Large Angles Of Incidence Research Articles

Line Defects as Valley Filters in Graphene

Hexagonal two‐dimensional crystals typically feature two identical but inequivalent conduction band minima—so‐called valleys—at the corners of the Brillouin zone. Exploiting this additional quantum number to process information—the goal of the field of valleytronics—requires directly manipulating the valley quantum number. Line defects emerging at grain boundaries between regions of pristine graphene could potentially act as valley filters. Quantum transport is quantitatively simulated through two such typical defects in a fully ab initio parameterized tight‐binding model to assess the validity of such a strategy. Pronounced valley polarization effects are found in transport through the line defect, that sensitively depend on the type and electronic structure of the line defect. In particular, a 5‐8‐5 defect features several strongly localized states breaking the valley symmetry, and, accordingly, leads to pronounced valley filtering effects. By contrast, a comparatively similar 5‐7 defect does not feature localized states, instead showing pronounced valley filtering at large angles of incidence. A much weaker energy dependence can still be exploited to select a specific valley. These results highlight the potential of defect engineering for building valleytronic devices, yet also the importance of realistic, quantitative models for judging the properties of tailored defects.

Angle insensitive filters based on Fabry–Pérot resonance structures

Filters based on plasmon resonance suffer from low efficiency due to inevitable metal loss. Moreover, their operational limitations, particularly their inability to function effectively at large incident angles, significantly restrict their practical applications. To address these challenges, we propose an all-dielectric filter characterized by relatively small Ohmic loss and remarkably high efficiency, even at wide incident angles. The filter is based on Fabry–Pérot cavity resonance, with narrow bandwidth and high transmittance. The transmittance in the near-infrared band is as high as 99%, and the transmittance in the shortwave band is highly suppressed. This filter offers ease of manufacture coupled with exceptional efficiency. It is expected to gain application prospects in different fields such as liquid crystal displays, optical communications, sensor detection, and imaging.

Dual-Phase Singularity at a Single Incident Angle with Spectral Tunability in Tamm Cavities.

The phase singularity, a sudden phase change occurring at the reflection zero is widely explored using various nanophotonic systems such as metamaterials and thin film cavities. Typically, these systems exhibit a single reflection zero with a phase singularity at a specific incident angle, particularly at larger angles of incidence (>50 degrees). However, achieving multiple phase singularities at a single incident angle remains a formidable challenge. Here, the existence of a dual-phase singularity is experimentally demonstrated at a lower incident angle using a tunable Tamm plasmon polariton (TPP) cavity that consists of gold-coated ultralow-loss phase change material Sb2S3-based distributed Bragg reflector. It can excite narrowband TPP resonances from normal incidence to a wide angle of incidence for both s- and p-polarizations of light. Notably, this TPP cavity shows dual-phase singularity at lower angles of incidence since the excited TPP for s- and p-polarizations exhibits zero reflection at slightly different wavelengths for the same incident angle. A TPP cavity-based scalable hydrogen sensor is proposed and shows that the dual-phase singularity can further improve the sensitivity of singular phase-based sensing approaches. Moreover, spectrally tunable dual-phase singularity is experimentally demonstrated at a lower incident angle using a metal-free Tamm cavity.

Treatment envelope of transcranial histotripsy: challenges and strategies to maximize the treatment location profile

A 750 kHz, 360-element ultrasound array has been built for transcranial histotripsy applications. This study aims to evaluate its performance to determine whether this array is adequate for treating a wide range of brain locations through a human skull. Treatment location profiles in 2 excised human skulls were experimentally characterized based on passive cavitation mapping. Full-wave acoustic simulations were performed in 8 human skulls to analyze the ultrasound propagation at shallow targets in skulls with different properties. Results showed that histotripsy successfully generated cavitation from deep to shallow targets within 5 mm from the skull surface in the skull with high SDR and small thickness, whereas in the skull with low SDR and large thickness, the treatment envelope was limited up to 16 mm from the skull surface. Simulation results demonstrated that the treatment envelope was highly dependent on the skull acoustic properties. Pre-focal pressure hotspots were observed in both simulation and experiments when targeting near the skull. For each skull, the acoustic pressure loss increases significantly for shallow targets compared to central targets due to high attenuation, large incident angles, and pre-focal pressure hotspots. Strategies including array design optimization, pose optimization, and amplitude correction, are proposed to broaden the treatment envelope. This study identifies the capabilities and limitations of the 360-element transcranial histotripsy array and suggests strategies for designing the next-generation transcranial histotripsy array to expand the treatment location profile for a future clinical trial.

