Wide Incident Angle Research Articles

Ultra-Broadband Infrared Metamaterial Absorber for Passive Radiative CoolingSupported by the National Natural Science Foundation of China (Grant Nos. 52022018 and 52021001), and the Program for Changjiang Scholars and Innovative Research Team in University.

Infrared metamaterial absorber (MMA) based on metal-insulator-metal (MIM) configuration with flexible design, perfect and selective absorption, has attracted much attention recently for passive radiative cooling applications. To cool objects passively, broadband infrared absorption (i.e. 8–14 μm) is desirable to emit thermal energy through atmosphere window. We present a novel MMA composed of multilayer MIM resonators periodically arranged on a PbTe/MgF2 bilayer substrate. Verified by the rigorous coupled-wave analysis method, the proposed MMA shows a relative bandwidth of about 45% (from 8.3 to 13.1 μm with the absorption intensity over 0.8). The broadband absorption performs stably over a wide incident angle range (below 50°) and predicts 12 K cooling below ambient temperature at nighttime. Compared with the previous passive radiative coolers, our design gets rid of the continuous metal substrate and provides an almost ideal transparency window (close to 100%) for millimeter waves over 1 mm. The structure is expected to have potential applications in thermal control of integrated devices, where millimeter wave signal compatibility is also required.

Design and analysis of an ultra-thin polarization-insensitive wide-angle triple-band metamaterial absorber for X-band application

This paper presents an ultra-thin triple-band metamaterial absorber (TMMA) for the X-band which is insensitive to polarization and has high absorbance to wide incident angles. The TMMA is a three-layer structure based on the combined resonator which is made of a slotted rhomboid ring and a double cross pattern structure. The calculation indicates that the proposed absorber has three absorption peaks at 7.48 GHz, 9.70 GHz and 11.27 GHz with absorption rates 99.88%, 99.99% and 99.97% respectively. The TMMA’s structure is simple and ultra-thin, its thickness is only 0.0258 λ (the wavelength at the minimum absorption frequency can be expressed as λ). Also, it has the characteristics of being insensitive to the polarization of incident waves and it attains high absorbance to wide incident angle. The distribution of electric field, magnetic field and surface current in the TMMA is discussed, and the absorption mechanism of the proposed TMMA is expounded. Experiment results indicate that theoretical calculations and simulations are effective. The MMA can be used in broadcasting, communications, meteorology and earth observation satellites.

Gradient 3D-printed honeycomb structure polymer coated with a composite consisting of Fe3O4 multi-granular nanoclusters and multi-walled carbon nanotubes for electromagnetic wave absorption

Electromagnetic wave radiation has adverse health effects on humans and can hamper nearby electronic devices’ regular operations. Therefore, the need to develop new materials with a substantial wave absorption property, a broad absorption bandwidth in a wide incident angle, and lightweight is of enormous technological interest worldwide. Here we report the experimental frequency-dependent permittivity and permeability behaviors of the composite consisting of Fe3O4 multi-granular nanoclusters (MGNCs) and multi-walled carbon nanotubes (MWCNTs). We also apply the results for the starting point for the gradient honeycomb structure design and optimization. We study the effect of the tilted angle on the reflection loss (RL) by finite element analysis (FEM). We obtain a flat RL over the X-band for the regular honeycomb structure with a 0° tilted angle. An increase in the tilted angle from 0° to 4° increases the RL, and a peak is formed at the maximum tilted angle of 4°. Coating the regular honeycomb structure with a thin layer of MGNC/MWCNT enhances the electromagnetic shielding (EMI). Furthermore, as tilted angle increases for the coated honeycomb structure from 0° to 4°, SE increases dramatically over the frequency range from 8.2 to 12.4 GHz at incident wave angles of 0° and 90°.

