Wide Range Of Incident Angles Research Articles

Metamaterial perfect absorber using elliptical nanoparticles in a multilayer metasurface structure with polarization independence.

A metamaterial perfect absorber (MPA) using elliptical silver nanoparticles is proposed and investigated to provide 100% absorption for both transverse electric and transverse magnetic polarizations with a wide range of incident angles and polarization independence. Metamaterial absorbers with narrow absorption performance over a wide frequency range are significantly desired in sensing applications. Incident angle insensitivity and polarization angle independence are key features of MPAs. The output characteristics are examined using the three-dimensional finite difference time domain method. The effective medium theory and transmission line theory are applied to investigate the simulation results. Here, the 100% absorption occurs at resonance wavelength of λres = 2290 nm, and maximum sensitivity and figure of merit become 200 nm/RIU and 720 RIU-1, respectively. The results show that an absorption spectrum is insensitive to the incident angle of 0°-60°. The proposed device can be used as a high-performance biosensor and photodetector.

Huygens' Metasurface With Stable Transmission Response Under Wide Range of Incidence Angle

Metasurfaces, as two-dimensional (2-D) metamaterials, show great potentials in electromagnetic wave manipulation. Here, an angular nondispersive transmission-type Huygens’ meta-atom, made up of an I-shaped metal patch and a square split ring, is proposed for wide incidence angle holographic metasurfaces. Nine varieties of the Huygens’ meta-atom with high transmission amplitude (higher than 0.9) and stable transmission phase (vary less than 10°) under transverse magnetic wave illumination from 0° to 60° are selected. To verify the performance of the meta-atoms proposed, two holographic metasurfaces are simulated and fabricated to reconstitute identical holographic images under normal and oblique linearly polarized wave illuminations, respectively. The experimental demonstrations show imaging efficiencies higher than 48.28% with signal-to-noise ratio as high as 22.57. Our designs provide a solution for angular nondispersive metasurface designs, which suggest great practical applications in beamforming, and information storage systems.

Tunable spin hall effect via hybrid polaritons around epsilon-near-zero on graphene-hBN heterostructures

We present a tunable spin Hall effect of light (SHEL) by introducing a monolayer of graphene on the hexagonal boron nitride (hBN). The interaction between the phonon polaritons in hBN and the plasmon polaritons in graphene can significantly enhance the SHEL around epsilon-near-zero (ENZ) near reststrahlen band-I (RB-I) in a very wide range of incident angles and compress it obviously in reststrahlen band-II (RB-II). The spin shifts can be obtained by adjusting the ratio of reflective coefficient and the phase difference for s- and p-waves which can be easily manipulated by the chemical potential. The effect of the tilted angle, thickness of hBN and incident angle on the SHEL is also discussed. These findings may lead to the potential applications for the nano-optical device based on the spin optics and the control of photons.

Broadband refractive index sensor based on localized surface plasmon for highly sensitive detection of fluid pressure

Localized surface plasmon resonance (LSPR) in gold (Au) nanoparticles is the research hotspot in various fields, due to its high chemical and physical stability, ease of preparation and multiple optical properties. However, the anisotropic response to polarized light of LSPR has been rarely studied. Here, we report optical refractive index sensors based on the polarization sensitive response of LSPR in gold nanoparticles, and its application in the highly sensitive detection of fluid species and hydraulic pressure. The polarization-sensitive absorption characteristics of the interfacial nanoparticles rendering it different absorption properties for TM and TE mode waves under the condition of total reflection, which is also strongly affected by the refractive index of medium n2 in contact with it. On this basis, the refractive index sensor based on LSPR in gold nanoparticles is successfully realized. In addition, the refractive index sensor features broadband response (from 200 to 1000 nm) in a wide range of incident angle δ (from −10° to 25°), exhibiting great application potential in various scenarios. To evaluate the practical applicability of the proposed sensor, we constructed an optical refractive index sensing system that can distinguish between liquid species and accurately detect hydraulic pressure in real time.

