Normal Incidence Research Articles

Broadband perfect Littrow diffraction metasurface under large-angle incidence

Abstract Littrow diffraction devices are commonly used in the laser field (e.g., laser resonators and spectrometers), where system integration requires larger incidence angles and perfect broadband efficiency. Compared to traditional diffraction devices, which struggle to manipulate light paths under large-angle incidence, metasurfaces has the potential to enhance the broadband efficiency. Despite quasi three-dimensional metasurfaces effects, only perfect anomalous reflection under normal incidence at limited wavelengths was achieved due to energy flow mismatch in the broadband Littrow configuration. Here, we propose a supercell metasurface capable of regulating broadband non-local responses. The metasurface effectively suppresses non-local responses under Littrow mounting, while providing sufficient non-local responses through strong structural coupling effects when the incidence deviates from the Littrow mounting. A large-angle broadband Littrow diffraction metasurface in the mid-infrared spectrum (3.11 µm ∼ 3.52 µm) has been successfully realized, with 99 % efficiency at Littrow angle of 70°. Our results break through the bandwidth limitations of perfect diffraction, providing robust support for the practical applications of metasurfaces in Littrow diffraction devices.

Monte Carlo simulations of backscattered electron coefficients and average penetration depths for Cu, Au, and Al under electron irradiation

Understanding and analyzing backscattered electron coefficients (BSCs) and average penetration depths (APDs) are critical in material science and electron microscopy. However, despite their importance, there are relatively few studies that cover a wide range of energies and a variety of materials for accurately calculating BSCs and APDs. Therefore, a Monte Carlo (MC) simulation was conducted to examine and explore the electron backscattering coefficients and average penetration depths of copper (Cu), gold (Au), and aluminum (Al) when bombarded by energetic electrons with energies ranging from 0 to 60 kilo-electron volts (keV) at normal incidence. The results showed strong agreement with experimental data. First, for the BSCs, we have deviations ranging from 0.3 to 5.4%. Second, the empirical calibration adjustment resulted in an excellent agreement with the experimental data of APDs. For Cu, the deviation was 3.32% at 5 keV. The exceptional agreement was observed at 9 keV for Au, with a deviation of just 0.08%. In the case of Al, the adjustment achieved a strong agreement with a deviation of 2.01%. These findings improve our understanding of backscattered electrons behavior by providing accurate simulations for Cu, Au, and Al across a wide energy range, resolving discrepancies, especially for low-Z materials such as aluminum (Z = 13). The improved accuracy in predicting APD and the original BSC results support scanning electron microscopy (SEM) applications, particularly in compositional and topographic imaging.

Green water loads on prismatic obstacles

Green water loads on prismatic obstacles (representing topside structures) mounted on the raised deck of a simplified vessel are investigated using computational fluid dynamics simulations and physical model testing with emphasis on examining different structure shapes, orientation angles and relative structure size. For each scenario investigated, several flow features are identified that characterize the green water interaction with the structure and influence loads, namely delayed flow diversion, formation of a vertical jet, scattered wave formation and the development of complex wake patterns. Comparing across structures, these interactions are more pronounced for blunt objects, and the associated force impulse is larger. For example, a cube with flow at normal incidence is found to experience approximately twice the force impulse of a circular cylinder of the same projected area. Equally, rotation of the cube leads to reduced run-up height and streamwise force on the structure. To explain these trends, a theoretical model based on Newtonian flow theory is adopted. This model provides an estimate of the streamwise force exerted on obstacles in high-Froude-number flows and shows good agreement with the numerical results when the flow is supercritical, shallow (small water depth relative to structure width) and the structure is tall (large structure height relative to water depth). Despite some limitations, the model should provide an efficient force prediction tool for practical use in design.

Photon-stimulated desorption from laser-etched vacuum structural materials of accelerators

Abstract The electron multipacting (EM) and the electron cloud (e-cloud) can have detrimental impact on the operation of the high energy or high current intensity particle accelerators. The enhanced performance of new generation particle accelerators has necessitated the development of advanced structural materials and vacuum systems. The EM and e-cloud effect have emerged as significant challenges which cannot be ignored during the design and construction of these advanced accelerators. Laser-etching technique represents a promising approach for the suppression of secondary electron emission from materials, which can lower the secondary electron yield (SEY) of the structural materials to below 1. Nevertheless, the deployment of laser-etching technique in the next generation of large particle accelerators requires a comprehensive investigation into its vacuum performance. In this work, a photo-stimulated desorption (PSD) experimental beamline was constructed on the Hefei Light Source II. The PSD yields of the structural materials (oxygen-free copper, chrome zirconium copper, stainless steel, and aluminum alloy) with and without laser-etching of the vacuum chambers were tested at normal incidence using the throughput method. The main gases desorbed from all sample surfaces are H2, CO, CO2, CH4, and H2O. The initial PSD yield of laser-etched material is higher than that of unetched material. However, the PSD yield of the laser-etched material decreases at a faster rate, and after a certain photon dose, the PSD yield of the laser-etched material is in the same order of magnitude as that of the unetched material.

