Wide Incident Angle Research Articles

Experimental investigation on flow-induced vibration of flexible multi cylinders in atmospheric boundary layer

Experimental investigations of the flow-induced vibration (FIV) of flexible multi cylinders in tandem, side-by-side and staggered arrangements were conducted in an atmospheric boundary layer wind tunnel over a wide spacing ratio and incidence angles. All cylinders were cantilever supported and allowed to vibrate in cross-flow and inline directions. The vibration characteristics and transition features are identified among different configurations, and the responses dependent on reduced velocity under each configuration are discussed. Four regimes of the FIV of tandem cylinders are classified, where cylinder 1 vibrates divergently similar to galloping in Regime Ⅰ (l < 1.6, α = 0°) but is suppressed in Regime Ⅱ (1.6 ≤ l < 3, α = 0°), cylinder 2 always vibrates obviously in all four regimes, and cylinder 3 has a distinct vibration in only Regimes Ⅱ, Ⅲ (3 ≤ l ≤ 5, α = 0°) and Ⅳ (l >5, α = 0°), in which l is the nondimensional center-to-center distance and α is the incidence angle. There are two regimes for side-by-side cylinders, in which a divergent vibration of cylinder 1 is observed in Regime V (l < 1.6, α = 90°) and only cylinder 3 vibrates significantly in Regime VI (1.6 ≤ l ≤ 3.2, α = 90°). The aerodynamic properties analyzed through a prediction method are used to explain the vibration mechanism, in which the negative aerodynamic damping ratio branches sustain the divergent vibration in Regime I. Based on the comprehensive study above, the FIV phenomenon of flexible multi cylinders can be understood in atmospheric environment with nonuniform inflow, high turbulence intensity and subcritical Reynolds numbers. Finally, the potential of harvesting wind energy via this technique, showing the highest efficiency 52% but limited at Ur≥30, is discussed.

A dynamically temperature tunable broadband infrared absorber with cross square nanocolumn arrays

A dynamically tunable broadband infrared absorber is designed by integrating VO2/SiO2/VO2 nanocolumn array on Al2O3 substrate with W/VO2/W nanocolumn array on W substrate. The absorption rates under different structural parameters and temperatures are simulated by using finite-difference time-domain method. The absorption rate can be dynamically adjusted through thermally induced VO2 phase transition. The absorber is polarization independent under perpendicular incidence and possesses wide incidence angle absorption characteristics. It has good application prospects in the fields of thermal photoelectric devices.

An ultra-wideband, polarization insensitive metamaterial absorber based on multiple resistive film layers with wide-incident-angle stability

AbstractIn this work, we propose a broadband, polarization-insensitive and wide incident angle stable metamaterial absorber (MA) based on the resistive film. The absorber consists of a three-layer structure with each layer of dielectric substrate printed with different shapes of resistive film. The multilayer structure not only extends the absorption bandwidth but also maintains high absorption under large wave incident angles. Numerical simulation shows that the absorptivity of a normal incident wave is above 90% in the frequency range 2.34–18.95 GHz, corresponding to a relative absorption bandwidth of 156%. Moreover, the whole MA structure has a total thickness of 11.3 mm, corresponding to 0.09 λ0 at its lowest absorption frequency. Due to the high symmetry of the structure, the absorber has good polarization insensitivity. In addition, for both transverse electric and transverse magnetic incidence, the proposed absorber achieves an absorptivity of more than 80% at incident angles of up to 45° and thus has good stability for wide incident angles. The absorption principle of the absorber is analyzed by the surface current and power loss density distribution. Parameter analysis is also performed for bandwidth optimization. Due to its advantages of wideband absorption with high efficiency, the proposed absorber has the potential to be applied to the energy-harvesting and electromagnetic stealth fields.