Effect of endwall passage vortex generator on corner stall of a tandem compressor cascade

The control mechanism of endwall passage vortex generator (EVG) on corner stall was investigated in a highly-loaded tandem cascade. Results show that utilizing EVG can reduce corner stall efficiently. The optimal EVG scheme in this study was located at 8% axial chord and 3.7 mm (14% of tandem cascade pitch) away from suction surface, which achieved a maximum loss coefficient reduction of 33.7%. The control mechanism was that the high-circulation fluid of the EVG induced vortex (EVGV) helped the corner stall fluid to move toward another direction, thus escaping from the streamwise adverse pressure gradient. As the location of the EVG moved downstream, the control effectiveness on corner stall was enhanced due to the fact that the flow separation at downstream of the EVG was reduced with the decrease of intersection angle between the flow and the EVG. As the location of the EVG moved toward pressure surface, the loss coefficient showed a tendency of first increasing and then decreasing; the increasing loss coefficient of P2S1 scheme was caused by the weaker contracted flow in the front blade passage and the enlarged corner separation in the rear blade passage; the corner stall could be effectively reduced with the EVG even it was far from the suction surface, which was due to the contracted flow passage between the EVG and the suction surface. The control effectiveness of the EVG was also numerically investigated, which showed that the EVG reduced the corner stall and loss coefficient in a wide range of incidence angle.

Wideband Radomes for Millimeter-Wave Automotive Radars

We demonstrate the efficacy of periodic corrugations, fabricated as part of the radar cover to serve the dual purpose of protecting the radar circuitry and maximizing transmission in millimeter-wave automotive radars. A 2-dimensional array of square-base pyramids engraved on the plastic radar cover was optimized to achieve minimum reflection for a wide range of incident angles, ± 40° and across the operating frequency band of 76 to 81 GHz. More importantly, we present an alternative radome texture, namely the inverse-pyramid corrugations that results in improved structural integrity as compared to the pyramidal corrugations. This novel inverse-pyramid radome structure is also much easier to manufacture and achieves similar performance as the more conventional pyramid structure. Both designs are shown to significantly reduce radar cover reflections, which is perhaps the most significant bottleneck in modern millimeter-wave automotive radar sensor performance.

Tunable perfect dual-narrowband absorber based on graphene-photonic crystal heterostructure

A dual-narrowband absorber in the terahertz (THz) band is proposed and numerically analyzed, which consists of a sheet of graphene on a photonic crystal (PC) heterostructure. We find a huge enhancement of absorption because optical Tamm states (OTSs) are excited at the interfaces of graphene-PC and PC heterostructure. The full width at half maximum (FWHM) of the absorption peak near the wavelength of 300 μm can be as narrow as 1.25 μm, and narrower FWHM can be obtained by adapting the wavelength and incident angle of the light beam. The wavelengths of absorption peaks and the absorptivities can be tuned by controlling the Fermi energy of graphene and the periods of PCs. In addition, the absorber can maintain high absorptivity over a wide incident angle range for both TE and TM polarization. The proposed optical absorber has potential applications in THz biosensors, filters, and switches.

Perfect transmission of elastic waves obliquely incident at solid–solid interfaces

Perfect wave transmission across dissimilar media is critical in wave devices. However, the existing impedance matching theory allows perfect wave transmission only in special cases dealing with electromagnetic waves or acoustic waves in fluids. For the cases where elastic waves are incident at an arbitrary angle from one solid to another, even elaborately designed layers satisfying the conventional theory cannot realize perfect wave transmission. Elastic waves in isotropic solids always carry waves of both longitudinal and transverse modes and this multimodality generally couples the two wave modes at solid–solid interfaces, making the conventional impedance matching theory inapplicable for elastic waves. Here, we establish a novel theory for the perfect transmission through solid–solid interfaces and propose a unique nonresonant anisotropic single-phase metamaterial realizing the theory. The theory dictates intriguing interferences among longitudinal–shear coupled waves existing inside a metamaterial. The theory is shown to be valid for any form of wave transmissions, either mode-preserving or mode-converting. Experiments using highly impedance-mismatched elastic plates were conducted to verify the validity of the proposed theory. A further analysis confirmed transmittance enhancement over a wide range of frequencies and incidence angles. Our findings will be useful in developing new types of powerful wave devices.

