Metallic Reflector Research Articles

A broadband tunable THz sensor based on graphene metasurface

In this paper, we design a polarization-independent and angle-insensitive wideband THz graphene metamaterial sensor. The proposed metasurface sensor consists of six layers, which are silicon dioxide substrate, metal reflector, air layer, graphene metasurface with microstructure, ion-gel layer and silicon dioxide dielectric layer from bottom to top. The sensor was simulated by using CST Microwave Studio software, and the absorption and sensing performance were analyzed. The results show that the absorption rate is more than 95 % in the range of 1.05–4.07 THz. In addition, when the Fermi energy level of graphene is changed, the absorption rate can be adjusted from 20 % to more than 95 %. At the same time, the absorptivity of the device can be maintained above 80 % before 60° when the TE and TM waves are incident, and it is insensitive to the polarization angle due to the central symmetry of the graphene surface microstructure. By changing the thickness of the air layer, the device can achieve maximum absorption at h2=30μm, where the maximum frequency shift sensitivity is 2.179 THz/RIU. The influence of structure parameters and incident angle on the absorption spectrum shows that the proposed structure has good stability. The proposed metasurface sensor has potential application prospects in biological detection and medical monitoring, broadening the application range of THz functional devices.

Dual band high gain circularly polarized frequency tunable silicon-graphene based MIMO antenna for THz wireless applications

Abstract The two-port aperture linked silicon-graphene radiator in THz regime is investigated in this study. The primary characteristics of the radiator design are as follows: (i) a perturbed circular aperture that offers two working spectrums, 2.85-3.58 THz and 2.31-2.45 THz, with the support of a circular E-field within the two operating bands; (ii) an aperture mirror arrangement that enhances polarization diversity and raises the isolation level to over 25 dB; and (iii) a graphene layer over the ceramic that allows for tunability in the entire working band as well as a circularly polarized frequency range. The broadsided radiation pattern is produced by the dual hybrid mode, or HEM11δ and HEM12δ. A metallic reflector positioned underneath the radiator enhances the dual working spectrum's gain value, or around 7.5 dBi. With the aid of CST-MWS software, the final results are confirmed. The developed radiator may be used for a variety of wireless applications in the THz domain thanks to its stable far-field pattern and strong diversity parameter values.

Light-emitting AlGaAs/GaAs diodes based on ingaas strain-compensated quantum wells with minimized internal losses OF 940 nm radiation absorption

IR light-emitting diodes based on InGaAs/AlGaAs multiple quantum wells and AlxGa1–xAsyP1–y-layers that compensate stresses in the active region have been developed. The optical losses caused by absorption of radiation generated by the active region (λ = 940 nm) were studied at different doping levels of n-GaAs substrates. It has been shown that reducing the donor doping level from 4 × 1018 to 5 × × 1017 cm–3 gives an increase in the quantum efficiency of LEDs by ~ 30%. A technology that eliminates optical losses caused by absorption during radiation output has been developed. By removing the growth substrate and transferring the device structure to a carrier substrate with the formation of a rear metal reflector, LEDs were created that demonstrate a twofold increase in external quantum efficiency and efficiency (~ 40%) compared to the technology of outputting radiation through an n-GaAs substrate.

Feline eye-inspired artificial vision for enhanced camouflage breaking under diverse light conditions.

Biologically inspired artificial vision research has led to innovative robotic vision systems with low optical aberration, wide field of view, and compact form factor. However, challenges persist in object detection and recognition against complex backgrounds and varied lighting. Inspired by the feline eye, which features a vertically elongated pupil and tapetum lucidum, this study introduces an artificial vision system designed for superior object detection and recognition in a monocular framework. Using a slit-like elliptical aperture and a patterned metal reflector beneath a hemispherical silicon photodiode array, the system reduces excessive light and enhances photosensitivity. This design achieves clear focus under bright light and enhanced sensitivity in dim conditions. Theoretical and experimental analyses demonstrate the system's ability to filter redundant information and detect camouflaged objects in diverse lighting, representing a substantial advancement in monocular camera technology and the potential of biomimicry in optical innovations.

