Strain- and Electron Doping-Induced In-Plane Spin Orientation at Room Temperature in Single-Layer CrTe2.

Ferromagnets with a Curie temperature surpassing room temperature (RT) are highly sought after for advancing planar spintronics. The ultrathin CrTe2 is proposed as a promising two-dimensional (2D) ferromagnet with a Curie temperature above 300 K. However, its single-layer film is highly susceptible to specific external perturbations, leading to variable magnetic features depending on the environment. The magnetic ordering of single-layer CrTe2 remains a topic of debate, and experimental confirmation of ferromagnetic order at RT is still pending. In our study, we utilized molecular beam epitaxy to create a single-layer 1T-CrTe2 on bilayer graphene, demonstrating ferromagnetism above 300 K with in-plane magnetization through superconducting quantum interference devices (SQUID) measurements. Our density functional theory (DFT) calculations suggest that the ferromagnetic properties stem from epitaxial strain, which increases the distance between adjacent Cr atoms within the layer by about 1.6% and enhances the Cr-Te-Cr angle by approximately 1.6°. Due to its interaction with the graphene substrate, the magnetic moment transitions from an out-of-plane to an in-plane orientation, while electronic doping exceeds 1.5 e/u.c. Combining DFT calculations with in situ scanning tunneling microscopy (STM) characterizations allowed us to determine the configuration of the CrTe2 single layer on graphene. This discovery presents the first experimental proof of ferromagnetic order in single-layer CrTe2 with a Curie temperature above RT, laying the groundwork for future applications of CrTe2 single-layer-based spintronic devices.

Full-Stokes polarization photodetector based on the hexagonal lattice chiral metasurface.

A hexagonal lattice silicon (Si) metasurface formed by the displacement of two mirrored isosceles trapezoid blocks in opposite directions is integrated into an InGaAs/InP photodetector to sense the circularly polarized light, whose optical properties mainly are controlled by the Fabry-Pérot (FP) cavity mode supported in the air slit called the Tunnel A. The Si metasurface can also be equivalent to the combination of the electric quadrupole (EQ) and the magnetic quadrupole (MQ) for the right circularly polarized (RCP) mode and the magnetic quadrupole for the left circularly polarized (LCP) mode. The external quantum efficiency of the circular polarization photodetectors is 0.018 and 0.785 for the RCP and LCP incidence, respectively. In addition, the full Stokes pixel based on the six-image-element technique can almost accurately measure arbitrary polarized light at 1550 nm operation wavelength, whose errors of the degree of linear polarizations (Dolp) and the degree of circular polarizations (Docp) are less than 0.01 and 0.15, respectively.

The Ultra-Large-Bandwidth Cascade Full-Stokes-Imaging Metasurface Based on the Dual-Major-Axis Circular Dichroism Grating.

A dual-major-axis grating composed of two metal-insulator-metal (MIM) waveguides with different dielectric layer thicknesses is numerically proposed to achieve the function of the quarter-wave plate with an extremely large bandwidth (1.0-2.2 µm), whose optical properties can be controlled by the Fabry-Pérot (FP) resonance. For the TE incident mode wave, MIM waveguides with large (small) dielectric layer thicknesses control the guided-mode resonant channels of long (short) waves, respectively, in this miniaturized optical element. Meanwhile, for the TM incident mode wave, the propagation wave vector of this structure is controlled by the hybrid mode of two gap-SPPs (gap-surface plasmon polaritons) with different gap thicknesses. We combine this structure with a thick silver grating to propose a circularly polarizing dichroism device, whose effective bandwidth can reach an astonishing 1.65 µm with a circular polarization extinction ratio greater than 10 dB. The full Stokes pixel based on the six-image element technique can almost accurately measure arbitrary polarization states at 1.2-2.8 µm (including elliptically polarized light), which is the largest bandwidth (1600 nm) of the full Stokes large-image element to date in the near-infrared band. In addition, the average errors of the degree of linear polarizations (Dolp) and degree of circular polarizations (Docp) are less than -25 dB and -10 dB, respectively.

