Metallic Reflector Research Articles

Light management design in ultra-thin chalcopyrite photovoltaic devices by employing optical modelling

In ultra-thin chalcopyrite solar cells and photovoltaic modules, efficient light management is required to increase the photocurrent and to gain in conversion efficiency. In this work we employ optical modelling to investigate different optical approaches and quantify their potential improvements in the short-circuit current density of Cu(In, Ga)Se2 (CIGS) devices. For structures with an ultra-thin (500 nm) CIGS absorber, we study the improvements related to the introduction of (i) highly reflective metal back reflectors, (ii) internal nano-textures applied to the substrate and (iii) external micro-textures by using a light management foil. In the analysis we use CIGS devices in a PV module configuration, thus, solar cell structure including encapsulation and front glass. A thin Al2O3 layer was considered in the structure at the rear side of CIGS for passivation and diffusion barrier for metal reflectors. We show that not any individual aforementioned approach is sufficient to compensate for the short circuit drop related to ultra-thin absorber, but a combination of a highly reflective back contact and textures (internal or external) is needed to obtain and also exceed the short-circuit current density of a thick (1800 nm) CIGS absorber.

High rear reflectance and light trapping in textured graphene based silicon thin film solar cells with back dielectric-metal reflectors

In this paper, we report the design of a graphene/silicon thin film solar cell with a novel array of textured dielectric-metal rear side reflectors. In order to minimize the surface recombination and parasitic absorption losses and to increase rear reflections, a unique design of rear side of solar cell reflectors is explored. Our proposed structure geometry has achieved extreme light trapping ability, maximum rear reflections, and high inner scattering, resulting in absorption of up to 90% at a 40° angle of incidence when 1 nm thick graphene is used on the top and textured SiO2-Ag was used as the dielectric-metal back reflector. Contrasted with the analogous reference cell devices, the light absorption in the proposed textured solar cell with a back dielectric-metal reflector is essentially improved in the visible to infrared region from 600 nm to 1200 nm with maximum achieved inner rear reflectance >89% and an attained absorption in the absorber layer from 80% to 90%. The electromagnetic field propagation, reflection, and transmission are calculated by using 2D Maxwell’s and Fresnel equations discretised by the finite element method (FEM). Different configurations, with a plane back reflector/textured dielectric-metal reflector, varied dielectric material/back metal reflector material/texture profile are investigated and reported to attain the best structure configuration. Improved light trapping in the absorber layer and an increase in rear reflection angle (rr(θ)) and photon absorption are accomplished under standard solar irradiation spectrum AM1.5 conditions. The proposed design would have significant impact in promoting optimum solar cell layer assembly with a high light trapping response in a wide variety of silicon thin film solar cells.

Development of a Wireless Surface Acoustic Wave Methane Sensor Using a Metal-Organic Framework As a Sensing Layer for Natural Gas Pipelines

Methane emission from the processing and transportation of fossil fuels can create serious environmental and safety issues as well as a financial burden. The need to monitor the energy infrastructure creates an opportunity for developing low-cost and robust sensors with wireless options. Wireless surface acoustic wave (SAW) devices, which are highly sensitive, cost-efficient, and passive, are a potential candidate for these applications. Nanoporous metal-organic frameworks (MOFs), which can selectively adsorb large amounts of gases in engineered pores at ambient conditions, are potential sensing layers. Here, we present on the development of a wireless and passive SAW sensor using the MOF zeolitic imidazolate framework-8 (ZIF-8) as the sensing layer for methane detection at ambient conditions. The SAW delay lines (nominal operating frequency = 436 MHz) were custom fabricated on Y-Z LiNbO3 piezoelectric substrates and coated with a 500 nm thick ZIF-8 layer using a dip coating method. The devices were excited by 428 MHz RF pulses to test for various concentrations of CO2 and CH4 in N2 background at a constant gas flow rate. Wireless measurements were performed via dipole antennas of nominal resonant frequency 433 MHz. When the sensor was exposed to the gases, surface acoustic wave phase shifted in a negative direction relative to the N2 baseline. This shift was attributed to the decrease in SAW velocity due to the increased mass of the MOF film upon gas adsorption. The CO2 and CH4 sensitivities extracted as the slopes of the respective calibration curves were 1.26 deg/vol-% and 0.03 deg/vol-%, respectively. It is important to note that currently tested minimum concentrations of CO2 and CH4 were 1% and 20%, respectively. Also, the wireless measurements were highly affected by the housing and metal reflectors around it. The demonstration on wireless methane sensing is of great importance for remote monitoring of methane emission sites and transmission pipelines. Our current investigations are focused in advancing the detection limit for leak detection and addressing the electromagnetic interferences in wireless measurement caused by the housing.