Selectivity analysis of femtosecond laser polarizer by three-layer grating

The novel three-layer dielectric transmission grating under the Bragg incidence, which can be used as a polarizer, has been designed. According to the modal method, under the conditions of operating wavelength of 800 nm, duty cycle of 0.5 and period of 552 nm, the thickness of three grating layers can be roughly calculated. After that, the rigorous coupled-wave analysis method based on Maxwell’s theory is used to obtain more suitable grating parameter, which makes the grating diffraction efficiency higher. Under the Bragg incidence, the diffraction efficiencies of TE polarization in −1st order and TM polarization in 0th order can both exceed 96%. The novel three-layer grating has not only high diffraction efficiency but also the wide incident angle bandwidth. The optimized results show that the grating has good performance and good application value in the field of multi-channel laser device.

A near-ideal solar selective absorber with strong broadband optical absorption from UV to NIR

A near-ideal solar absorber, which is composed of a metal nanowire array and a planar multilayer system, is proposed and investigated. Both numerical and analytical results show that the proposed nanostructure can achieve over 90% optical absorption throughout the wavelength range of 300–1909 nm due to the coupled effect of multiple resonance modes, and can maintain a good absorption stability over a wide incident angle regardless of the polarization states. Meanwhile, for practical applications, the total photothermal conversion efficiency can reach 95.57% at operating temperature of 373.15 K, which is particularly useful in solar energy harvesting. The absorption performance is also strongly dependent on the diameter and height of nanowire as well as the thicknesses of dielectric layers, enabling the further improvement of both the operating bandwidth and absorption efficiency. Moreover, by adjusting the period of the multilayer or nanowire materials, the selective absorption properties of this nanostructure can be flexibly controlled to satisfy more spectral requirements. These features make the presented designs hold promise for a series of solar-dependent optical applications, such as photothermal energy generation and thermal emitters.

3-D Printed Swastika-Shaped Ultrabroadband Water-Based Microwave Absorber

In this letter, a swastika-shaped ultrabroadband metamaterials absorber is proposed. The designed absorber can be fabricated with the polylactic acid by 3-D printing, and packaged after filling with water. The absorber can achieve greater than 90% absorption in the frequency band of 9.3–49.0 GHz, with a relative bandwidth of 136%, which has been well demonstrated in the microwave experiment. Moreover, it also performs well at wide angles of incidence and the absorption efficiency has almost no obvious changes over the water temperature range of 0 °C–100 °C. The absorber proposed in this letter benefits from ultrawideband absorption, low cost and environment-friendly materials, it can be applied in the fields of electromagnetic (EM) stealth and EM energy harvesting.

Broadband water-based metamaterial absorber with wide angle and thermal stability

In order to widen the absorption band of the metamaterial absorber to expand its application field, using the electromagnetic characteristics of water in the microwave band, a water-based metamaterial absorber with a structure different from the traditional “metal layer–dielectric layer–metal layer” one is proposed. The numerical simulation results show that its absorptivity of incident electromagnetic waves exceeds 90% in the ultra-wide operating band of 7.84–74.16 GHz and the absorption bandwidth can cover five bands including the X-band, Ku-band, K-band, Ka-band, and U-band. Based on the theories of equivalent medium and impedance matching, the mechanism of broadband absorption was explored by analyzing the electromagnetic field distribution and power loss density of the absorber. The designed absorber has the absorption performance of polarization independence, and it can work under a wide incident angle. In addition, the absorber has excellent thermal stability which can still maintain stable operation when the temperature changes from 0 °C to 100 °C. This water-based metamaterial absorber is of low cost, and its manufacturing process is relatively convenient, so it has broad application prospects in the fields of radar stealth, microwave darkroom, and prevention of electromagnetic radiation.

Electromagnetic simulations of polarization-insensitive and wide-angle multiband metamaterial absorber by incorporating double asterisk resonator

A novel multiband metamaterial absorber (MTMA) is proposed which is capable of well presenting a polarization-insensitive and wide incident angle stability in the microwave frequency range. The proposed MTMA comprising two vertically stacked asterisk-based copper resonators separated by dielectric layer of flame retardant type four and backed with a thin copper film. High-frequency structure simulator was used to simulate the absorber and to depict surface current distribution. The results showed that the absorber is operated at three narrow bands with high absorptivity of about 92, 100 and 100% at frequencies of 2.75, 4.3 and 9.5 GHz, respectively. The position of the resonant peaks can be effectively tuned by adjusting the geometry parameters of the structure, thereby achieving a multiband, polarization-independent and wide-angle absorber. The designed structure is important for the application of sensors, thermal images and in constructing broad multiband signals for electromagnetic compatibility/interference.