High Performance Wide Angle DBR Design for Optoelectronic Devices

To improve the performance of optoelectronic devices, a high performance wide angle SiO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> /Ti <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> O <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">5</sub> Distributed Bragg Reflector (DBR) mirror was designed and fabricated on double polished sapphire substrate. 17-pairs gradually varied DBR structure was applied in order to acquire a broad stopband reflectivity over 95% from 400 nm to 800 nm, covering the whole visible light region. The DBR mirrors deposited at the backside of optoelectronics devices are also demonstrated to remain a high reflectivity at wide incident angles, which improves the emission efficiency of LEDs and light absorption efficiency of InGaN/GaN photoelectrochemical cell (PEC).

Simple design of efficient broadband multifunctional polarization converter for X-band applications

A simple design of a broadband multifunctional polarization converter using an anisotropic metasurface for X-band application is proposed. The proposed polarization converter consists of a periodic array of the two-corner-cut square patch resonators based on the FR-4 substrate that achieves both cross-polarization and linear-to-circular polarization conversions. The simulated results show that the polarization converter displays the linear cross-polarization conversion in the frequency range from 8 to 12 GHz with the polarization conversion efficiency above 90%. The efficiency is kept higher than 80% with wide incident angle up to 45°. Moreover, the proposed design achieves the linear-to-circular polarization conversion at two frequency bands of 7.42–7.6 GHz and 13–13.56 GHz. A prototype of the proposed polarization converter is fabricated and measured, showing a good agreement between the measured and simulated results. The proposed polarization converter exhibits excellent performances such as simple structure, multifunctional property, and large cost-efficient bandwidth and wide incident angle insensitivity in the linear cross polarization conversion, which can be useful for X-band applications. Furthermore, this structure can be extended to design broadband polarization converters in other frequency bands.

3D rampart-based dual-band metamaterial absorber with wide-incident-angle stability

We propose a dual-band metamaterial absorber (MA) based on a stereo resonant structure. The unit cell of the proposed MA is composed of double metallic rings with a rampart on each ring into a dielectric substrate. In contrast to the conventional plane resonator, our proposed metamaterial exhibits wider incident angle stability. The power loss density distribution shows that the standing ramparts play a key role in enhancing the electromagnetic wave absorption. Experimental results verify the wide-incident-angle stability of the absorber. Our proposed strategy paves a new way on developing MAs with wide incident angle stabilities.

Graphene-based dual-band tunable perfect absorber in THz range

ABSTRACT We design and simulate a tunable dual-band perfect absorber. This absorber is composed of a circular graphene layer with an ellipse cut out in the centre, separated by a dielectric layer in the middle and a gold mirror at the bottom. Results reveal that there exist two perfect absorption peaks in the terahertz band. The position of the absorption peak can be changed by adjusting the Fermi level and geometric parameters of the graphene pattern. Furthermore, the absorber is insensitive to the polarized incident light with a wide incident angle. Functionalities are achieved based on a simple and symmetrical single-layer graphene pattern, which can greatly reduce the manufacturing costs. The proposed absorber can be applied in the field of graphene-based photonic devices, such as detectors, sensors, and filters.

Broadband absorption of infrared dielectric resonators for passive radiative cooling

Infrared resonator metamaterials, exhibiting spectral selective absorption of light, have recently been of great interest for passive radiative cooling. In this approach, coolers radiate power passively, with emissivity equals to its absorptivity. However, high-efficient energy dissipation requires radiation windows of broad bandwidth, i.e. 8–14 μm, that challenges the narrow-band feature of resonators. Here, we numerically realize the broadband absorption (above 80% in 8–11 μm) for dielectric resonators over a wide incident angle range (0°–60°), predicting 12 °C cooling below the ambient temperature at nighttime. The electromagnetically resonating eigenmodes of the dielectric resonators are perturbated by losses to reshape the dispersion relation in radiation windows, which are implemented by two methods: inserting metal components and coating lossy dielectrics. Retrieval constitutive parameters, as well as S parameters, map the reshaping process for broadband considerations. Additionally, ideal transparency (nearly 100%) above the wavelength of 0.5 mm is achieved. These crucial features offer an effective solution to the microwave signals shielding problem generally encountered in previous radiative coolers, regarding applied scenes such as buildings and cars.