Switchable trifunctional terahertz absorber for both broadband and narrowband operations.

In this paper, we proposed a multilayer terahertz absorber composed of hybrid graphene and vanadium dioxide (VO2). Based on electrical controlling of graphene and thermal tuning of VO2, three different switchable absorption states are achieved in one structure. When VO2 is in the metal phase and the Fermi level of graphene is set as 0eV, high-frequency broadband (bandwidth, 5.45THz) absorption from 4.5 to 9.95THz is demonstrated. While VO2 is switched to the insulator state, absorption states depend on the Fermi energy of graphene. As the Fermi level changes from 1eV to 0eV, the absorption can be switched from low-frequency broadband (bandwidth, 2.86THz) to dual-frequency absorption. The effect of geometric parameters and fabrication tolerance on the robustness of the absorption properties is explored. The proposed absorber has three switchable states through modulation of graphene and VO2, which is expected to realize potential applications in modulating, filtering, detecting, and other fields.

Mutual coupling of corner-localized quasi-BICs in high-order topological PhCs and sensing applications.

Recently, high-order topological photonic crystals (PhCs) have attracted huge research attention due to their novel physics mechanism and the application potential in integrated photonics. Based on the two-dimensional Su-Schrieffer-Heeger model, we construct and study the mutual coupling between the high-order corner states in 2D dielectric PhCs. Simulation results show that the Q-factor of such corner-localized quasi-bound states in the continuum (BICs) could be enhanced following mutual coupling in finite size. Furthermore, we study the side-coupled structure based on defect-hybrid waveguides and the edge state microring, the quasi-BIC microcavity. The refractive index sensing application based on corner-localized quasi-BICs shows outstanding simulated sensitivity (312.8 nm/RIU) and figure of merit (∼103 1/RIU). The robustness against fabrication errors due to its topologically protected nature makes it competitive compared with other quasi-BICs sensors.

Flexible alternating current electroluminescent devices integrated with high voltage triboelectric nanogenerators.

Flexible alternating current electroluminescent (ACEL) devices have attracted growing interest as promising wearable displays for their uniformity of light emission, low power consumption, and excellent reliability. However, the requirement of high-voltage power sources for driving ACEL devices greatly impedes their portability and commercialization. Here, we developed flexible ACEL devices integrated with high output-voltage triboelectric nanogenerators (TENG) using easy and low-cost crumpled Al electrodes. The output voltage and current could reach as high as 490 V and 71.74 µA, corresponding to the maximum instantaneous output power density of 1.503 mW cm-2, which was demonstrated to power an integrated flexible ACEL patterned display. In addition, through signal acquisition and transmission, ACEL can display the compression frequency of TENG in real time. Such self-powered ACEL devices are very promising as flexible displays in wearable electronics.

Mid-Infrared Sensor Based on Dirac Semimetal Coupling Structure.

A multilayer structure based on Dirac semimetals is investigated, where long-range surface plasmon resonance (LRSPR) of a dielectric layer/Dirac semimetal/dielectric layer are coupled with surface plasmon polaritons (SPPs) on graphene to substantially improve the Goos−Hänchen (GH) shift of Dirac semimetals in the mid-infrared band. This has important implications for the study of mid-infrared sensors. We studied the reflection coefficient and phase of this multilayer structure using a generalized transport matrix. We established that subtle changes in the refractive index of the sensing medium and the Fermi energy of the Dirac semimetal significantly affected the GH shift. Our numerical simulations show that the sensitivity of the coupling structure is more than 2.7×107 λ/RIU, which can be used as a potential new sensor application. The novelty of this work is the design of a tunable, highly sensitive, and simple structured mid-infrared sensor that takes advantage of the excellent properties of Dirac semimetals.

Large bandwidth and high-efficiency plasmonic quarter-wave plate.