Broadband, wide-angle antireflection in GaAs through surface nano-structuring for solar cell applications

We demonstrate broadband and wide-angle antireflective surface nanostructuring in GaAs semiconductors using variable dose electron-beam lithography (EBL). Various designed structures are written with EBL on a positive EB-resist coated GaAs and developed followed by shallow inductively coupled plasma etching. An optimized nanostructured surface shows a reduced surface reflectivity down to less than 2.5% in the visible range of 450–700 nm and an average reflectance of less than 4% over a broad near-infrared wavelength range from 900–1400 nm. The results are obtained over a wide incidence angle of 33.3°. This study shows the potential for anti-reflective structures using a simpler reverse EBL process which can provide optical absorption or extraction efficiency enhancement in semiconductors relevant to improved performance in solar photovoltaics or light-emitting diodes.

Achromatic metasurface doublet with a wide incident angle for light focusing.

Benefiting from the excellent capabilities of arbitrarily controlling the phase, amplitude and polarization of the electromagnetic wave, metasurfaces have attracted much attention and brought forward the revolution of fields ranging from device fabrications to optical applications. Cascaded metasurfaces have been demonstrated to correct the monochromatic aberration and enable a near-diffraction-limited focusing spot over a wide incident angle. However, they can only work under the design wavelength and suffer from the axial chromatic aberration at another wavelength. Here, an achromatic metasurface doublet is proposed to eliminate the axial achromatic aberration and enable high-quality focusing with a wide incident angle at distinct wavelengths. It consists of square nanopillar arrays with spatially varying width to simultaneously realize wavelength-dependent phase controls. The constructed metasurface doublet is further verified numerically and near-diffraction-limited foci are exactly formed at the same plane with an incident angle up to 20° for design wavelengths. We expect that our proposed approach can find optical applications in the fields of holograms, photograms, microscopy and machine vision.

Ultra-broadband and wide-angle perfect solar absorber based on TiN nanodisk and Ti thin film structure

At present, solar energy is widely used as a kind of clean energy. The main solar radiation range under AM 1.5 is about 300 ~ 3000 nm. In this paper, we designed an efficient, ultra-broadband perfect solar absorber to have as long absorption bands in this range as possible to help alleviate the energy problem. The simulation calculations and experiments of the solar absorber show that the absorption bandwidth with absorption greater than 90% is greater than 2100 nm. It is worth noting that the perfect absorption bandwidth with absorption greater than 99% has more than 1600 nm. The absorption rate over the whole wavelength range (300 nm–3000 nm) (weighted directly around the sun by solar AM 1.5) is more than 90%. We can effectively control the absorption spectrum by adjusting the structural parameters. In addition, the proposed solar absorber is polarization independent, both the transverse electrical (TE) mode and the transverse magnetic (TM) mode, Absorption remains above 80% when the wide incidence angle is as high as 50°. Our propose design has high broadband absorption and great potential for solar thermal energy harvesting, thermoelectrics, and thermal emitters applications.

Metamaterials absorber for multiple frequency points within 1 GHz

Designing high efficiency microwave absorbers with frequencies in 1 GHz is still a huge challenge in practical applications. We design and present a novel microwave metamaterials absorber at six frequency points of 160, 442, 530, 723, 765 and 965 MHz with absorptances up to 90%, which is insensitive to polarization, viable at wide incidence angle and relatively simple in structure. We demonstrate the absorption characteristics of the absorber and explain the mechanism by means of equivalent impedance theory, surface current and energy distribution. The experimental results verify the correctness of the simulation and provide valuable reference for the design of broad-band microwave absorber in lower frequency band.