Experimental Investigation of Meter-Level Resolution Radar Measurement at Ka Band in Yellow Sea

The backscatter characteristics of ocean surfaces are of great importance in active marine remote-sensing fields. This paper presents the high spatial and temporal resolution dual co-polarized (VV and HH) and cross-polarized (HV) Ka-band sea-surface backscattering measurements taken from the Yellow Sea research platform at incidence angles ranging from 30° to 50° and in the wind speed range from 5.8 to 8.6 m/s. The experimental results show that the backscattering coefficient in HH polarization is close to (or even surpassing) that in VV polarization within a wind speed range of 7.1 to 8.6 m/s for Ka band under high resolution at medium incidence angles (30°–50°). Further analysis of the 10-ms short-time observation samples found that the sea surface echoes in VV polarization are more sensitive to wave motions, exhibiting more complex scattering characteristics such as multi-peaks and reducing scattering energy, especially at high wind speeds and large incident angles. The Doppler velocity analysis also confirms that rapid ocean wave changes can be detected within a short observation period, especially in VV polarization. The research in this article not only demonstrates the high spatial and temporal resolution capabilities of Ka-band radar for ocean surface observation but also reveals its great potential in interpreting and inversing rapidly evolving marine phenomena.

Radio absorption in structures like artificial magnetic conductors at large angles of incidence of TM-polarized waves

The frequency-angular characteristics of the reflection of TM-polarized waves from thin (thickness up to 1/200 wavelength) artificial magnetic conductor (AMC) and radio absorber (RA) based on band-reflecting and band-passing gratings are presented. It is shown that the operating frequency bands of the AMC and RA expand tens of times when the angle of incidence changes from 0 to 89 degrees. In this case, the value of the ratio (is the difference in wavelengths at the edges of the absorption band and is the thickness of the RA) increases to 30.

Near-infrared multi-band small-angle TE-polarization nonreciprocal thermal emitter

Multi-band small-angle irreversible radiation is significantly demanded in applications of high-resolution infrared sensing and infrared filtering. However, current nonreciprocal thermal radiation is either multichannel radiating with large incident angles or radiating at small angles within one single band. To address this issue, a multi-band nonreciprocal thermal emitter operated by exciting multiple resonances within the near-infrared band is proposed. The adopted microstructure arrays consist of periodic silicon square-ring arrays that are supported by magneto-optical medium and metal layer. With the physical mechanisms study through electromagnetic field energy distribution, the TE-polarized multi-band nonreciprocity of thermal radiation is over 70 % at small incident angle of 4°. Equally important, this strategy is effective and generalizable for dual-polarized small-angle multi-band nonreciprocal radiation with adopted microstructure arrays. It is believed that this scheme can promote the development of nonreciprocal thermal photonics.

Full-scale application of dimensional quality assessment on precast slabs: A scan planning approach

Ensuring precise dimensional quality of individual precast concrete (PC) slabs is crucial to guarantee smooth assembly at construction sites. Although many studies have been conducted to increase the accuracy of dimensional quality assessment (DQA), there has been few studies that systematically determine best locations where accuracy is maximized and scanning time is minimized. To tackle this issue, this study presents a scan planning technique for rapid and accurate DQA of PC slabs. A two-scan strategy along the longitudinal direction is proposed as PC slabs are lengthy to avoid the effect of large incident angle. A series of experimental tests are conducted and the results show a DQA accuracy of 3.1 mm within 582 s for a full-scale slab with 12-m long, outperforming the previous study by 4.6 %. The findings of this study are expected to be useful for accurate and rapid DQA of PC slabs.

Turbulent flow field in maglev centrifugal blood pumps of CH-VAD and HeartMate III: secondary flow and its effects on pump performance.

Secondary flow path is one of the crucial aspects during the design of centrifugal blood pumps. Small clearance size increases stress level and blood damage, while large clearance size can improve blood washout and reduce stress level. Nonetheless, large clearance also leads to strong secondary flows, causing further blood damage. Maglev blood pumps rely on magnetic force to achieve rotor suspension and allow more design freedom of clearance size. This study aims to characterize turbulent flow field and secondary flow as well as its effects on the primary flow and pump performance, in two representative commercial maglev blood pumps of CH-VAD and HeartMate III, which feature distinct designs of secondary flow path. The narrow and long secondary flow path of CH-VAD resulted in low secondary flow rates and low disturbance to the primary flow. The flow loss and blood damage potential of the CH-VAD mainly occurred at the secondary flow path, as well as the blade clearances. By contrast, the wide clearances in HeartMate III induced significant disturbance to the primary flow, resulting in large incidence angle, strong secondary flows and high flow loss. At higher flow rates, the incidence angle was even larger, causing larger separation, leading to a significant decrease of efficiency and steeper performance curve compared with CH-VAD. This study shows that maglev bearings do not guarantee good blood compatibility, and more attention should be paid to the influence of secondary flows on pump performance when designing centrifugal blood pumps.