Chiral Guided Mode Resonance with Independently Controllable Quality Factor and Circular Dichroism.

Chiroptical resonances with high quality factors (Q factors) have recently garnered extensive attention due to their broad applications in lasing and optical sensing. However, the independent manipulation of the Q factor and circular dichroism (CD) of chiroptical resonances has rarely been proposed. Here, we demonstrate that the Q factor and CD of guided mode resonance (GMR) can be independently manipulated by simply varying two structural parameters in a diatomic dielectric metasurface grating, offering a new paradigm for chiroptical resonance manipulation. We reveal that the independent manipulation of the Q factor and CD of the GMR is attributed to the modulation of the collective interference of guided mode fields excited by the two orthogonal linearly polarized normal incidence. GMRs with a Q factor of 183 and CD of ±0.62 have been experimentally validated, which is comparable to state-of-the-art chiral quasi-BICs. These findings provide a powerful platform for the realization of high-Q chiroptical resonances.

Hybrid phase-correction metasurfaces for accurate near-field manipulation

Although metasurface has attracted significant attention in recent years due to its ability to manipulate electromagnetic waves, discrepancies are often observed when comparing the designed and realized near-field performances. Among various contributing factors, one important issue is related to the fact that the elements arranged on a metasurface are typically designed under the assumption of normal incident excitation, whereas in practice they are actually excited at various oblique incidence angles depending on the element's relative position to the feed antenna. This introduces amplitude and phase errors in the reflection or transmission coefficients of the elements within the metasurface, thereby affecting its overall performance. In this paper, a hybrid phase-correction method is proposed to compensate for the effect of oblique incidence on metasurfaces. The actual oblique incidence angles are decomposed into elevation and azimuth directions: In the elevation direction, the element characteristics under oblique incidence replace those under normal incidence, while in the azimuth direction, element rotation is introduced to compensate for the local effects. Near-field focusing and holographic metasurfaces are designed to verify the proposed hybrid phase-correction method. Full-wave electromagnetic simulations show that the proposed metasurface has the advantage in sidelobe levels without significant costs in terms of the amount of element-level computations. Finally, the near-field focusing metasurfaces are fabricated, and the measured results further demonstrate the potential of the proposed design for accurate near-field manipulation.

Estimation of surface doses in the presence of an air gap under a bolus for a 6 MV clinical photon beam - a phantom study.

Goal of the present study was to develop and build a phantom that replicates the air gaps under a gel bolus and to estimate the surface dose (Dsurf) under normal incidence with a 6 MV photon beam. For this, an acrylic phantom with 10 plates, each including five open slots (one in the centre and four off axis) with a size of 2cm × 2cm at depths of 0.54cm, 0.72cm, 0.90cm, 1.26cm, and 1.62cm from the phantom's surface was used. Computed tomography image sets were obtained without and with a gel bolus (thickness: 2mm, 4mm, and 6mm) placed on top of the phantom. Dose calculations were performed with the XiO treatment planning system (TPS) for a 6 MV photon beam at normal incidence and a field size of 15cm × 15cm that covered all the slots. A virtual bolus in TPS was employed in CT picture sets that did not include a bolus. Six points of interest at a depth of 1mm from the surface contour of each slot were used to determine the mean surface dose (Dsurf) estimated by the TPS with and without the presence of a bolus. It turned out that, as the depth of the air gap (between skin surface and bolus surface) increased from 0.54cm to 1.62cm, there was a 25.2% increase in Dsurf without bolus, followed by an increase of 7.6%, 6.4%, and 7.7% for a virtual bolus with 2mm, 4mm, and 6mm thickness, while corresponding increases were 14.8%, 14.3%, and 8.3% for an actual bolus, respectively. However, as the thickness of the air gap increased, Dsurf under the bolus decreased (from - 17.5% to -18.8%, and from - 10.4% to -16.9%, for a virtual and a physical bolus, respectively). It is concluded that, to ensure a homogeneous Dsurf across the treatment area, extra attention should be given while utilizing a bolus in clinical radiation applications, to avoid any air gaps under the bolus.