Enhancing sensing capacity of terahertz metamaterial absorbers with a surface-relief design

Metamaterial absorbers (MAs) serve as important electromagnetic wave-absorbing devices that have captured the attention of researchers for a long term. Functioning as sensitive detectors to determine perturbations in an ambient environment is another significant subsidiary function. Here, we theoretically propose an optimized fabrication method to implement terahertz MAs with fewer steps and also evaluate both absorption and sensing performances of such MAs realized by the new method. Simulation findings demonstrate that such MAs can basically maintain the original absorption features perfectly, including near-complete absorption at resonance as well as strong robustness to wide incident angles. Specifically, the full width at half-maximum and quality factor of the absorption resonances attenuate less than 26% and 8% with this new method, remaining in the ranges of ∼ 0.03 – 0.04 THz and ∼ 20 – 27 for two selected example MAs. More significantly, sensing capacities of this type of MA, in terms of maximum detection range (enhancing at least 9%), observable spectral modulation (increasing at least 6.3%), and refractive index sensitivity, are improved to a large extent because of more intense coupling between resonant field and matter in the case of surface-relief MAs. This stronger coupling results from exposing more spots of the resonantly high field to direct contact with an approaching analyte, which is illustrated by field profiles of the MAs at resonance in this work. Additionally, other desirable absorber features are also explored with such MAs, like functioning as building blocks to configure multiband MAs and strong robustness against fabrication errors. Such new-style terahertz MAs shown in the paper, acting as good examples, not only prove that terahertz MAs can be fabricated by the proposed time- and cost-saving route in contrast to the traditional MA fabrication process, but also can serve as novel platforms to explore other intriguing terahertz photonic effects, such as the field enhancement effect.

Second-Order Approximation of the Seismic Reflection Coefficient in Thin Interbeds

As most of the lithostratigraphic reservoirs in China are thin interbeds, the study of seismic responses in thin interbeds is an integral part of lithologic reservoir exploration. However, at present, the research on seismic reflection coefficients of thin interbeds in exploration seismology is still weak, which leads to the lack of theoretical basis for the subsequent interpretation of amplitude variation with offset (AVO) related to thin interbed. To solve this problem, in this paper, we proposed second-order approximate equations of the seismic reflection coefficients in thin-bed and thin-interbed layers. Under the assumption of a small impedance contrast in layered media, we made a second-order approximation with a more evident physical meaning to the reflection coefficient calculation method proposed by Kennett. Then, based on the test of the single thin-layer theoretical model, it was confirmed that the second-order approximation equation of the PP-wave (reflected compressional wave) is accurate at incident angles less than 30°, and that of the PS-wave (converted shear wave) is accurate at wider incident angles. Finally, based on the single-thin-bed equations, the approximate equations of seismic reflection coefficients in thin interbeds were established, the validity of which was verified by the theoretical model. Our equations will be applicable to the calculation of PP- and PS-wave reflection coefficients in thin interbeds where internal multiples are difficult to suppress and transmission loss is hard to accurately compensate. This lays a theoretical foundation for improving the seismic prediction accuracy of lithologic reservoirs.

Light Harvesting at Oblique Incidence Decoupled from Transmission in Organic Solar Cells Exhibiting 9.8% Efficiency and 50% Visible Light Transparency

AbstractFor many years, it has been recognized that potential organic photovoltaic cells must be integrated into elements requiring high transparency. In most of such elements, sunlight is likely to be incident at large angles. Here it is demonstrated that light transmission can be largely decoupled from harvesting by optically tailoring an infrared shifted nonfullerene acceptor based organic cell architecture. A 9.67% power conversion efficiency at 50° incidence is achieved together with an average visual transmission above 50% at normal incidence. The deconstruction of a 1D nanophotonic structure is implemented to conclude that just two λ/4 thick layers are essential to reach, for a wide incidence angle range, a higher than 50% efficiency increase relative to the standard configuration reference. In an outdoor measurement of vertically positioned 50% visible transparent cells, it is demonstrated that 9.80% of sunlight energy can be converted into electricity during the course of 1 day.