Controllable Terahertz Switch Using Toroidal Dipolar Mode of a Metamaterial

We present a controllable terahertz (THz) metamaterial switch by manipulating toroidal dipolar mode in simulation. The metamaterial switch consists of periodically patterned metallic split rings with photosensitive silicon stripes. The excitation of toroidal dipolar resonance is closely dependent on the transition between the dielectric and conductive phases of photosensitive silicon. The toroidal dipolar mode is exploited to modulate the ON/OFF-switching transmission of THz wave under wide incident angles. The operation mechanism and frequency tunability are discussed.

Dual-Band Plasmonic Perfect Absorber Based on the Hybrid Halide Perovskite in the Communication Regime

Due to the weak absorption of (CH3NH3)PbI3 in the communication regime, which restricts its optoelectronic applications, we design a adjustable dual-band perfect absorber based on the (CH3NH3)PbI3 to significantly enhance its absorption capability. Since the localized plasmon (LP) mode and surface plasmon (SP) mode are excited in the structure, which can both greatly enhance light absorption of the (CH3NH3)PbI3 layer, dual-band perfect absorption peaks are formed in the communication regime, and the absorption of (CH3NH3)PbI3 layer is increased to 43.1% and 64.2% at the dual-band absorption peaks by using finite-difference time-domain (FDTD) methods, respectively. By varying some key structural parameters, the dual-band absorption peaks of (CH3NH3)PbI3 can be separately shifted in a wide wavelength region. Moreover, the designed absorber can keep good performance under wide angles of incidence and manifested polarization correlation. Furthermore, not just for (CH3NH3)PbI3, the physical mechanism in this absorber can also be utilized to strengthen the absorption of other halide perovskites.

Generalized Kerker's conditions under normal and oblique incidence using the polarizability tensors of nanoparticles.

The polarizability tensors of a particle are its characteristic parameters, which once obtained, can be applied as equivalent representations of the particle in any problems involving plane wave illuminations. In this paper, the generalized Kerker's conditions for unidirectional scattering are derived, in the case of normal and oblique incidence, in terms of the polarizability tensors of any arbitrary nanostructures in homogeneous media and located on dielectric substrates. In order to present structures that corroborate the conditions derived from such polarizabilities, first, the effect of constituent material on the frequency response of the nanoparticle is investigated. Then, the dimensions of nanostructures that satisfy the first and second Kerker's conditions are evaluated, while it is also ascertained that by varying the excitation wavelengths in an individual nanoparticle, switching between forward and backward unidirectional scattering can be achieved. This creates numerous attractive possibilities for the manipulation of optical pressure forces. Moreover, the influence of impinging direction upon the forward-to-backward scattering ratio is studied. Since, in many applications, nanoparticles are situated on dielectric substrates to make the structures more practically feasible, in this work, the effect of substrates on the Kerker's conditions are evaluated. It is shown that the presence of a substrate adds new dimensions of polarizability to the structure. Despite this new polarizability, two structures are engineered, here, which create strong asymmetrical scattering over a wide frequency range and wide angle of incidence.

Reflectivity reduction of nanopatterned c-Si solar cells with antireflective coatings exposed to a wide range of incidence angles

The scope of the present study is to assess through numerical calculations the effect of nanopatterning the photovoltaics (PV) and that of imposing additional superstrate layers to the efficiency of the PV’s, under normal and wide incidence angles. The numerical calculations were performed with the Rigorous Coupled-Wave Analysis (RCWA) method where we examined several types of Si wafers including additional Si3N4 and SiO2 superstrate layers, and periodic patterns of grooves affecting either the superstrates or both the superstrates and Si substrate. A very broad parameter space was investigated (thickness of the superstrates, and the depth, diameter and periodicity constant of hole and pillar-like patterns) from which we derived the optimal set of parameters for each wafer type. The performance of the optimized wafers was then evaluated over a broad range of incident angles. Both the deposition of the superstrates as well as the imposition of periodic patterns improve drastically the absorption efficiency of the wafers and they allow for maintaining the maximum efficiency across the high incidence angle regime.