Polarization beam splitter based on double-groove slanted grating under normal incidence

Polarization beam splitter based on double-groove slanted grating under normal incidence

Analysis of the defect mode features in an asymmetric and symmetric acoustic system using expansion chambers

In this paper, the transfer matrix method is used to study the dispersion of acoustic waves in a finite periodic expansion chambers system with a defect. Two kinds of structures are studied. The first one is formed by expansion chambers, which are symmetrical concerning a defect, and the second one is asymmetrical with a defect. Studying the properties of consistent peak (defective mode) at normal incidence is the objective of these calculations. Consistent peaks arise in the forbidden frequency intervals when a fault in the perfectly periodic structure is present. Concerning the defected asymmetrical configuration, there is a consistent peak located inside acoustic band gap. On the other hand, in the symmetrical structure, two consistent peaks appear in the band gap. In the asymmetrical structure, the consistent peak height depends on the variance in the cross-section of the defect guide. Moreover, the impact of the defect guide’s length on the number of consistent peaks that appear in the band gap will be examined. These results can be used in several applications, such as the design of new kinds of acoustic filters.

Effect of seabed condition on the hydrodynamic performance of a pile-restrained H-shaped floating breakwater

The present study investigates the hydrodynamic analysis of pile-restrained H-shaped porous breakwater for various seabed conditions using the small amplitude wave theory. The Multi-Domain Boundary Element Method (MDBEM) is employed to investigate the influence of parametric variations on the hydrodynamic coefficients and horizontal wave force under normal and oblique incident waves. The numerical accuracy is ensured by comparing it with the available literature. The numerical investigation on the hydrodynamic performance of the H-shaped breakwater is performed for various seabed configurations considering different angles of slope, the width of slope/step/obstacle, step height, number of steps, soil permeability, angle of wave incidence, the width of flange and submergence draft of the web of the H-shaped structure. The findings indicate that the seabed undulation has a higher wave impact on the breakwater than the horizontal seabed. In addition, the study suggests that the sloped seabed is preferable in deeper water depths to reflect waves efficiently and the seabed permeability can affect the hydrodynamic coefficients in shallow and intermediate water depths. The study performed on the H-shaped breakwater for varying seabed topography will be helpful in the design and construction of a suitable H-shaped breakwater for an effective wave absorber in coastal regions.

Topologically Engineered High-Q Quasi-BIC Metasurfaces for Enhanced Near-Infrared Emission in PbS Quantum Dots.

Enhancing photoluminescence (PL) efficiency in colloidal quantum dots is pivotal for next-generation near-infrared photodetectors, imaging systems, and photonic devices. Conventional methods, especially metal-based plasmonic structures, suffer from large optical losses, which limits their practical use. Here, we introduce a quasi-bound state in the continuum (quasi-BIC) metasurface on a silicon-on-insulator platform, tailored to provide high-quality factor resonances with minimized losses. Utilizing topological charge engineering and controlled in-plane asymmetry in silicon cylinder arrays, we developed a robust quasi-BIC capable of maintaining a high Q factor across a broad angular range, achieving an experimental Q factor of 3031 at normal incidence. This approach significantly enhances near-field interactions, achieving a ≤110-fold increase in PL for PbS quantum dots at 33 K and a 41-fold enhancement at room temperature. Our findings offer a scalable, cost-effective solution for enhancing light emission in advanced optoelectronic applications.

Conformal reconfigurable holographic metasurface for multifunctional radiation and scattering modulation.

A reconfigurable holographic metasurface (HM) with multifunctional modulation of radiation and scattering for conformal applications is designed in this paper. Based on optical holography theory, a holographic conformal modulation mechanism is proposed, and the conformal surface impedance distribution of HM is derived. To illustrate this mechanism, the designed conformal reconfigurable HM is used to demonstrate a series of radiation and scattering modulation functions, with its reconfigurable property enabling dynamic beam control. In radiation mode, beam scanning with wide angle from -50° to 50° is achieved. In scattering mode, specific responses are generated under different incident angles, including beam steering under oblique incidence within ±60°, multi-beam splitting within ±60° under normal incidence, and diffuse reflection. Low radar cross section (RCS) is exhibited over a wide frequency band from 7.2 to 25 GHz. The designed conformal reconfigurable HM shows high adaptability to cylindrical platforms, insensitivity to oblique incidence, and stability in beam deflection angles, which provides an innovative technical approach for information transmission and stealth in conformal devices.