A large bandwidth and high-efficiency subwavelength quarter-wave plate (QWP) is an indispensable component of an integrated miniaturized optical system. The bandwidth of existing plasmonic quarter-wave plates with a transmission efficiency of more than 50% is less than 320 nm in the near-infrared band. In this paper, a metallic quarter-wave plate with a bandwidth of 600 nm (0.95-1.55 µm) and an average transmittance of more than 70% has been designed and shows excellent potential to be used in miniaturized optical polarization detection systems and as an optical data storage device. For TE mode incident waves, this miniaturized optical element can be equivalent to a Fabry-Pérot (FP) resonator. Meanwhile, for the TM mode incident wave, the transmission characteristics of this structure are controlled by gap surface plasmon polaritons (G-SPPs) existing in the symmetric metal/insulator/metal (MIM) configuration.

Polarization-Selective Bidirectional Absorption Based on a Bilayer Plasmonic Metasurface.

We propose an alignment-free and polarization-selective bidirectional absorber composed of a one-dimensional bilayer Au grating array buried in a silicon nitride spacer. The absorptivity of the designed structure is more than 95% (77%) under normal forward (backward) TM-polarized light incidence, and is more than 80% (70%) within a forward (backward) incident angle up to 30°. The great bidirectional absorption performance is illustrated by the resonance coupling of the surface plasmon polaritons (SPPs) resonance, the propagating surface plasmon (PSP) resonance and the localized surface plasmon (LSP) resonance under TM-polarized wave illumination. Moreover, the excitation of the Fano-like resonance mode of the proposed metasurface can produce two significantly different peaks in the absorption spectrum under the oblique TM-polarized incidence, which is beneficial for the plasmon-sensing application. Therefore, the proposed bidirectional metasurface absorber can be a candidate in the application of optical camouflage, thermal radiation, solar cells and optical sensing.

Full-Stokes imaging polarimetry based on a metallic metasurface.

We use a single-layer thick metallic metasurface to design the 0-,45- and 90-degree polarizers with transmission efficiencies exceeding 95% based on the bright electric dipole resonance and dark magnetic dipole resonance. In addition, we utilize a bilayer metallic metasurface (forming an efficient Fabry-Perot resonator) to propose a circularly polarizing dichroism waveplate (CPDW). The circular polarization dichroism (CPD = IRCP - ILCP.) in the transmission mode at 1.6 µm wavelength reaches 89% and the extinction ratio (ER = IRCP/ILCP) is 830:1. These four polarizing elements are integrated to form a full Stokes pixel that almost accurately measures arbitrary polarized light at λ0 = 1.6 µm (including elliptically polarized light).

Self-Powered Flexible Blood Oxygen Monitoring System Based on a Triboelectric Nanogenerator.

Flexible optoelectronics based on inorganic functional components have attracted worldwide attention due to their inherent advantages. However, the power supply problem presents a significant obstacle to the commercialization of wearable optoelectronics. Triboelectric nanogenerator (TENG) technology has the potential to realize self-powered applications compared to the conventional charging technologies. Herein, a flexible self-powered blood oxygen monitoring system based on TENG was first demonstrated. The flexibility of the TENG is mainly due to the inherent properties of polydimethylsiloxane (PDMS) and the continuously undulating surface of crumpled gold (Au) and the rough surface on the electrode and PDMS effectively increased the output performance. The output voltage, output current density, and power density were 75.3 V, 7.4 µA, and 0.2 mW/cm2, respectively. By etching the sacrificial layer, we then derived a flexible blood oxygen and pulse detector without any obvious performance degradation. Powered by the TENG, the detector is mounted onto the thumbnail, from where it detects a stable photoplethysmography (PPG) signal which can be used to calculate the oxyhemoglobin saturation and pulse rate. This self-powered system provides a new way to sustainably monitor physiological parameters, which paves the way for development of wearable electronics and battery-free systems.

Theoretical System of Contact-Mode Triboelectric Nanogenerators for High Energy Conversion Efficiency.