A Circularly Polarized High-Gain Planar 2 × 2 Dipole-Array Antenna Fed by a 4-Way Gysel Power Divider for WLAN Applications

In this letter, a circularly polarized high-gain planar 2 × 2 dipole-array antenna for wireless local area networks, radio frequency identification, radar, and satellite communication applications is proposed. The proposed array antenna consists of four planar dipole antennas with integrated microstrip-to-parallel-stripline linear tapered balun and feeding network. The designed array antenna is fed by a modified 4-way Gysel power divider. The proposed antenna uses three stacked metal layers, and a metal reflector under the antenna boosts the overall antenna gain. The size of the designed antenna on 3.2 mm thick FR4 substrate was 150 mm × 150 mm × 20 mm including the reflector. The designed antenna had left-hand circular polarization (LHCP), and its measured LHCP gain value was 8.70–9.62 dBic at 2.4–2.6 GHz band. The measured axial ratio was 0.3–2.2 dB at the same frequency band of interest. A thorough theoretical analysis and design equations of the proposed array antenna are presented in this letter.

Highly efficient GaN-based high-power flip-chip light-emitting diodes

High-power flip-chip light-emitting diodes (FCLEDs) suffer from low efficiencies because of poor p-type reflective ohmic contact and severe current crowding. Here, we show that it is possible to improve both the light extraction efficiency (LEE) and current spreading of an FCLED by incorporating a highly reflective metallic reflector made from silver (Ag). The reflector, which consists of an Ag film covered by three pairs of TiW/Pt multilayers, demonstrates high reflectance of 95.0% at 460 nm at arbitrary angles of incidence. Our numerical simulation and experimental results reveal that the FCLED with Ag-based reflector exhibits higher LEE and better current spreading than the FCLED with indium-tin oxide (ITO)/distributed Bragg reflector (DBR). As a result, the external quantum efficiency (EQE) of FCLED with Ag-based reflector was 6.0% higher than that of FCLED with ITO/DBR at 750 mA injection current. Our work also suggests that the EQE of FCLED with the Ag-based reflector could be further enhanced 5.2% by replacing the finger-like n-electrodes with three-dimensional (3D) vias n-electrodes, which spread the injection current uniformly over the entire light-emitting active region. This study paves the way towards higher-performance LED technology.

Electrically Tunable Metal–Semiconductor–Metal Terahertz Metasurface Modulators

We propose metal-semiconductor-metal cavity arrays as active elements of electrically tunable metasurfaces operating in the terahertz spectrum. Their function is based on reverse biasing the Schottky junction formed between top metal strips and the n-type semiconductor buried beneath. A gate bias between the strips and a back metal reflector controls the electron depletion layer thickness thus tuning the Drude permittivity of the cavity array. Using a rigorous multiphysics framework which combines Maxwell equations for terahertz waves and the drift-diffusion model for describing the carrier behavior in the semiconductor, we find a theoretically infinite extinction ratio, insertion loss of around 10%, and picosecond intrinsic switching times at 1 THz, for a structure designed to enter the critical coupling regime once the depletion layer reaches the bottom metal contact. We also show that the proposed modulation concept can be used for devices operating at the higher end of the terahertz spectrum, discussing the limitations on their performance.