Polarization-Insensitive, High-Efficiency Metasurface with Wide Reception Angle for Energy Harvesting Applications.

This research presents an innovative polarization-insensitive metasurface (MS) harvester designed for energy harvesting applications at 5 GHz, capable of operating efficiently over wide reception angles. The proposed MS features a novel wheel-shaped resonator array whose symmetrical structure ensures insensitivity to the polarization of incident electromagnetic (EM) waves, enabling efficient energy absorption and minimizing reflections. Unlike conventional designs, the metasurface achieves near-unity harvesting efficiency, exceeds 94% under normal incidence, and maintains superior performance across various incident angles for TE and TM polarizations. To validate the design, a 5 × 5-unit cell array of the MS structure was fabricated and experimentally tested, demonstrating excellent agreement between simulation and measurement results. This work significantly advances metasurface-based energy harvesting by combining polarization insensitivity, wide-angle efficiency, and high absorption, making it a compelling solution for powering wireless sensor networks in next-generation applications.

Acoustic absorption of 3D printed samples at normal incidence and as a duct liner

Prediction of the acoustic performance of 3D printed materials is investigated at normal and grazing incidence. A direct numerical (microscopic) simulation that solves the full set of Navier-Stokes equations is used as a reference. It is compared with a macroscopic approach in which the material is represented by an equivalent fluid. The materials have a periodic microstructure, consisting either of a single network of spherical or cubic cavities connected by cylindrical channels or of a double-nested network. The samples are printed using the stereolithography technique and are tested using an impedance tube and a duct test bench. For single network geometries, the results of sound absorption at normal and grazing incidence predicted using the equivalent fluid approach are in good agreement with those obtained by the microscopic approach. Comparisons with impedance tube measurements confirm that both approaches can accurately predict the absorption coefficient of the samples. For the in-duct liner configuration, the transmission loss measurements and predictions show similar evolution with frequency change, despite the discrepancy in amplitude. For the double network geometry, the equivalent fluid approach cannot exactly reproduce the results obtained with the direct numerical simulation. Finally, while the predictions with the microscopic approach provide a good match with the impedance tube measurements, only a poor agreement is obtained using the duct testing bench.

Optimization of parallel coiled cavities of different depths in microperforated panel sound absorbers

This paper presents a microperforated panel (MPP) sound absorber with parallel coiled-up-cavities of different-depths (PCD) and the corresponding optimization on their cavities. In this study, an analytical model is initially proposed for estimating the cavity depths of the PCD-MPP absorber upon normal incidence absorption coefficient evaluation at given resonance frequencies. Cavity effective depths and normal incidence absorption coefficient are evaluated after coiling up cavities for a compact structure. Numerical simulations with the finite element method and experiments are conducted for validations. Subsequently, a design process is suggested on the basis of the proposed model for sound absorption optimization. Results show that, absorption coefficient from the proposed analytical model agrees well with finite element simulations and experiments. It is also shown that, for the effective depth evaluation of the coiled-up cavities of PCD-MPP absorbers, the diagonal lines of subchannels of coiled-up cavities with a coiled-up angle of 180° can accurately represent the effective depths, while the combination of centerlines of subchannels and quarter arc inside the coiled-up area is more suitable for those with a coiled-up angle of 90°. Optimization investigation shows that, PCD-MPP absorbers can have high absorption performance with the averaged and maximum absorption coefficient of 0.91 and 0.98 within the target bandwidth of 400–1600 Hz, where the absorber thickness can stay below 65 mm. This work can provide valuable guidelines for the design of sound absorbers for broadband absorption.

An ultrasensitive angular interrogation metasurface sensor based on the TE mode surface lattice resonance

The localized surface plasmon resonance metasurface is a research hotspot in the sensing field since it can enhance the light-matter interaction in the nanoscale, but the wavelength sensitivity is far from comparable with that of prism-coupled surface plasmon polariton (SPP). Herein, we propose and demonstrate an ultrasensitive angular interrogation sensor based on the transverse electric mode surface lattice resonance (SLR) mechanism in an all-metal metasurface. In theory, we derive the sensitivity function in detail and emphasize the refraction effect at the air-solution interface, which influences the SLR position and improves the sensitivity performance greatly in the wide-angle. In the measurement, a broadband light source substitutes the single-wavelength laser generally used in traditional angular sensing, and the measured SLR wavelength of broadband illuminant at normal incidence is defined as the single wavelength, avoiding the sensitivity loss from the large angle. The experimental sensitivity can reach 4304.35°/RIU, promoting an order of magnitude compared to those of SPP-sensors. This research provides a novel theory as well as the corresponding crucial approach to achieving ultrasensitive angular sensing.