Machine–learning-enabled metasurface for direction of arrival estimation

Abstract Metasurfaces, interacted with artificial intelligence, have now been motivating many contemporary research studies to revisit established fields, e.g., direction of arrival (DOA) estimation. Conventional DOA estimation techniques typically necessitate bulky-sized beam-scanning equipment for signal acquisition or complicated reconstruction algorithms for data postprocessing, making them ineffective for in-situ detection. In this article, we propose a machine-learning-enabled metasurface for DOA estimation. For certain incident signals, a tunable metasurface is controlled in sequence, generating a series of field intensities at the single receiving probe. The perceived data are subsequently processed by a pretrained random forest model to access the incident angle. As an illustrative example, we experimentally demonstrate a high-accuracy intelligent DOA estimation approach for a wide range of incident angles and achieve more than 95% accuracy with an error of less than 0.5 ° $0.5{}^{\circ}$ . The reported strategy opens a feasible route for intelligent DOA detection in full space and wide band. Moreover, it will provide breakthrough inspiration for traditional applications incorporating time-saving and equipment-simplified majorization.

Moiré graphene nanoribbons: nearly perfect absorptions and highly efficient reflections with wide angles.

The strong absorption and reflection from atomically thin graphene nanoribbons has been demonstrated over the past decade. However, due to the significant band dispersion of graphene nanoribbons, the angle of incident wave has remained limited to a very narrow range. Obtaining strong absorption and reflection with a wide range of incident angles from atomically thin graphene layers has remained an unsolvable problem. Here, we construct a tunable moiré superlattice composed of a pair of graphene nanoribbon arrays to achieve this goal. By designing the interlayer coupling between two graphene nanoribbon arrays with mismatched periods, the moiré flat bands and the localization of their eigen-fields was realized. Based on the moiré flat bands of graphene nanoribbons, highly efficient reflection and nearly perfect absorption was achieved with a wide range of incident angles. Even more interesting, is how these novel phenomena can be tuned through the adjustment of the graphene's Fermi energy, either electrostatically or chemically. Our designed moiré graphene nanoribbons suggest a promising platform to engineer moiré physics with tunable behaviors, and may have potential applications in the field of wide-angle absorbers and reflectors in the mid-infrared region.

In-Wing Pressure Measurements for Airspeed and Airflow Angle Estimation and High Angle-of-Attack Flight

Small uncrewed aerial vehicles (UAVs) capable of stable, controlled stalled descent offer the possibility for a wider operating envelope and for precise, steep approaches and landings. However, the aerodynamics of stalled flight are inherently complex and underexplored compared to flight regimes with attached flow. This paper presents a number of advancements in these areas—particularly focusing on pressure sensors for estimating flow states. This paper presents a modular, open-source design for in-wing, flow-based differential pressure sensor arrays for estimating flow states without the requirement of fragile pitot tubes or airflow vanes. An extensive, publicly available set of wind tunnel and flight test results from a small UAV with 24 in-wing pressure sensors is provided. The data contain a wide range of incidence angles, including deep stalled states. This paper also presents data-driven regression models for predicting airflow angles and airspeed using only pressure information. Our results demonstrate predictions for airspeed, angle of attack (AoA), and angle of slip (AoS) with root mean square error (RMSE) of , 0.20°, and 2.83°, respectively, using wind tunnel data and all 24 sensors. Finally, this paper provides a placement analysis to determine the optimal sensor locations for estimating airflow data with fewer pressure sensors. Models trained and validated with real flight data show airspeed, AoA, and AoS predictions with RMSE of , 1.54°, and 4.07°, respectively, using data from only three pressure sensors each.

Narrowband-to-broadband switchable and polarization-insensitive terahertz metasurface absorber enabled by phase-change material

A terahertz absorber with controllable and switchable bandwidth that is insensitive to polarization is of great interest. Here, we propose and demonstrate a metasurface absorber with switchable bandwidth based on a phase-change material of vanadium dioxide (VO2) and verify its performance by finite element method simulations. The metasurface absorber is composed of a hybrid cross fractal as a resonator separated from a gold ground plane by a polyimide spacer. Switching from narrowband to broadband absorber is achieved via connecting VO2 patches to the gold first-order cross fractal converting the resonator to a third-order cross fractal. In the insulator phase of VO2, the main narrowband absorption occurs at the frequency of 6.05 THz with a 0.99 absorption and a full-width half-maximum (FWHM) of 0.35 THz. Upon insulator-to-metal transition of VO2, the metasurface achieves a broadband absorption with FWHM of 6.17 THz. The simulations indicate that by controlling the partial phase transition of VO2, we can tune the bandwidth and absorption level of the absorber. Moreover, the designed absorber is insensitive to polarization due to symmetry and works well for a very wide range of incident angles. In the metallic state of VO2, the absorber has an absorption exceeding 0.5 in the 3.57–8.45 THz frequency range with incident angles up to 65°.