As the rapid expansion of next-generation electronics, portable and efficient energy sources has become one of the most important factors impeding the market development. Triboelectric nanogenerators (TENGs) are a potential candidate for its unsurpassed features. Herein, we deeply analyzed the power and conversion efficiency of contact-mode TENGs considering the whole energy conversion process. Firstly, reaching beyond the conventional analysis, a compressive force was introduced to derive a more versatile motion profile, which provided a better understanding of the working principle of contact-separation process. Then, we deeply analyzed the influence of various parameters on its performance. Especially, the maximum efficiency TENGs can be obtained under optimum force. It is realistic and useful for more efficient TENGs. Furthermore, this research stands a good chance of establishing standards for quantifying the efficiency of TENGs, which lays the basis for the further industrialization and multi-functionization of TENGs technology.

A fully verified theoretical analysis of strain-photonic coupling for quantum wells embedded in wavy nanoribbons.

For optoelectronic devices, an attractive research field involves the flexible adjustment of the band gap in semiconductor quantum well (QW) structures by strain engineering. However, rigid wafer-based technology enables lattice-misfit strain during epitaxial growth, which is biaxial, unchangeable, and not sufficient for the devices fitted on various irregular surfaces. Therefore, exploiting the strain produced by externally deformed configurations offers unique opportunities to continuously and non-defectively tune the QW's band structure. Here, we propose a strategy to induce uniaxially distributed strain in elaborately designed wavy quantum well nanoribbons (QWNRs). Meanwhile, a numerically solved strain-photonic coupling model based on the theory of elasticity and the eight-band k·p method is established to illustrate the strain distribution coupled with the strain-induced band gap shift of the wavy QWNR. The µ-photoluminescence measurements reveal a periodically varied band gap in the QW along the uniaxial tensile direction, which is consistent with the result of theoretical calculations. This model demonstrates the potential application of a wrinkled configuration in arbitrarily controlling and tuning the band gap and thus the optoelectronic performances of quantum well systems.

Metallic metasurfaces for high efficient polarization conversion control in transmission mode.

A high efficient broadband polarization converter is an important component in integrated miniaturized optical systems, but its performances is often restricted by the material structures, metallic metasurfaces for polarization control in transmission mode never achieved efficiency above 0.5. Herein, we theoretically demonstrate that metallic metasurfaces constructed by thick cross-shaped particles can realize a high efficient polarization transformation over a broadband. We investigated the resonant properties of designed matesurfaces and found that the interaction between double FP cavity resonances and double bulk magnetic resonances is the main reason to generate a high transmissivity over a broadband. In addition, through using four resonances effect and tuning the anisotropic optical response, we realized a high efficient (> 0.85) quarter-wave plate at the wavelength range from 1175nm to 1310nm and a high efficient (> 0.9) half-wave plate at the wavelength range from 1130nm to 1230nm. The proposed polarization converters may have many potential applications in integrated polarization conversion devices and optical data storage systems.

Epidermal Inorganic Optoelectronics for Blood Oxygen Measurement.

Flexible and stretchable optoelectronics, built-in inorganic semiconductor materials, offer a wide range of unprecedented opportunities and will redefine the conventional rigid optoelectronics in biological application and medical measurement. However, a significant bottleneck lies in the brittleness nature of rigid semiconductor materials and the performance's extreme sensitivity to the light intensity variation due to human skin deformation while measuring physical parameters. In this study, the authors demonstrate a systematic strategy to design an epidermal inorganic optoelectronic device by using specific strain-isolation design, nanodiamond thinning, and hybrid transfer printing. The authors propose all-in-one suspension structure to achieve the stretchability and conformability for surrounding environment, and they propose a two-step transfer printing method for hybrid integrating III-V group emitting elements, Si-based photodetector, and interconnects. Owing to the excellent flexibility and stretchability, such device is totally conformal to skin and keeps the constant light transmission between emitting element and photodetector as well as the signal stability due to skin deformation. This method opens a route for traditional inorganic optoelectronics to achieve flexibility and stretchability and improve the performance of optoelectronics for biomedical application.

Buckling-Based Method for Measuring the Strain-Photonic Coupling Effect of GaAs Nanoribbons.