Multi-mode hybridization in the MXene tetramer metasurface for ultra-broadband solar energy utilization

MXene is promising in photothermal or photovoltaic conversion, while high-performance MXene metasurface solar absorbers based on simple and feasible structures are still lacking. This study aims to design a solar absorber with ultra-broadband absorption capability in the visible and near-infrared wavelength ranges based on the MXene nanoblock tetramer/silica film/MXene substrate structure. The average absorptivity of this proposed metasurface absorber is 96.9% in the wavelength range of 300–2500 nm covering the whole solar spectrum. The physics behind the high absorption results from multiple-mode hybridization in different resonant bands, including the coupling between the surface plasmons, cavity resonances, and guided-mode resonances. The broadband and high-absorption performance remains stable under large-angle incidence and structural parameter variations with the average absorption above 90% in the whole wavelength region of interest. The calculated energy absorption ratio of the AM1.5 solar radiation spectrum can reach up to 96.3%, indicating low solar energy loss and efficient solar energy capture. In summary, these results provide great application prospects in the fields of photothermal and photovoltaic conversion.

Time delay of reflected and transmitted laser pulse at laser action on a dense plasma slab

The time delay phenomenon of reflected and transmitted laser pulse under the action of s-polarized laser radiation on a supercritical plasma slab is considered. The dependence of the time delay and peak intensity of reflected and transmitted signals on the slab thickness, plasma density, and angle of laser radiation incidence is investigated. It is shown that with increasing thickness of the plasma slab, the saturation of the transmitted signal delay time (Hartman effect) does not occur due to the exponential decrease in its intensity. The tunneling speed of the laser pulse energy through the slab of supercritical plasma is calculated, and it is shown that at large angles of laser radiation incidence, it can exceed the speed of light in vacuum.

Ultra-wideband solar absorber via vertically structured GDPT metamaterials

We explore a new solar absorber via vertically structured metamaterials integrating 2D graphene, dielectric layers of silica (SiO2), aluminum oxide (Al2O3), and phase-transition vanadium dioxide (VO2) graphene, dielectric, phase transition (GDPT), achieving broadband absorption and polarization independence. Applying the finite-difference time-domain (FDTD) method, the optical performance of the absorber is analyzed under the normal incidence of a plane wave spanning wavelengths from 250 to 4000 nm. Particularly, our investigation reveals a broad bandwidth exceeding 90 % absorption, spanning 3559 nm from 250 to 3809 nm, with an impressive average absorption efficiency of over 95.9 %. Significantly, the absorber demonstrates polarization independence and insensitivity to large angles of incidence, enhancing its practical capability. We illuminate the important role of Fabry-Perot resonance in absorption by precisely tuning layer thicknesses to induce interference effects. The integration of graphene, dielectric layers, and VO2, augmented by Fabry-Perot resonance, provides a vital foundation for high-performance solar energy conversion technology. This technology has important applications in efficient and broadband solar absorbers, and these absorbers can maintain high performance under different conditions. Our goal is to solve current technological challenges and contribute to the development of next-generation solar energy conversion for various photon technologies, such as energy harvesting, electromagnetic wave capture, and photovoltaic devices.

Biomimetic Wings for Micro Air Vehicles.

In this work, micro air vehicles (MAVs) equipped with bio-inspired wings are investigated experimentally in wind tunnel. The starting point is that insects such as dragonflies, butterflies and locusts have wings with rigid tubular elements (corrugation) connected by flexible parts (profiling). So far, it is important to understand the specific aerodynamic effects of corrugation and profiling as applied to conventional wings for the optimization of low-Reynolds-number aerodynamics. The present study, in comparison to previous investigations on the topic, considers whole MAVs rather than isolated wings. A planform with a low aperture-to-chord ratio is employed in order to investigate the interaction between large tip vortices and the flow over the wing surface at large angles of incidence. Comparisons are made by measuring global aerodynamic loads using force balance, specifically drag and lift, and detailed local velocity fields over wing surfaces, by means of particle image velocimetry (PIV). This type of combined global-local investigation allows describing and relating overall MAV performance to detailed high-resolution flow fields. The results indicate that the combination of wing corrugation and profiling gives effective enhancements in performance, around 50%, in comparison to the classical flat-plate configuration. These results are particularly relevant in the framework of low-aspect-ratio MAVs, undergoing beneficial interactions between tip vortices and large-scale separation.

