beneficial exciton transfer, (3) engineering the energy level of donors to form the blocking barrier for reducing hole transfer into the donor with high recombination loss. The work unveils the device physics which is fundamentally important for designing and optimizing high-performance ternary blend OSCs.

Previous studies on orbital angular momentum (OAM) communication mainly considered line-of-sight environments. In this letter, however, it is found that OAM communication with high-order modulation can be achieved in highly reverberant environments by combining the OAM multiplexing with a spatial equalizer. The OAM multiplexing exhibits comparable performance of conventional multiple-input multiple-output (MIMO) system., 4 pages, 4 figures

We report a highly efficient graphene-based modulator by using an edge plasmonic effect in this paper. The modulation efficiency of the proposed modulator can be as large as 1.58 dB/μm, which is several times larger than that of previous reported modulators. By enhancing the gap plasmon mode and the edge plasmonic effect in a well-designed diagonal waveguide, a wedge-to-wedge SPP mode is strongly confined in both horizontal and vertical directions in terms of a small mode area (Aef /A0

The development of transition-metal sulfides, such as nickel sulfides (e.g., Ni3S2), as catalysts for the hydrogen evolution reaction is one potential solution to environmental pollution and energy crisis. However, its limited utilization of visible light and high recombination ratio of photoinduced electron–hole pairs suppress its photocatalytic activity. The key issue in improving photocatalytic efficiency lies in fabricating a p–n heterojunction with efficient interfacial contact and broadband absorption. Here, we developed a method for fabricating an interlaced Ni3S2/MoS2 heterostructure with close interfacial contact. In our fabrication approach, a porous Ni3S2 scaffold is prepared by chemical vapor deposition and a hydrothermal method is used to prepare a Ni3S2/MoS2 photocatalyst with close interfacial contact. The numerous interfaces of the interlaced Ni3S2/MoS2 heterostructures promote effective electron–hole pair separation and facilitate electron transfer. Meanwhile, the hybrid Ni3S2/MoS2 nanostructures favor broadband absorption extending from 300 to 800 nm. As a result, the hybrid Ni3S2/MoS2 exhibits a remarkable rate of hydrogen evolution (540.75 μmol g−1 h−1), which is 5.71 and 3.89 times greater than those of pure Ni3S2 and MoS2, respectively, under otherwise identical conditions. The results of this work are significant for developing promising transition-metal sulfide heterostructures in the field of hydrogen evolution by photocatalytic water splitting.

Innovative multiplexing technologies based on nano-optics for anti-counterfeiting have been proposed as overt and covert technologies to secure products and make them difficult to counterfeit. However, most of these nano-optical anti-counterfeiting materials are metasurfaces and metamaterials with complex and expensive fabrication process, often resulting in materials that are not damage tolerant. Highly efficient anti-counterfeiting technologies with easy fabrication process are targeted for intuitive and effective authentication of banknotes, secure documents, and goods packing. Here, a simple strategy exploiting self-assembling and nanoimprinting technique to fabricate a composite lattice photonic crystal architecture featuring full spatial control of light, multiplexed full-pixel imaging, and multichannel cryptography combined with customized algorithms is reported. In particular, the real-time encryption/recognition of mobile quick response codes and anti-counterfeiting labels on a postage stamp, encoded by the proposed photonic architecture, are both demonstrated. The wave optics of scattering, diffraction, and polarization process involved are also described, validated with numerical simulations and experiments. By introducing a new degree of freedom in the 3D space, the multichannel image switching exhibits unprecedented variability of encryption, providing a promising roadmap to achieve larger information capacity, better security, and higher definition for the benefit of modern anti-counterfeiting security.