Design of ultra-wideband and near-unity absorption water-based metamaterial absorber

In this study, a water-based metamaterial absorber (MMA) with ultra-wideband microwave absorption has been proposed and investigated. Using a special structural design, the absorption rate of the absorber exceeds 90% in the frequency range 5.5–27.5 GHz, and the absorber’s thickness is only 5.8 mm. The angular tolerance of the MM absorber shows that the absorber works well under a wide angle of incidence. When bent, the absorber shows omnidirectional absorption characteristics and polarization insensitivity. As the proposed water MM absorber is low cost and easy to manufacture, it could be widely used in electromagnetic stealth and energy harvesting.

Numerical study of a broadband metamaterial absorber using a single split circle ring and lumped resistors for X-band applications

We report a numerical study on the design of a broadband metamaterial absorber (MMA) with a single layer of metal–dielectric–metal based on an FR-4 substrate for X-band applications. The MMA structure consists of a periodic array of a split circle ring and lumped resistors coupled within split segments. The MMA structure achieves a broadband absorption response in the frequency range of 7.8–12.6 GHz with an absorptivity of above 90% under normal incidence for all polarization angles. The absorptivity remains above 70% in the frequency range of 6.8–11.8 GHz at wide incident angles from 0° to 30° for both transverse electric and transverse magnetic polarizations. The physical mechanism of the absorber is explained by the electric and the surface current distributions that, in turn, are significantly affected by magnetic resonance.

Metallic nanomesh for high-performance transparent electromagnetic shielding

This paper reports a high-performance transparent electromagnetic shielding material based on an ultrathin and large-area metallic nanomesh, which was fabricated by a facile and rational process utilizing ultraviolet lithography and the ion beam etching technique. Measurements reveal that a single-layer metallic nanomesh can harvest excellent shielding effectiveness exceeding 40 dB in the wide frequency range from 500 MHz to 40 GHz. Besides, efficient light transmittance (85% at 550 nm) is achieved in both visible and near-infrared regions. Furthermore, the proposed structure remains excellent performance at wide incident angles even up to 50°. Hence, it is believed that this metallic nanomesh with easy fabrication can be a potential alternative in the transparent electromagnetic shielding domain.

An optical-transparent metamaterial for high-efficiency microwave absorption and low infrared emission

An optically transparent metamaterial structure with simultaneously broadband microwave absorptivity and low infrared (IR) emissivity is proposed. Two specifically designed optically transparent metasurfaces were combined to realize radar and IR bi-stealth. One was designed to control the microwave absorption though properly modifying the impedance and resonance peaks of the meta-atom. The other was designed to control the IR radiation. Within a wide incident angle of ±60°, the proposed structure displays high absorptivity greater than 90% in the region of 6.28–12.29 GHz for TE polarization. For TM polarization, the absorptivity in the region of 7.19–15.26 GHz is greater than 90%. The IR emissivity of the metamaterial structure is about 0.30 in the IR region from 3 µm to 14 µm. The perfect consistency between experimental results and simulation results demonstrates that the proposal has practical application of multispectral stealth technology.

Triple-band perfect metamaterial absorber with good operating angle polarization tolerance based on split ring arrays

In this paper, a triple-band perfect metamaterial absorber based on Cu-dielectric-Cu triple-layer nanostructure is reported. The top metal film structure consists of a ring and four pairs of capacitor plates, which has a frequency selection effect, allowing the absorber to resonate in the near infrared range. Theoretical study shows that the absorption of the three absorption peaks (872.54 nm, 1008.69 nm and 1138.62 nm) are 87.1%, 99.9% and 99.6%, respectively. The average absorption is 95.53%, including two perfect absorption peaks. Changing the structural parameters can affect its absorption peaks and resonant wavelengths. At the same time, due to the high symmetry of the absorber, it is not sensitive to the polarization angle and incident angle. Whether in the TE mode or the TM mode, the absorber at a wide incident angle (0-60°) also exhibits good operating angle polarization tolerance. Therefore, the perfect metamaterial absorber we designed can be widely used in sensing.