Polarization independent perfect absorption of borophene metamaterials operating in the communication band

Two-dimensional (2D) materials, such as graphene and black phosphorus, support deeply confined and tunable plasmons, making them suitable for designing absorbers with ultra-compact size and flexible manipulation. However, the operating frequency of such plasmonic absorbers is difficult to control to the communication band. Here, we propose a metamaterials composed of a borophene array, a dielectric layer and a metal reflector to achieve an optical perfect absorber near the communication wavelength of 1550 nm. In order to overcome the polarization sensitivity caused by anisotropic borophene materials, another borophene layer is introduced to achieve a polarization independent absorber, which can be attributed to the fact that the energy of the electromagnetic field is transferred between two borophene arrays as the polarization angle changes. In addition, through the modulation of carrier density, it is feasible to fine-tune the resonance wavelength of the absorber to 1330 nm, which corresponds precisely to the second communication window. This work may provide a theoretical foundation for the development of polarization independent devices, potentially broadening the scope of their applications.

Design and analysis of Minkowski fractal shaped metasurface absorber with broadband and polarization-insensitive characteristics using a tunable graphene layer for terahertz applications

The proposed research article’s main goal is to demonstrate a terahertz (THz) broadband absorber for high-speed wireless communication applications. The proposed structure is a compact one and possesses three layers. The ground layer acts as a metal reflector, lossy silicon acts as a dielectric material and finally a graphene layer in the shape of a minkowski fractal acts as a radiating patch for the proposed design. We have chosen a thickness of 5 μm for the lossy silicon which has a dielectric constant of 11.90. The bottom layer of the proposed design contains a good conductive material like gold with a conductivity of 4.561 e+007 s m−1 with a thickness of 0.2 μm. The thickness of a monolayer graphene is one nanometer, and the overall unit cell size of the proposed structure is 12 × 12 μm2. Because of its symmetrical nature, the proposed absorber offers a broad response to both TM and TE modes irrespective of any polarization angle. The proposed absorber can operate in the terahertz frequency range and has achieved two broad frequency bands from 3.34–3.98 THz and 4.6–5.30 THz, with an absorption percentage greater than 90. We can observe peak absorption frequencies at 3.60 THz and 5.04 THz, which exhibit an absorption percentage close to unity. Additionally, we validated the proposed broadband absorber using an equivalent circuit approach and verified it using the ADS tool.

Tamm-cavity terahertz detector

Efficiently fabricating a cavity that can achieve strong interactions between terahertz waves and matter would allow researchers to exploit the intrinsic properties due to the long wavelength in the terahertz waveband. Here we show a terahertz detector embedded in a Tamm cavity with a record Q value of 1017 and a bandwidth of only 469 MHz for direct detection. The Tamm-cavity detector is formed by embedding a substrate with an Nb5N6 microbolometer detector between an Si/air distributed Bragg reflector (DBR) and a metal reflector. The resonant frequency can be controlled by adjusting the thickness of the substrate layer. The detector and DBR are fabricated separately, and a large pixel-array detector can be realized by a very simple assembly process. This versatile cavity structure can be used as a platform for preparing high-performance terahertz devices and opening up the study of the strong interactions between terahertz waves and matter.

High-Resolution Modeling Without Computation Slowdown for PETALE in CROCUS

In a collaboration between Ecole Polytechnique Fédérale de Lausanne (EPFL) and CEA, in the fall of 2020, the experimental Programme d’Étude en Transmission de l’Acier Lourd et ses Eléments (PETALE) was successfully carried out in the CROCUS reactor of EPFL. This article presents and compares the methods tested in the modeling of the experiments, specifically focusing on the metal reflectors installed at the periphery of CROCUS. A basic design model consisting of a few cuboids was refined to a fully detailed version, without impacting the run time of simulations. Notably, each reflector sheet of PETALE was segmented into 121 voxels based on topological measurements. This detailed voxelization did not affect calculation times, thanks to the use of three-dimensional lattices as available in Serpent 2. Profiling of the simulations revealed the high computational surface transformations associated with Serpent 2 and highlighted the efficiency benefits of factorizing these into universe transformations. As the CROCUS simulations were carried out using a modified build of Serpent 2, additional simulations were also performed using a standard version of Serpent 2 with a GODIVA model as a neutron source to ensure that the findings are generalizable. These additional tests confirmed the initial results, with significant performance variations observed between the models, particularly larger in surface-tracking mode than in delta-tracking mode. Consequently, the modeling method may therefore be applied to future high-fidelity modeling of neutron transmission and shielding experiments.