Ultrawideband high‐gain dual‐polarized antenna with anti‐interference capability

In this article, a high-gain dual-polarized antenna with band-rejection capability for ultrawideband (UWB) applications is proposed. Tapered dipoles are chosen as a primary radiator to achieve UWB operation and it is reflected by a metallic cavity reflector for high gain radiation. A notch at WLAN band is realized by etching a set of four bent slots in the radiating elements. The measured results demonstrate that the proposed design with overall dimensions of 0.69λ L × 0.69λ L × 0.16λ L (λ L is free-space wavelength at the lowest operating frequency) has operating bandwidth of 95.1% (3.2-9.0 GHz) and the rejected frequency band from 5.0 to 5.9 GHz. Additionally, good unidirectional radiation patterns with a broadside gain from 8.1 to 11.5 dBi and radiation efficiency of better than 90% are also achieved.

A circularly polarized wide-band magneto-electric dipole antenna with simple structure for BTS applications

A circularly polarized magneto-electric dipole antenna with wide frequency band for base transceiver station (BTS) is presented. The structure of the proposed antenna is composed of a cavity metallic reflector with defected side walls, two pairs of vertical and trapezoidal horizontal copper plates and a Г-shaped feed line. Defected side walls of the metallic reflector are utilized to improve the gain, front to back ratio (FBR) and impedance matching. Electric dipole and quasi magnetic dipole are realized with horizontal plates and shorted vertical plates, respectively. The trapezoidal horizontal plates are employed to obtain circular polarization (CP) characteristic with 3-dB axial ratio bandwidth of 41% from 1.7 to 2.6 GHz. Experimental results show that the proposed antenna operates at frequency range of 1.4–2.8 GHz with S11 < −10 dB and 3-dB axial ratio of 1.8 dB and 2.1 dB for frequencies 1.9 GHz and 2.1 GHz, respectively. Furthermore, half-power beamwidth (HPBW) wider than 60.5° and the gain higher than 6.2 dBi are realized.

High‐gain dual‐polarized antenna for ultrawideband applications

In this article, a high-gain and dual-polarized antenna with UWB operation is proposed. The antenna is composed of two tapered dipoles as radiating elements, which are arranged orthogonally and fed perpendicularly to achieve polarization diversity. A metallic cavity reflector is placed behind the radiator for high gain radiation entire the operating bandwidth. To validate the design method, an antenna prototype is designed, fabricated, and measured. The measured results demonstrate that the proposed design has good performance with |S11| ≤ −10 dB and isolation ≥20 dB over a frequency band 3.2-8.8 GHz, equivalently to about 93.3%. In addition, unidirectional radiation pattern and broadside gain of from 8.1 to 11.8 dBi are obtained across the operating bandwidth.

Atomically thin planar metasurfaces

Enhancing light absorption in two-dimensional (2-D) materials using simple planar structures is important for the development of large-area atomic-scale photonic devices but also has been a major challenge. We theoretically present a general strategy to create an atomically thin planar metasurface consisting of a semiconductor monolayer (replacing the array of subwavelength elements in classical metasurfaces), a transparent spacer, and a metallic back reflector, to achieve near-perfect, polarization-sensitive absorption in 2-D materials. This strategy is validated by experimentally demonstrating planar metasurfaces based on atomically thin Ge films. Higher than 90% exclusive absorption, 2.5 times greater than prior demonstration, is achieved in the MoS2 monolayer for TE polarization. Our strategy can be extended to the infrared spectrum and thus has potential for thermal energy applications.

Composite graphene-metal microstructures for enhanced multiband absorption covering the entire terahertz range

We numerically demonstrate a novel route to effectively enhancing multi-band terahertz absorption enabled by a variety of tunable polarization-insensitive multiband terahertz absorbers based on composite graphene and metal microstructures. In these devices, the multiband plasmon resonance absorption in graphene, resulted from the Fabry–Pérot cavity between the continuous graphene and the underneath metal reflector, can be effectively enhanced by the designed metal microstructures. As a demonstration, we simulate several multiband absorbers based on composite graphene and several patterned metal microstructures (spiral, ring, disk, square). It is interesting to find that the number of absorption bands can be arbitrarily manipulated by the dielectric spacer hight. By setting the same spacer height of 60 μm, all absorbers exhibit identical absorption properties with six near-unity absorption bands in the whole terahertz region ranging from 0.1 to 10.0 THz under normal incidence regardless of the shapes of microstructures. These absorption bands also show clear independence of polarization under normal incidence and the absorbance peaks can be flexibly adjusted by changing the graphene chemical potential. Our work opens up a new avenue for the development of various multiband graphene absorbers, which may have enormous potential applications in terahertz photoelectric detectors, sensors, modulators, and switches.