The ability to continuously and reversibly tune the band gap and the strain-photonic coupling effect in optoelectronic materials is highly desirable for fundamentally understanding the mechanism of strain engineering and its applications in semiconductors. However, optoelectronic materials (i.e., GaAs) with their natural brittleness cannot be subject to direct mechanical loading processes, such as tension or compression. Here, we report a strategy to induce continuous strain distribution in GaAs nanoribbons by applying structural buckling. Wavy GaAs nanoribbons are fabricated by transfer printing onto a prestrained soft substrate, and then the corresponding photoluminescence is measured to investigate the strain-photonic coupling effect. Theoretical analysis shows the evolution of the band gap due to strain and it is consistent with the experiments. The results demonstrate the potential application of a buckling configuration to delicately measure and tune the band gap and optoelectronic performance.

Active metasurface terahertz deflector with phase discontinuities.

Metasurfaces provide great flexibility in tailoring light beams and reveal unprecedented prospects on novel functional components. However, techniques to dynamically control and manipulate the properties of metasurfaces are lagging behind. Here, for the first time to our knowledge, we present an active wave deflector made from a metasurface with phase discontinuities. The active metasurface is capable of delivering efficient real-time control and amplitude manipulation of broadband anomalous diffraction in the terahertz regime. The device consists of complementary C-shape split-ring resonator elements fabricated on a doped semiconductor substrate. Due to the Schottky diode effect formed by the hybrid metal-semiconductor, the real-time conductivity of the doped semiconductor substrate is modified by applying an external voltage bias, thereby effectively manipulating the intensity of the anomalous deflected terahertz wave. A modulation depth of up to 46% was achieved, while the characteristics of broadband frequency responses and constant deflected angles were well maintained during the modulation process. The modulation speed of diffraction amplitude reaches several kilohertz, limited by the capacitance and resistance of the depletion region. The scheme proposed here opens up a novel approach to develop tunable metasurfaces.

Tunable plasmon-induced transparency in hybrid waveguide-magnetic resonance system.

We present a hybrid waveguide-magnetic resonance system with split ring resonators (SRRs) periodically arranged on top of a waveguide layer. Due to the destructive interference between the electric coupling to the magnetic resonance mode generated in the SRRs and the TE/TM waveguide modes supported by the waveguide layer, double plasmon-induced transparency is obtained at the infrared wavelength. Furthermore, the PIT resonance can be dynamically tuned by the incident angle. An ultranarrow PIT window with an FWHM of 7 nm is observed at the wavelength of 1.448 µm. The group index at the narrow PIT window can reach up to 100. We also demonstrate that the refractive index sensitivity and the figure of merit value can reach up to 640 nm/RIU and 64 in the sensing range, respectively. The proposed hybrid waveguide-magnetic resonance system with a high-quality factor PIT window is promising for efficient optical sensing, optical switching, and slow-light device design.

Tunable coupling-induced transparency band due to coupled localized electric resonance and quasiguided photonic mode in hybrid plasmonic system.

A numerical and theoretical study is presented on the exhibition of tunable narrow band coupled-induced transparency phenomenon in a hybrid waveguide-plasmon system consisting of gold twin nanowires array embedded in a slab waveguide. We show that, at slightly non-normal incidence, a properly designed splitting of transmission with narrow transparency peaks may occur at a given wavelength, depending on the angle of incidence. This leads to the wavelength-selective high quality coupled-induced transparency resonance at optical frequencies. By adjusting the gap distance of the pair gratings, the coupled-induced transparency band can be switched between on-state and off-state, which provides us possibilities to develop controllable plasmonic functional devices employing plasmonic nanostructures.

Hybrid plasmonic waveguide with gain medium for lossless propagation with nanoscale confinement.

In this Letter, we propose a hybrid plasmonic nanosystem consisting of a silver cladding layer with a semicylinder bump on top of InGaAsP nanowire. Because of the coupling between the dielectric waveguide mode and surface plasmon polariton mode, the hybrid plasmonic mode can exhibit low loss with strong field localization. The finite element method numerical simulations are employed to evaluate the performances of the hybrid mode. In order to achieve the lossless propagation of the hybrid mode with the mode area of 0.0058(λ²/4) at 1.55 µm, the material gain of 200 nm × 300 nm InGaAsP nanowire should reach 1223 cm⁻¹.