Drift-diffusion model is an indispensable modeling tool to understand the carrier dynamics (transport, recombination, and collection) and simulate practical-efficiency of solar cells (SCs) through taking into account various carrier recombination losses existing in multilayered device structures. Exploring the way to predict and approach the SC efficiency limit by using the drift-diffusion model will enable us to gain more physical insights and design guidelines for emerging photovoltaics, particularly perovskite solar cells. Our work finds out that two procedures are the prerequisites for predicting and approaching the SC efficiency limit. Firstly, the intrinsic radiative recombination needs to be corrected after adopting optical designs which will significantly affect the open-circuit voltage at its Shockley-Queisser limit. Through considering a detailed balance between emission and absorption of semiconductor materials at the thermal equilibrium, and the Boltzmann statistics at the non-equilibrium, we offer a different approach to derive the accurate expression of intrinsic radiative recombination with the optical corrections for semiconductor materials. The new expression captures light trapping of the absorbed photons and angular restriction of the emitted photons simultaneously, which are ignored in the traditional Roosbroeck-Shockley expression. Secondly, the contact characteristics of the electrodes need to be carefully engineered to eliminate the charge accumulation and surface recombination at the electrodes. The selective contact or blocking layer incorporated nonselective contact that inhibits the surface recombination at the electrode is another important prerequisite. With the two procedures, the accurate prediction of efficiency limit and precise evaluation of efficiency degradation for perovskite solar cells are attainable by the drift-diffusion model., 32 pages, 11 figures

Electromagnetic (EM) waves with helical wavefront carry orbital angular momentum (OAM), which is associated with the azimuthal phase of the complex electric field. OAM is a new degree of freedom in EM waves and is promising for channel multiplexing in communication system. Although the OAM-carrying EM wave attracts more and more attention, the method of OAM generation at microwave frequencies still faces challenges, such as efficiency and simulation time. In this work, by using the circuit theory and equivalence principle, we build two simplified models, one for a single scatter and one for the whole metasurface to predict their EM responses. Both of the models significantly simplify the design procedure and reduce the simulation time. In this paper, we propose an ultrathin complementary metasurface that converts a left-handed (right-handed) circularly polarized plane wave without OAM to a right-handed (left-handed) circularly polarized wave with OAM of arbitrary orders and a high transmission efficiency can be achieved., Comment: 6 pages, 9 figures

Vortex beams carrying orbital angular momentum are extensively studied owing to its potential to expand channel capacity of microwave and optical communication. By utilizing the Pancharatnam-Berry phase concept, an ultra-wideband single layer metasurface is proposed to realize the conversion from incident plane waves to reflected vortex beams covering a considerable bandwidth from 6.75 to 21.85 GHz . An equivalent circuit model combined with broadband phase shift network is developed to effectively design the meta-atoms in metasurface. It is the first time to design wideband metasurfaces with the phase-based characteristics. To verify the proposed model, some deformed square loop meta-atoms are proposed to construct the metasurfaces with broadband OAM characteristic. Moreover, the vortex beams with the integer (l = -3), fractional (l = -1.5), and high-order (l = -10) OAM mode are generated respectively. Based on an OAM spectral analysis, the mode purity of the generated vortex waves is discussed in detail. The experimental results achieve a good agreement with those obtained from the simulation, thus proving the effectiveness and practicability of the proposed method., 10 pages, 12 figures

Transient electromagnetic-thermal co-simulation based on discontinuous Galerkin time-domain (DGTD) method is studied in this paper. The electromagnetic field and thermal field are coupled through temperature-dependent material properties. The power loss of the electromagnetic field acts as a heat source. A waveguide example is used to demonstrate the capability of the proposed method.

This paper proposes a new source reconstruction method (SRM) based on deep learning. The conventional SRM usually requires oversampled measurements data to ensure higher accuracy. Thus, conventional SRM numerical system is usually highly singular. A deep convolutional neural network (ConvNet) is proposed to reconstruct the equivalent sources of the target to overcome difficulty. The deep ConvNet allows us to employ less data samples. Besides, the ill-conditioned numerical system can be effectively avoided. Numerical examples are presented to demonstrate the feasibility and accuracy of the proposed method. Its performance is also compared with the traditional neural network and interpolation method. Moreover, we further expand the proposed method to measure the permittivity of dielectric scatterers.

We share personal views on future of electromagnetic (EM) education in this conference paper. Firstly, several approaches and examples regarding the enhancement of depth and breadth of existing EM courses are presented. Secondly, study and research in interdisciplinary fields correlated to electromagnetics are analyzed. Finally, the training strategies for commercial software use are discussed.