Probing the Néel order in altermagnetic RuO2 films by X-ray magnetic linear dichroism

Abstract The emerging altermagnetic RuO2 with both compensated magnetic moments and broken time-reversal symmetry possesses nontrivial magneto-electronic responses and nonrelativistic spin currents, which are closely related to magnetic easy axis. To probe the Néel order in RuO2, we conducted Ru M 3-edge X-ray magnetic linear dichroism (XMLD) measurement. For epitaxial RuO2 films, characteristic XMLD signals can be observed in either RuO2(100) and RuO2(110) at normal incidence or RuO2(001) at oblique incidence, and the signals disappear when test temperature exceeds Néel temperature. For nonepitaxial RuO2 films, the flat lines in the XMLD patterns of RuO2(100) and RuO2(110) demonstrate that there is no in-plane uniaxial alignment of Néel order in these samples, due to the counterbalanced Néel order of the twin crystals evidenced by X-ray diffraction phi-scan measurements. Our experimental results unambiguously demonstrate the antiferromagnetism in RuO2 films and reveal the spatial relation of Néel order to be parallel with RuO2 [001] crystalline axis. These research findings would deepen our understanding of RuO2 and other attractive altermagnetic materials applied in the field of spintronics.

Dual‐band flexible FSS conformal to elliptic cylindrical surface

AbstractThis paper introduces a conformal frequency selective surface (FSS) with dual passbands from 9 to 9.3 GHz and 13.1–13.7 GHz, as well as one stopband from 10.4 to 10.9 GHz. The frequency response of the curved FSS coincides with the performance of the planar FSS with the same unit cell under normal incidence. The unit cell comprises two compact flower‐shaped metallic slots patched on the dielectric substrate. This design has excellent stability of the incident angle up to 70°. The equivalent circuit model considering the mutual coupling between the two slots is illustrated to analyse the resonance mechanism. The whole antenna system enclosed by the conformal FSS is simulated to evaluate the loss in the far‐field beam's directivity and aberration. The impact caused by the different curvatures is investigated. The samples in planar and elliptic cylindrical surfaces are fabricated. The insertion losses of both polarisations are less than 1.5 dB, as the angle of incidence varies to 60°. The frequency response of curved FSS is identical to the planar FSS under normal incidence. The maximum attenuations of both polarisations in the passbands are less than 1.38 dB, and the average transmission coefficient in the stopband is −22 dB. The conformal FSS brings slight aberration in the half power beamwidth (HPBW) of the incident beam in the passbands. Furthermore, it suppresses the main beam in the stopband by at least 25 dB.

Angle and Polarization Insensitive RCS Reduction Metasurface Based on Hybrid Mechanism of Polarization Conversion and Absorption

AbstractThis article introduces the concept, theory, and design of an angle and polarization insensitive radar cross section (RCS) reduction metasurface, using a hybrid mechanism of polarization conversion and absorption. By introducing ladder‐ and rectangle‐shaped metallic patches in the vertical dimension of a 3‐D structure, polarization conversion rate (PCR) deterioration, brought by the increase of equivalent substrate thickness at oblique incidences, can be suppressed. Furthermore, lumped resistors are loaded at proper places in each polarization conversion cell, to achieve the power absorption while maintain the angular insensitivity of the PCR. With the above hybrid mechanism, a stable 10‐dB RCS reduction can be achieved regardless of the angle of incidence in a wide range and polarization directions. An equivalent circuit model is established for explaining the physical mechanism of the proposed metasurface. For validation, a prototype is fabricated and tested. Measurement results indicate that, for both monostatic RCS at the normal incidence and specular RCS of off‐normal incidences from 0° to 45°, a 10‐dB TE‐ and TM‐mode RCS reduction can be achieved in the entire X‐band (8–12 GHz) and Ku‐band (12–18 GHz).

Normal incidence impacts by hollow projectiles produce vertical plumes: Application to planetary exploration missions

Normal incidence impacts by hollow projectiles produce vertical plumes: Application to planetary exploration missions