All‐Dielectric Fabry–Pérot Cavity Design for Spectrally Selective Mid‐Infrared Absorption

All‐dielectric metamaterials that exhibit broadband absorption of infrared (IR) radiation are promising platforms for the development of modern and applied nanophotonic technologies ranging from precise biosensing to high‐resolution imaging and high photon‐yield light detection. Herein, bulk meta‐absorbers comprising dielectric layers to provide broadband absorption of mid‐IR light are structured and studied. Through computational investigations and experimental assessments, the geometric parameters of the metastructure are judiciously selected and the peak of absorption is tuned to encompass the mid‐wave IR (MWIR) to long‐wave IR (LWIR) wavelengths, where absorbance of over ≈95% is achieved in the conducted analyses. These results are obtained using numerical optimization to achieve the highest absorption peak values. Thus, based on the mechanistic understanding of the conducted study, the reported percentage is the highest possible absorption efficiency obtained by the absorber metastructure. Importantly, the projected meta‐absorber is polarization insensitive and performs consistently over a wide range of incidence angles. The engineered multilayer architecture based on artificial media provides new possibilities for the broadband manipulation and control of the IR light.

Coiling-Up Space Metasurface for High-Efficient and Wide-angle Acoustic Wavefront Steering

The recent advent of acoustic metasurface displays tremendous potential with their unique and flexible capabilities of wavefront manipulations. In this paper, we propose an acoustic metagrating made of binary coiling-up space structures to coherently control the acoustic wavefront steering. The acoustic wave steering is based on the in-plane coherent modulation of waves in different diffraction channels. The acoustic metagrating structure with a subwavelength thickness is realized with 3D printed two coiling-up space metaunits. By adjusting structural parameters of the metaunits, the −1st-order diffraction mode can be retained, and the rest of the diffraction orders are eliminated as much as possible through destructive interference, forming a high-efficiency anomalous reflection in the scattering field. The anomalous reflection performance of the designed metagrating is achieved over a wide range of incident angles with high efficiency.

An Ultrahigh Narrowband Absorber Close to the Information Communication Window

In this paper, we demonstrate a plasmonic ultrahigh narrowband perfect absorber, which realizes an absorption intensity of up to 99.99% in the near-infrared electromagnetic spectrum regime. Different dimensional effects on absorption properties are studied using computer simulation technology (CST) with finite element method (FEM) solver. For both transverse electric (TE) and magnetic (TM) polarization, the absorber shows high stability over a wide range of incident angles. This ultrahigh absorption is attributed to synergy effect of magnetic resonance and surface plasmon resonance. Furthermore, the sensitivity goes to 300 nm/RIU for various refractive indices of different analytes. Our proposed absorber working wavelength is very close to the communication window of information; therefore, except for remarkable sensing abilities, it can also be well utilized in optoelectronic applications like optical switching, amplifiers, and all-optical plasmonic modulators.

Ion incidence angle dependent pattern formation at AZ 4562® photo resist by Ar+ ion beam erosion

Interactions between ion beams and polymers have great potential to tailor the properties of the polymer surface by modifying the morphology as well as the chemical composition. The application of a wide range of ion incidence angles (0° - 75°) for the erosion of the commercially available photoresist AZ 4562® revealed a multitude of nanopatterns as investigations using atomic force microscopy and scanning electron microscopy have shown. A sequence of angle-dependent patterns (nanoholes, ripples, facets) can be observed, which are already known from inorganic materials. Additionally, the variation of the erosion time (5 min – 60 min) showed an evolution of the nanopatterns. A changed composition of the polymer surface could be detected by means of X-ray photoelectron spectroscopy.

Multifunctional Waterborne Acoustic Metagratings: From Extraordinary Transmission to Total and Abnormal Reflection

Wavelength-thick multifunctional metagratings for waterborne sound are highly desirable in various application scenarios, such as human health monitoring and ocean exploration. In this article, we present a class of metagratings with which distinct and switchable wave manipulation functionalities, such as extraordinary transmission, total reflection, and abnormal reflection can be achieved through a single acoustic grating. Simultaneous and high-efficiency control over both reflected and transmitted waves is achieved through a systematic design approach in which wave diffraction theory, effective medium theory, and intelligent optimization algorithms are concurrently utilized. Switching between different functionalities can be achieved simply by changing the operation frequency, or by changing the incident angles. Robust and perfect wave-front manipulation effects are obtained over a wide range of incident angles and operation frequencies. Our work not only provides a design paradigm of planar acoustic devices for multiple manipulation purposes, but also presents a feasible solution for the development of integrated acoustic devices for distinct underwater applications.