Ultrathin absorber with independently tunable dual-resonances for terahertz sensing

A technique is implemented for the generation and alteration of two independently tunable resonance peaks in a graphene based ultrathin absorber structure designed for the terahertz spectrum. The proposed structure is a multi-layered stacked arrangement of two resonator layers of graphene grown over the top of two isolated dielectric (SiO2) substrate layers shielded by a gold metal reflector at the bottom. The structure exhibits very low thickness profile and can be implemented with a thickness equivalent to λ/19.6 where λ is the free space wavelength. Two patterned graphene layers contributes to two independently tunable narrowband/near-perfect absorption peaks at 9.56 THz and 10.96 THz. This polarisation insensitive absorber structure also exhibits frequency ratio tunability of higher order mode with respect to lower order mode by distinctly/separately varying the Fermi level of graphene layers. The response is stable for a wider angle of wave incidence up to 70°. Its narrowband response makes it a suitable candidate to be utilized as biosensor for refractive index sensing.

Coherent optical coupling to surface acoustic wave devices

Surface acoustic waves (SAW) and associated devices are ideal for sensing, metrology, and hybrid quantum devices. While the advances demonstrated to date are largely based on electromechanical coupling, a robust and customizable coherent optical coupling would unlock mature and powerful cavity optomechanical control techniques and an efficient optical pathway for long-distance quantum links. Here we demonstrate direct and robust coherent optical coupling to Gaussian surface acoustic wave cavities with small mode volumes and high quality factors (>105 measured here) through a Brillouin-like optomechanical interaction. High-frequency SAW cavities designed with curved metallic acoustic reflectors deposited on crystalline substrates are efficiently optically accessed along piezo-active directions, as well as non-piezo-active (electromechanically inaccessible) directions. The precise optical technique uniquely enables controlled analysis of dissipation mechanisms as well as detailed transverse spatial mode spectroscopy. These advantages combined with simple fabrication, large power handling, and strong coupling to quantum systems make SAW optomechanical platforms particularly attractive for sensing, material science, and hybrid quantum systems.

Broadband terahertz absorber based on patterned slotted vanadium dioxide

This study aims to explore broadband terahertz absorbers based on metamaterials by introducing a design featuring patterned slotted vanadium dioxide. This absorber structure includes a vanadium dioxide resonator layer, an intermediate Topas dielectric layer, and an underlying metal reflector layer. To enhance the absorption bandwidth, the vanadium dioxide resonant layer is strategically patterned to optimize impedance matching. Simulation outcomes reveal that, under positive incidence, the absorption rate exceeds 90 % within the frequency range of 1.5–4.2 THz. The absorber demonstrates a substantial absorption bandwidth of 2.7 THz, centered at 2.85 THz, corresponding to a relative bandwidth of 94.7 %. Additionally, through modulation of the conductivity of vanadium dioxide, the absorption peak can be finely tuned, spanning from approximately 2.5 %–99 %. In addition, it's angularly symmetrical and exhibits absorption properties for wide-angle incidence. The tunable terahertz absorber designed in this paper simultaneously achieves high absorption over a wide frequency range, is dynamically tunable, and has a simple structure. From the application point of view, it can provide a way to realize detection, and imaging in the terahertz range.

Enhancing light extraction efficiency of the inclined-sidewall-shaped DUV micro-LED array by hybridizing a nanopatterned sapphire substrate and an air-cavity reflector

In this work, we hybridize an air cavity reflector and a nanopatterned sapphire substrate (NPSS) for making an inclined-sidewall-shaped deep ultraviolet micro light-emitting diode (DUV micro-LED) array to enhance the light extraction efficiency (LEE). A cost-effective hybrid photolithography process involving positive and negative photoresist (PR) is explored to fabricate air-cavity reflectors. The experimental results demonstrate a 9.88% increase in the optical power for the DUV micro-LED array with a bottom air-cavity reflector when compared with the conventional DUV micro-LED array with only a sidewall metal reflector. The bottom air-cavity reflector significantly contributes to the reduction of the light absorption and provides more escape paths for light, which in turn increases the LEE. Our investigations also report that such a designed air-cavity reflector exhibits a more pronounced impact on small-size micro-LED arrays, because more photons can propagate into escape cones by experiencing fewer scattering events from the air-cavity structure. Furthermore, the NPSS can enlarge the escape cone and serve as scattering centers to eliminate the waveguiding effect, which further enables the improved LEE for the DUV micro-LED array with an air-cavity reflector.