Numerical investigation of rip currents near a barred beach induced by irregular waves

Rip currents pose a common natural hazard at coastal tourist beaches, presenting a significant threat to the safety of beachgoers and beach management. To better understand the characteristics of rip currents, particularly those induced by irregular waves on a barred beach, this study utilized a Boussinesq-type, phase-resolving hydrodynamic model for numerical simulations. We investigated various factors, including significant wave height, peak period, incident angle, and bottom friction, to assess their impact on rip currents formation. The results of our research reveal some key insights. Firstly, an increase in significant wave height and peak period fosters the development of rip currents. However, beyond a certain threshold, the intensity of the rip currents decreases. Conversely, a larger incident angle tends to reduce the intensity of rip currents. Additionally, higher bottom friction leads to increased energy dissipation during wave propagation, consequently resulting in decreased rip currents intensity. In an effort to create a more precise representation of real sea conditions, multi-directional waves were employed as incident waves in this study. An analysis of different standard deviations of directional distributions revealed that as the directional spread of incident waves widens, the waves become more susceptible to breaking, ultimately leading to a reduction in the intensity of rip currents.

Terahertz wide-band wide-angle absorber and polarization converter based on VO2 metamaterial

In this study, a Terahertz wide-band wide-angle absorber and polarization converter based on VO2 metamaterial is proposed. The device is simulated by using full-wave electromagnetic computing software CST. The device functions as a wide-band absorber while VO2 is in the metallic mode. In the 1.82 THz to 4.27 THz band, the absorption efficiency exceeds 90% and the relative bandwidth reaches 81.4%. The absorption mechanism is analyzed bying equivalent circuit theory. Additionally, the absorber can work at large polarization angle and incident angle. While VO2 is in the insulated mode, the design can convert the incident y-polarized terahertz wave into the x-polarized state and realize the transformation of linear polarization. Polarization conversion efficiency (PCR) exceeds 90% in the band of 1.57 THz to 3.82 THz, with a relative bandwidth of 83.5%. The device also supports large incidence angle tolerances. These results show that the disigned metamaterial device has potential applications in absorption, polarization conversion and imaging.

A new seismic wave input method for canyon sites based on the finite element method-indirect boundary integral equation method

The seismic input is the basis for the seismic safety analysis of dams, but current seismic input methods have not reasonably considered the influence of canyon scattering effects on the dynamic response of dams. In this paper, the total motion field of the canyon is deconstructed into the free field of a uniform half space and the scattering field of the canyon and they are obtained separately. The indirect boundary integral equation method (IBIEM) is used to determine the scattering field of the canyon, which is superimposed with the free field to obtain the seismic ground motion field (total motion field) of the canyon. The accuracy of the total motion field of the canyon is verified through reference solutions. In the finite element analysis of seismic ground motion field in the canyon, the total motion field at the truncation boundary of the canyon foundation is transformed into the equivalent seismic input loads and the interior of the canyon are simulated using the finite element method (FEM). Then, based on the wave input for the free field combined with the viscoelastic artificial boundary, a new wave input method for the total motion field combined with the viscoelastic artificial boundary is established. The seismic wave input method based on the finite element-indirect boundary integral equation method is applied to numerically determine the seismic response of a trapezoidal canyon. The differences in the seismic response of the canyon under free-field input and total motion field input are discussed. The results show that there is a certain deviation between the displacement and waveform obtained from the free-field input and the solutions obtained from the indirect boundary integral equation method. There is a large error for a large angle of incidence, and the errors increase with increasing seismic wave frequency. When the incident angle is 60°, the maximum error in the frequency domain reaches 92.3 %, and the maximum error in the time domain reaches 250 %. The displacement amplitude and waveform of the canyon obtained from the total motion field input showed agreement with the results of the indirect boundary integral equation method. The wave input method established in this paper has high calculation accuracy and reasonably considers the scattering effect of canyons, which provides a basis for more accurate predictions of the seismic response of dams on canyon foundations.