Multi-functional polarization conversion manipulation via graphene-based metasurface reflectors

In this work, we present an efficient polarization conversion device via using a hollow graphene metasurface. The platform can simultaneously realize a series of excellent performances, including the broadband x-to-y cross polarization conversion (CPC) function with near unity polarization conversion ratio (PCR), dual-frequency linear-to-circular polarization conversion (LTC-PC) function, and highly sensitive polarization conversion function manipulation under wide oblique incidence angle range. For instance, the proposed device obtains an x-to-y CPC function with the bandwidth up to 1.83 THz (χPCR ≥98.8%). Moreover, the x-to-y CPC function can be switched to LTC-PC function via artificially tuning the Fermi energy of graphene. The maximal frequency shift sensitivity (S) of polarization conversion function reaches 23.09 THz/eV, suggesting a frequency shift of 2.309 THz for the LTC-PC function when the chemical potential is changed by 0.1 eV. Based on these superior performances, the polarization converter can hold potential applications in integrated and compact devices, such as polarization sensor, switches and other optical polarization control components.

Highly Transparent Broadband and Polarization-Insensitive Absorber Based on Metasurface

A highly transparent polarization-insensitive metamaterial absorber with wideband microwave absorption is presented. The broadband absorption (6.0~16.7 GHz, absorptance &gt; 85%) is achieved using three patterned resistive metasurfaces. The visible light transmittance of the absorber is as high as 85.7%. The thickness of the absorber is 4.42 mm, which is only 0.088 times of the upper-cutoff wavelength. A prototype sample is fabricated and measured to demonstrate its excellent performance. The experimental results agree well with the simulation results. In view of its wide band absorption, high transmittance, low profile, polarization insensitivity and wide incidence angle stability, the presented absorber has a wide range of potential applications.

Compact Dual-Band, Wide-Angle, Polarization- Angle -Independent Rectifying Metasurface for Ambient Energy Harvesting and Wireless Power Transfer

A dual-band and polarization-angle-independent rectifying metasurface (MS) with a miniaturized dimension and a wide incident angle range are presented in this article. The proposed structure consists of a single layer of periodic cell arrays with integrated diodes, a $dc$ feed, and a load. A novel method of incorporating surface-mount components (e.g., diodes) into the texture is developed to simplify the structure. The matching network between MS and the nonlinear rectifier can be eliminated directly due to the multimode resonance and adjustable high-impedance characteristics of the MS. Moreover, the proposed MS can maintain high conversion efficiency by using different diodes without changing the overall topology. In addition, the proposed design can effectively capture incoming waves with arbitrary polarizations and a wide incident angle range of 60°. The $4\times 4$ MS array is fabricated and measured. Experimental results show that the proposed structure can achieve maximum efficiency of 58% at 2.4 GHz and 50% at 5.8 GHz with an input power of 0 dBm under different polarizations and incident angles. Importantly, it is also shown that the rectifying MS can maintain high efficiency over a wide power range from −3 to 10 dBm. The proposed design concept is very suitable for the adaptive wireless power supply of portable devices.

Graphene-Based Tunable Wideband Metamaterial Absorber With Polarization Insensitivity and Wide Incident Angle

A graphene-based tunable wideband metamaterial absorber with polarization insensitivity and wide-incident-angle is presented in this paper. The results show that the absorption is over 90% from 5GHz to 8GHz when the fermi level of graphene is 0.5eV, and the absorption can be tuned by electrically changing the Fermi level of graphene. The mechanism of wideband and tunable absorption is explained by calculating the normalized input impedance and monitoring the surface current. Finally, the results adequately verify that the absorption of metamaterial absorber has the advantages of polarization insensitivity and wide-incident-angle. Thus, it holds great potential application value on many fields such as electromagnetic stealth, electromagnetic shielding, communication and so on.