Wideband Low-Profile Fabry-Perot Cavity Antenna with Metasurface

A novel Fabry-Perot cavity (FPC) antenna with metasurface is presented, which can achieve broad bandwidth and low profile. Traditional FPC antennas, with rectangular microstrip antennas as feeds, have limited impedance bandwidth and struggle to make a compromise in the gain bandwidth and maximum gain value. To obtain wide bandwidth, the FPC antenna proposed in this paper utilizes a feed antenna loaded with parasitic patches. To widen impedance bandwidth and gain bandwidth and reduce the profile, a positive phase gradient partially reflective surface (PRS) and an artificial magnetic conductor (AMC) are located above and below the feed antenna, respectively. The phase property of the PRS and AMC also brings in a more smooth gain value curve. To further increase gain values, four metal reflector plates are located around the proposed antenna. The overall dimension of the antenna is 2.5λ0×2.5λ0×0.25λ0 (λ0 is the free space wavelength at 7.5 GHz). Simulated results show that the resonant cavity antenna proposed in this letter exhibits an impedance bandwidth of 13.3% (7–8 GHz) and a 3 dB gain bandwidth of 14.3% (7.02–8.10 GHz). The maximum gain in the whole operating band is 14.5 dBi. The measured results are in good agreement with the simulated ones.

Bandwidth enhancement of circularly polarised slot global navigation satellite systems antenna using an integrated filter‐antenna approach

Abstract:A technique to enhance the bandwidth of a circularly polarised antenna using the filter‐antenna design approach is presented. The design relies on adding a resonator coupled to the radiation element—an L‐shaped slot antenna in the case of global navigation satellite systems. This change in the feeding structure broadens both the impedance and the axial ratio (AR) bandwidths and allows more design flexibility. Compared with a reference L‐shaped slot antenna with a conventional feed, the proposed antenna design achieves a doubling of the impedance matching bandwidth from 19% to 41% over the frequency range from 1.2 to 1.81 GHz and a wider AR bandwidth from 35% to 44.5% in the range from 1.17 to 1.84 GHz. The AR bandwidth covers the entire impedance bandwidth. The effect of a metal reflector on antenna performance is also investigated and discussed.

High-power and high-efficiency operation of 1.3 µm-wavelength InP-based photonic-crystal surface-emitting lasers with metal reflector.

We demonstrate high-output-power and high-efficiency operation of 1.3-µm-wavelength InP-based photonic-crystal surface-emitting lasers (PCSELs). By introducing a metal reflector and adjusting the phase of the reflected light via optimization of the thickness of the p-InP cladding layer, we successfully achieve an output power of approximately 400 mW with the slope efficiency of 0.4 W/A and the wall-plug efficiency of 20% under CW conditions. In addition, this PCSEL exhibits a narrow circular beam with a divergence angle below 1.6° even at high output powers under CW conditions at temperatures from 15°C to 50°C. We have also demonstrated an output power of over 12 W under pulsed conditions at room temperature.

Design and development of speckle-free high-power laser-driven phosphor converted compact automotive headlamp module

The applicability of diode-lasers in automobile headlights is an advanced innovation for the automobile illumination industry due to the extraordinary properties of laser light over conventional light sources, such as high brightness, wide colour gamut, high directionality, low energy consumptions and long lifetime. Lasers are highly coherent in nature, so they encounter the problem of unwanted speckles and spurious fringes and always require a high level of opto-thermal engineering along with speckle reduction mechanisms for high lumen laser applications. Targeting such challenges, in this paper, we report an innovative design and development scheme for a high lumen laser-based automotive headlamp module. The headlamp prototype comprises a set of four cylindrical diffusers which distribute the high energy laser radiation via scattering along the length of the diffusers within a metallic mirro-based pyramidal cavity reflector. The scattered laser light from cylindrical diffusers interacts with a remote phosphor layer that prevents phosphor–resin burning. The pyramidal cavity reflector plays an important role in making the laser light uniform and speckle-free, via spatial and angular diversity, as light exits from the cavity after multiple internal reflections. This reflector redirects the highly concentrated white light over a long range without using any projection lens. The design and performance of the headlight system was studied using TracePro simulation software and tested experimentally in a photometric laboratory. The International Commission on Illumination (CIE) coordinates of the light generated by the headlamp was (0.3947, 0.4908) and the correlated colour temperature was 4240 K, which represents warm white light illumination.