Dual circularly polarized loop antenna using a pair of resonant even‐modes

Conceptual design of dual circularly polarized (CP) square loop antennas using a pair of resonant even-modes for wireless sensor application is presented. A pair of even-modes is simultaneously excited within a single, nonuniform square loop element and employed to realize a dual circular polarization characteristic. When the natural boundary conditions of the even-modes and the feed lines are matched, respectively, a bidirectional dual CP loop antenna can be attained at first. Then, a unidirectional dual CP loop antenna is designed by introducing a simple metallic reflector. Both bidirectional and unidirectional designs exhibit dual CP performance with available operation bandwidth of 10.0% and 8.5%. The design approach is expected to get applications in dual CP antenna designs for wireless sensor systems.

Influence of light absorption on the metallic nanotextured reflectors of GaN-based light emitting diodes

Metallic nanotextured reflectors have been widely used in light emitting diodes (LEDs) to enhance the light extraction efficiency. However, the light absorption loss for the metallic reflectors with nanotexture structure is often neglected. Here, the influence of absorption loss of metallic nanotextured reflectors on the LED optoelectronic properties were studied. Two commonly used metal reflectors Ag and Al were applied to green GaN-based LEDs. By applying a Ag nanotextured reflector, the light output power of the LEDs was enhanced by 78% due to the improved light extraction. For an Al nanotextured reflector, however, only a 6% enhancement of the light output power was achieved. By analyzing the metal absorption using finite-difference time-domain (FDTD) and the metal reflectivity spectrum, it is shown that the surface plasmon (SP) intrinsic absorption of metallic reflectors with nanotexture structure play an important role. This finding will aid the design of the high-performance metal nanotextured reflectors and optoelectronics devices.

Design and Simulation of the Bifacial III-V-Nanowire-on-Si Solar Cell

ABSTRACTRigorous coupled wave analysis (RCWA) simulation was used to model the absorption in periodic arrays of GaAs0.73P0.27 nanowires (NWs) on Si substrates dependent upon the diameter (D), length (L), and spacing (center-to-center distance, or pitch, P) of the NWs. Based on this study, two resonant arrangements for a top NW array sub-cell having the highest limiting short-circuit current densities (Jsc) were found to be close to D = 150 nm, P = 250 nm and D = 300 nm, P = 500 nm, both featuring the same packing density of 0.28. Even though a configuration with thinner NWs exhibited the highest Jsc = 19.46 mA/cm2, the array with D = 350 nm and P = 500 nm provided current matching with the underlying Si sub-cell with Jsc = 18.59 mA/cm2. Addition of a rear-side In0.81Ga0.19As nanowire array with D = 800 nm and P = 1000 nm was found to be suitable for current matching with the front NW sub-cell and middle Si. However, with thinner and sparser In0.81Ga0.19As NWs with D = 700 nm and P = 1000 nm, the Jsc of the bottom sub-cell was increased from 17.35 mA/cm2 to 18.76 mA/cm2 using a planar metallic back surface reflector, thus achieving a current matching with the top and middle cells.

A Miniaturized 2*2 Antenna Array

Based on the research of the transmission line model of microstrip power splitter, based on the traditional T-type microstrip power splitter, a new symmetric T-type four-way microstrip power splitter is proposed, in which the front-end power split port. The phase difference is 180 degree, and the power distribution characteristics are good. By loading a rectangular patch microstrip antenna at each port of the power splitter and etching the metamaterial structure on the floor. A miniaturized 2*2 antenna array operating at 2.45 GHz was designed. The antenna size is 40*40mm, placed on a metal reflector, and its forward gain is about 8dB in the wide range of 2.1-2.7GHz.