Facile fabrication and optical characterization of nanoflake aluminum oxide film with high broadband and omnidirectional transmittance enhancement

Abstract In this study, the non-lithography fabrication of nanoflake aluminum oxide (Al2O3) film with omnidirectional broadband antireflection properties was demonstrated. The nanoflake Al2O3 films on glass slide and silicon wafer substrates were sequentially prepared by reactive magnetron sputtering and surface modification with alkali-treatment and hot water treatment processes. The morphologies, chemical compositions and optical properties of the prepared samples were characterized by field-emission scanning electron microscope (FE-SEM), photoelectron spectroscopy (PES), variable angle spectroscopic ellipsometry (VASE) and UV-VIS spectrophotometry with angle-dependent transmittance measurements, respectively. The results indicated that the thickness of the sputtered Al2O3 film was a crucial parameter for the growth of nanoflake for the nanostructure formed by alkali treatment with 0.1 M of potassium hydroxide (KOH) at room temperature and hot water treatment in deionized (DI) water at 80 °C for 3 min. The best optical performance was achieved with the single nanoflake film layer, resulting in enhanced optical transmittance exceeding 90% over the broad range of visible region at wide incident angle. This facile fabrication process of the nanoflake layer was offering the potential benefits for inexpensive omnidirectional anti-reflection (AR) coating in large area.

Ultra-Wideband Flexible Absorber in Microwave Frequency Band.

In this paper, an ultra-wideband flexible absorber is proposed. Based on a summary of the absorption mechanism, using lossless air to replace the heavy lossy dielectric layer will not substantially impact the absorption. The dielectric layer is only a thin layer of polyimide. The proposed absorber is a sandwich structure. The surface is a layer of copper metal ring and wire, and it is loaded with chip resistors to expand the absorber bandwidth. Simulated results show that the bandwidth of the proposed absorber, with an absorptivity of more than 90%, is 2.55–10.07 GHz, with a relative bandwidth over 119.2%. When the electromagnetic wave has a wide incident angle, the absorber still maintains a high absorption. This absorber has been fabricated by FPC (flexible printed circuit) technology. The proposed absorber was attached to the cylinder and measured. The measurement results are roughly the same as the simulation results. The fabricated absorber is easy to carry and flexible, such that it can easily be conformed to irregular objects. The proposed absorber is polarization-insensitive, low profile, thin, and portable, so it is easier to apply in a variety of practical fields.

Development of a solar energy based desalination system using a hyperboloid concentrator

Solar concentrating technologies are still relevant due to innovative designs, materials and manufacturing methods. With an increased demand for fresh water, such technologies can be effectively utilized for sea water desalination. Desalination methods utilizing solar energy can significantly contribute towards sustainable and renewable energy solutions.In this work, an attempt is made to design, develop and experimentally analyse the performance of a desalination unit with a hyperboloid concentrator and a helical copper coil receiver. The elliptic hyperboloid concentrator is a special design for concentrating and capturing radiation over a wide incident angle and thus eliminates tracking. Further a CFD analysis was carried out to design and optimize the helical coil receiver dimensions. The optimized receiver was then fabricated.Results showed that, using the hyperboloid concentrator, acceptable quality of potable water, with a salinity of 190 PPM was obtained. The system performance was also impressive with an efficiency improvement of 30% compared to the traditional parabolic dish concentrator. The output temperature obtained was 109 0C using the combined system comprising ETC, parabolic dish and hyperboloid concentrator.

Multiband linear and circular polarization rotating metasurface based on multiple plasmonic resonances for C, X and K band applications

In this work, a multiband polarization converting metasurface is presented which achieves cross-polarization conversion in five frequency bands while linear-to-circular and circular-to-linear polarization transformation in eight frequency bands. The polarization transforming functionality of the structure is spread over an ultra-wide frequency range (5–37 GHz) covering most of X, C, Ku, K and Ka bands. Such an extraordinary ultra-wideband operation originates from multiple plasmonic resonances occurring in the structure based on two coupled rectangular split-ring resonators. Moreover, the polarization transforming capability is stable within the frequency range 5–19 GHz for wide oblique incidence angles, which is up to 60°, both for transverse-electric and transverse-magnetic polarizations. Furthermore, the proposed structure acts as a meta-mirror which preserves handedness of the circular polarization upon reflection. Measurements performed on the fabricated metasurface are found to be consistent with numerical simulation results. The ability to perform three functionalities through a single compact structure with extraordinary wideband, qualifies the proposed design to be a promising candidate for integration with important microwave applications such as satellite, radar, and 5G communication.