Research on intelligent identification algorithm of coal and rock strata based on Hilbert transform and amplitude stacking

AbstractThe high precision identification of coal–rock layers is a significant challenge in intelligent mining. There is a large amount of electromagnetic noise and metal reflector signals in the full space detection environment of mining roadway, which makes it hard to distinguish the reflected waves at interface from a set of echo signals generated by the interface due to the similar amplitudes among them. So the method of identifying layers solely based on amplitude characteristics has poor stability and accuracy in coal mining environments. This paper proposes a method for identifying coal–rock layers based on Hilbert transform and tracking–scanning–stacking technology. There are two steps to achieve the recognition of air–coal–rock interfaces. First, by analysing the instantaneous amplitude spectrum obtained from the Hilbert transform, the first extreme point that is always the maximum value within a wavelength range is determined as the rough position of the air–coal interface. To solve the problem of recognition errors caused by noise and energy dispersion, the density difference method is used to remove discrete points. Second, the precise position of the air–coal interface is determined by tracking the extreme points within the 1.5 wavelength range around the rough position, and using the amplitude stacking method to quantitatively analyse and compare the degree of energy concentration. The data between zero time and the reflected waves at the air–coal interface is removed to avoid the impact of them on the recognition of the coal–rock interface. Results of physical model experiments and actual coal mine experiments show that this method yields better results and has high stability compared to conventional recognition method. Moreover, the average relative thickness errors are 4.5% for air layer and 4.2% for coal layer.

Dual‐Function Microwave Devices Based on Metamaterials

AbstractMicrowave detectors play a crucial role in modern technological applications, but traditional detectors suffer from limited functionality, limited detection accuracy, and difficulties in large‐scale deployment. This study proposes a dual‐function microwave detector based on metamaterials, capable of simultaneously measuring microwave power and polarization angle. The detector consists of a metal disc with 12 concave grooves, a dielectric layer, a metal reflector, and a resistive network. The designed detector achieves efficient absorption of 93.54% of microwave energy, with 75.4% dissipated in the resistive component and converted into heat. By detecting the temperature using a thermocouple thermometer, the incoming wave power can be determined, and by discriminating between different resistive temperatures, the polarization angle of the incoming wave can be identified. Simulation and experimental results demonstrate the outstanding detection efficiency and stability of this detector. It exhibits a linear response to different incident power densities and polarization angles, showing potential application value in the field of microwave wireless power transmission.

On-Demand Generation of Indistinguishable Photons in the Telecom C-Band Using Quantum Dot Devices.

Semiconductor quantum dots (QDs) enable the generation of single and entangled photons, which are useful for various applications in photonic quantum technologies. Specifically for quantum communication via fiber-optical networks, operation in the telecom C-band centered around 1550 nm is ideal. The direct generation of QD-photons in this spectral range with high quantum-optical quality, however, remained challenging. Here, we demonstrate the coherent on-demand generation of indistinguishable photons in the telecom C-band from single QD devices consisting of InAs/InP QD-mesa structures heterogeneously integrated with a metallic reflector on a silicon wafer. Using pulsed two-photon resonant excitation of the biexciton-exciton radiative cascade, we observe Rabi rotations up to pulse areas of 4π and a high single-photon purity in terms of g(2)(0) = 0.005(1) and 0.015(1) for exciton and biexciton photons, respectively. Applying two independent experimental methods, based on fitting Rabi rotations in the emission intensity and performing photon cross-correlation measurements, we consistently obtain preparation fidelities at the π-pulse exceeding 80%. Finally, performing Hong-Ou-Mandel-type two-photon interference experiments, we obtain a photon-indistinguishability of the full photon wave packet of up to 35(3)%, representing a significant advancement in the photon-indistinguishability of single photons emitted directly in the telecom C-band.

Laser Dynamic Control of the Thermal Emissivity of a Planar Cavity Structure Based on a Phase-Change Material.

Tailoring the thermal emission of a material in the long-wave infrared (IR) range of 8-13 μm is crucial for many IR-adaptive applications, including personal thermal management, IR camouflage, and radiative cooling. Although various materials and surface structures have been proposed for these purposes, space-selective and dynamic control of their emissivity is challenging. In this study, we present a planar surface cavity structure consisting of a Ge2Sb2Te5 (GST) film on top of a thin metal reflector to modulate its emissivity by using an ultraviolet laser beam. A laser-induced phase change in GST allowed for the local control of emissivity. The average emissivity in the long-wave IR range was tunable from 0.15 to 0.77 simply by changing the laser energy deposited on the GST film. This enabled the laser printing of high-contrast emissivity patterns, which were erasable by subsequent thermal annealing. Emissivity-modulated GST cavities could be fabricated on not only rigid substrates but also flexible plastic substrates such as polyimide. The GST surface cavity was highly flexible and remained stable upon repeated bending to a curvature radius of 0.5 cm. This study provides a promising route for realizing scalable and flexible thermal emitters with tunable surface emissivity.