Design of Polarization Reconfigurable Antenna Based on Metasurface and Metallic Reflector

In this paper, a polarization reconfigurable antenna is proposed, which is composed of a metasurface, a slot antenna and a metallic reflector. The reconfiguration of polarization can be achieved easily by rotating the metasurface which is made up of 16 identical patched. The slot antenna adopts a double slot structure for extending its S11 bandwidth. Moreover, the axial ratio bandwidth could be reconfigured by changing the distance among the metasurface, slot antenna and metallic reflector. Simulation results show that the antenna could operate in linear polarization, left-hand and right-hand circular polarizations from 4GHz to 5GHz(22.2%). The maximum gain could achieve 8.46dBi.

Ultra-Light Reduced Graphene Oxide Based Aerogel/Foam Absorber of Microwave Radiation.

We present the polarization-dependent highly absorptive in Ka-band composition of conventional polyurethane foam filled with in situ synthesized aerogel coated by reduced graphene oxide (rGO). The rGO-based aerogel was in situ prepared into the open-cell polyurethane foam (PUF) skeleton through a bidirectional freeze-drying process. The aerogel is composed of the flat lamellas stacks, possessing the anisotropic structure and unique electromagnetic properties. Further improvement of the electromagnetic shielding ability was possible by the rGO coating introduction as a coupling layer between PUF and rGO-based aerogel. This enhances the overall conductivity of the resulting composites: 1.41 + 3.33i S/m vs. 0.9 + 2.45i S/m for PUF loaded with in situ synthesized aerogel without rGO coating.With this mechanically robust plane easy to process coating one could achieve −20 dB by power with the record light structure (0.0462 g/cm2). That could compete in view of the weight per cm2 even with graphene-based absorbers comprising either dielectric matching elements or back metal reflectors, or both.

A Simplified Model for Characterizing the Effects of Scattering Objects and Human Body Blocking Indoor Links at 28 GHz

This paper presents a simple approach to characterize the effects of scattering objects around indoor links at 28 GHz while the link is fully blocked by a human body. The effects of scattering objects nearby the link were studied by conducting measurements with a metallic reflector and the human body. Here, the basic mechanisms of wave propagation, such as reflection and diffraction, were accounted for each scattering object. To predict the attenuation caused by the metallic reflector, a specular reflection model was employed in reflection modeling. In diffraction modeling, on the other hand, the double knife-edge diffraction (DKED) model was exploited to predict the attenuation by the human body. Simulations were then compared with measurements to evaluate the prediction accuracy of the models. Results indicate that the presented simple models work well for indoor links. Therefore, the results of this paper could be extended to model multiple human bodies near the indoor links of fifth generation (5G) systems.

Generating Multiple OAM Based on a Nested Dual-Arm Spiral Antenna

In this paper, a nested dual-arm spiral antenna (NDASA) is proposed to generate multiple orbital angular momentum (OAM) modes at the same operating frequency. The NDASA consists of the feeding network and three nested dual-arm spiral structures with different sizes. Meanwhile, a metal reflector is located between the spiral structures and the feeding network to enhance the radiation directivity, which can also reduce the impact of the radio frequency (RF) coaxial cables. Both the experimental and simulated results show OAM waves with topological charges of $l = 1$ , $l = 3$ and $l = 5$ can be independently generated at $f = 3$ GHz, and their maximum gains are 6.94dBi, 6.76dBi and 5.49dBi, respectively. In addition, the OAM modes multiplexing and demultiplexing technology are investigated in simulations and experiments. The isolations between the OAM modes of $l = 1$ , $l = 3$ and $l = 5$ and the other different OAM modes are greater than 13.8dB, 13.2dB and 25.1dB, respectively, which proves the potential ability of the proposed NDASA for multiplexing and demultiplexing multiple OAM modes.

Development of antenna arrays with a beam width 33 degrees at the level of half power

A new design of the antenna array for mobile base stations providing two orthogonal polarizations in 1.71-2.17GHz frequency band is proposed. The antenna array includes 16 radiators, 12 power dividers and two phase shifters disposed on a metal reflector having dimensions 320×1387 mm. The developed antenna array contains less radiators and power dividers than other known antennas of this kind therefore one is cheaper for production. The fabricated sample of the proposed antenna array has return loss is better than -18.5dB and isolation better than 33dB. Beam width in the horizontal plane is equal 36 ± 4 degrees and side lobes level doesn’t exceeding -20dB. The antenna array provides gain=20.5dBi.