Performance Enhancement of All-Inorganic Carbon-Based CsPbIBr2 Perovskite Solar Cells Using a Moth-Eye Anti-Reflector.

All-inorganic carbon-based CsPbIBr2 perovskite solar cells (PSCs) have attracted increasing interest due to the low cost and the balance between bandgap and stability. However, the relatively narrow light absorption range (300 to 600 nm) limited the further improvement of short-circuit current density (JSC) and power conversion efficiency (PCE) of PSCs. Considering the inevitable reflectance loss (~10%) at air/glass interface, we prepared the moth-eye anti-reflector by ultraviolet nanoimprint technology and achieved an average reflectance as low as 5.15%. By attaching the anti-reflector on the glass side of PSCs, the JSC was promoted by 9.4% from 10.89 mA/cm2 to 11.91 mA/cm2, which is the highest among PSCs with a structure of glass/FTO/c-TiO2/CsPbIBr2/Carbon, and the PCE was enhanced by 9.9% from 9.17% to 10.08%. The results demonstrated that the larger JSC induced by the optical reflectance modulation of moth-eye anti-reflector was responsible for the improved PCE. Simultaneously, this moth-eye anti-reflector can withstand a high temperature up to 200 °C, and perform efficiently at a wide range of incident angles from 40° to 90° and under various light intensities. This work is helpful to further improve the performance of CsPbIBr2 PSCs by optical modulation and boost the possible application of wide-range-wavelength anti-reflector in single and multi-junction solar cells.

Broadband and wide-angle metamaterial absorber based on the hybrid of spoof surface plasmonic polariton structure and resistive metasurface.

Electromagnetic (EM) wave absorber with broad and robust absorption performance over wide incident angle range is persistently desired in specific applications. In this work, we propose and demonstrate a broadband and wide-angle metamaterial absorber (MA) based on a hybrid of stereo spoof surface plasmonic polariton (SSPP) structure and planar resistive metasurface. At first, we design a broadband SSPP absorber by adjusting the dispersion and loss of the artificial plasmonic structure (PS) simultaneously. Furthermore, owing to utilize its spatial phase manipulation ability, we integrate a resistive metasurface on top of the PS to construct a modified circuit analog (CA) absorber with a dispersive metamaterial spacer. The absorption mechanism of the hybrid structure is analyzed theoretically. The results indicate that the hybrid MA is equipped with broad and robust absorption performance over a wide incident angle range due to the synergistic absorption of the PS and metasurface. Finally, a prototype of the hybrid MA is fabricated by silk-printing technic and its absorption performances are measured. The experimental results can verify the theoretic ones and indicate that proposed hybrid MA can achieve 90% absorptivity from 3.9 GHz to 10.6 GHz with thickness of 7.0 mm, which is only 106% times of the ultimate thickness corresponding to the absorption performance of MA. In general, the concept and design offer a distinct approach of utilizing SSPP to design absorbers with excellent performances from radio frequency to optic band, which are promising for extensive applications.

Modelling of beam energy absorbed locally in conduction mode laser metal fusion

Fluid dynamics models for laser material processing with metal fusion in conduction mode generally assume a constant absorptivity. This parameter is known to govern the process. However, it used to be pre-set by extrapolating absorptance measurements made at different conditions or adjusted to reproduce experimental bead shapes. In this study a new approach is developed. It consists in predicting the absorptance as a function of local surface conditions, including the surface temperature. The proposed absorptance model is applied to the metal alloy Ti-6Al-4V. It is found that the absorptance of this alloy changes with surface temperature over a wide range of beam incidence angles. Thermo-fluid simulations with tracking of the free-surface deformation are performed for conduction mode beam welding test cases with a Yb fibre laser and different travel speeds. It is found that the absorptivity coefficient commonly used for this process clearly underestimates the absorptance and the melt pool geometry predicted for the process conditions of this study. The computational results are also compared against experimental results and good quantitative agreement of the melt pool depth, width, length, free surface contour geometry, and the curvature of the end depression left after re-solidification at the laser switch-off location is obtained. The results show that the absorptance field predicted depends on the melt pool development stage, on the spatial location within the beam spot, and on the process conditions.