Research on multi-resonance mechanism to achieve ultra-wideband high absorption of a metamaterial absorber in the UV to MIR range

Wideband metamaterial perfect absorbers have been extensively researched due to their excellent properties. A four-layer ultra-broadband metamaterial perfect absorber engraved with a square cavity is proposed. A clever combination of grating and square cavity allows the absorber to manifests an overall absorption of over 92 % and an average absorption rate of 97.27 % within the operational band from 280 to 5400 nm. Through analysis, it can be concluded that the coupling effects of Rayleigh anomaly (RA), magnetic resonance (MR), cavity resonance (CR), propagating surface plasmon resonance (PSPR) and local surface plasmon resonance (LSPR) result in perfect absorption. The absorber is insensitive to polarization and large angle incidence. It is worth mentioning that the average absorption rate of our structure reaches 93.32 % at a large incidence angle of 60° in TE mode, which is much higher than that of other absorbers. At 1000 K, the total photothermal conversion efficiency of our absorber is still above 90 %. The designed structure has the following advantages: high absorption, ultra-wide bandwidth, high tolerance to fabrication errors and insensitivity to large angle incidence. Thus, the absorber can be used in the solar energy harvesting and perfect stealth. Additionally, it also can be served as a reference in thermophotovoltaics, thermal imaging and infrared detection.

A Highly Efficient Plasmonic Polarization Conversion Metasurface Supporting a Large Angle of Incidence

The angle of incidence of the compact polarization conversion device is crucial for practical use in integrated miniaturized optical systems. However, this index is often ignored in the design of quarter-wave plate based on metasurface. Herein, it is shown that a thick metallic cross-shaped hole array supports extraordinary optical transmission peaks controlled by a Fabry–Pérot (FP) resonator mode. The positions of these peaks have been proven to be independent over a large range of incidence angles. We numerically design a miniatured quarter-wave plate (QWP) with an 80 nm bandwidth (840~920 nm) and approximately 80% average efficiency capable of effectively functioning as a linear-to-circular (LTC) polarization converter at an incidence inclination angle of less than 30°. This angle-insensitive compact polarization conversion device may be significant in a new generation of integrated metasurface-based photonics devices.

Multi-functional switchable terahertz metasurface device prediction by K-nearest neighbor

In this paper, we present the design of a versatile terahertz (THz) device characterized by its multi-functional capabilities including broadband absorption and polarization modulation, achieved through leveraging the temperature-induced phase transition behavior of vanadium dioxide (VO2). Operating in the metallic state of VO2, the device exhibits broadband absorption properties, delivering a remarkable total effective absorption of 2.871 THz with absorption rates exceeding 90 % across the frequency ranges of 8.140∼9.405 THz and 12.740∼14.346 THz. Furthermore, it demonstrates excellent adaptability to large angle incidence. Conversely, in the dielectric state of VO2, the device transitions into a polarization modulation mode, facilitating linear-to-cross-polarization (LTX) and linear-to-circular-polarization (LTC) conversions. Notably, for LTX polarization, the conversion efficiency exceeds 90 % within the 8-11.5 THz range, while for LTC polarization, the ellipticity surpasses 0.8 within the 7.861∼8 THz range. Additionally, we introduce a machine learning (ML) approach to optimize the device parameters, presenting a novel strategy for enhancing the design and optimization of future multifunctional devices.

Multi-band enhanced nonreciprocal thermal radiation based on Weyl semimetals.

Previous studies manifested that the majority of structures that exhibit nonreciprocal thermal radiation in the mid-infrared are capable of achieving either single-band strong nonreciprocity or multi-band weak nonreciprocity at a large incidence angle. However, few structures can realize multi-band strong nonreciprocity at a small incidence angle. To address such scientific issues, we propose a tunable nonreciprocal thermal emitter based on gallium arsenide (GaAs)/graphene/Weyl semimetal (WSM). This device is capable of achieving strong nonreciprocity at 7.3 μm, 10 μm and 13.6 μm wavelengths at an incidence angle of 25.5°. It is shown that the field enhancement of the GaAs/graphene composite layer can improve the nonreciprocal response of the WSM layer. In addition, by changing the Fermi energy level of graphene and the axial vector b of the Weyl semimetal, tunable nonreciprocal thermal radiation can be realized. What's more, we find that the structure breaks Kirchhoff's law without lithography and an external magnetic field, which reveals the advantages of applying our research in the field of thermal radiation.