Normal Incident Wave Research Articles

Realization of Single-Layer Fourier Phased Metasurfaces for Wideband RCS Reduction

An efficient and fast strategy to design and realize single layer Fourier phased metasurfaces for wideband radar cross section (RCS) reduction when illuminated by a circular polarization (CP) plane wave is proposed in this letter. The scattering phase (between 0 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">o</sup> and 360 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">o</sup> ) required at each unit cell of the proposed metasurfaces was computed using the Fourier phase formula in which the focal length ( <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">F</i> ) is inversely proportional to the phase distribution. Pancharatnam-Berry (PB) phase theory was applied with unit cells of subwavelength periodicity to further enhance the scattering and RCS reduction characteristics. The proposed wideband Fourier phased metasurface has a square shape and contains 30 × 30 PB unit cells with subwavelength periodicity of 5 mm ≈ 0.26λ <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">16GHz</sub> . Both simulation and measured results show that the proposed Fourier phased metasurfaces can achieve more than 10 dB of RCS reduction under normal incidence of CP plane wave regardless of the value of <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">F</i> . Under oblique incidence, more than 10 dB of RCS reduction was maintained for incident angles up to 60°. In addition, the single-layer Fourier phased metasurface features wideband 10 dB RCS reduction bandwidth from 10 GHz to 24 GHz with a thickness of only 2 mm. This resulted in an 82.3% fractional bandwidth (FBW) of RCS reduction which is higher than other designs reported in the literature. The proposed design strategy provides a promising way to design and realize metasurfaces for wideband and stable RCS reduction performance without the need to use a computationally complex and/or time-consuming and slow running optimization algorithm.

Field Observation and Quasi-3D Numerical Modeling of Coastal Hydrodynamic Response to Submerged Structures

Even though submerged breakwater reduces incident wave energy, it redistributes the coastal area's wave-induced current, sediment transport, and morphological change. This study examines the coastal hydrodynamics and the morphological response of a wave-dominated beach with submerged breakwaters installed through field observation and quasi-3D numerical modeling. The pre-and post-storm bathymetry, water level, and offshore wave under storm forcing were collected in Bongpo Beach on the East coast of Korea and used to analyze the coastal hydrodynamic response. Four vertically equidistant layers were used in the numerical simulation, and the wave-induced current was examined using quasi-3D numerical modeling. The shore normal incident wave (east-northeast) generated strong cross-shore and longshore currents toward the hinterland of the submerged breakwater. However, the oblique incident wave (east-southeast) induced the southeastward longshore current and the sedimentation in the northeast area of the beach. The results suggested that the incident wave direction is a significant factor in determining the current and sediment transport patterns in the presence of the submerged breakwaters. Moreover, the quasi-3D numerical modeling is more appropriate for estimating the wave transformation, current, and sediment transport pattern in the coastal area with the submerged breakwater.

Freak wave in a two-dimensional directional wavefield with bottom topography change. Part 1. Normal incidence wave

In the propagation and evolution of sea waves, previous studies pointed out that the occurrence of the freak wave height is significantly related to the quasi-resonant four-wave interaction in the modulated waves. From numerical--experimental study over an uneven bottom, the nonlinear effect caused by the bathymetry change also contributes to the occurrence of extreme events in unidirectional waves. To comprehensively analyse the two-dimensional wavefield, this study develops an evolution model for a directional random wavefield based on the depth-modified nonlinear Schrödinger equation, which considers the nonlinear resonant interactions and the wave shoaling the shallow water. Through Monte Carlo simulation, we discuss the directional effect on the four-wave interaction in the wave train and the maximum wave height distribution from deep to shallow water with a slow varying slope. The numerical result indicates that the directional spreading has a dispersion effect on the freak wave height. In a shallow-water environment, this effect becomes weak, and the bottom topography change is the main influencing factor in the wave evolution.

Design of graphene-based tunable ultra-thin UWB metasurface for terahertz regime

This work presents a simple graphene-based tunable ultra-wideband (UWB) metasurface for terahertz application. The unit cell comprises a silicon dioxide (SiO2) substrate sandwiched between the top patterned graphene layer and the bottom continuous graphene layer, where each graphene layer is 1 nm thick. A separate DC bias configuration is also introduced, consisting of another SiO2 substrate backed by a gold plate to achieve independent biasing of both graphene layers. The metasurface is ultra-thin with a thickness of only 5.1μm, i.e. λ0588.235, where λ0 is calculated at the lowest operating frequency. The proposed absorber provides an absorption bandwidth (BW) of 13.9 THz (Fractional BW = 197.16%), covering the electromagnetic spectrum from 0.1 to 14 THz with an absorptivity of more than 90%. The periodicity of the proposed unit cell is found to be only 13.5μm, i.e. λ0222.22. Hence, it satisfies the effective homogeneity condition. The designed unit cell has structural symmetry, which makes the proposed absorber polarization insensitive to the normal incidence of the plane wave. The structure is also examined under a different incident angle (θ) for both TE and TM polarization. It is found to have absorptivity of more than 80% in the frequency range 0.1–14 THz for incidence angle up to 60∘. The variation in the chemical potential (μc) of top Gpat provides tunable absorption characteristic. The increase in μc of Gpat leads to the blue shift in the absorptivity response. The proposed absorber covers the entire ”Terahertz Gap” of the electromagnetic spectrum and is suitable for various terahertz regime applications.

Local Field Enhancement Due to the Edge States of Nanoplasmonic Crystal

In this paper, we describe the effect of local electric field enhancement due to the existence of photonic edge states in a nanoplasmonic crystal. These states can be excited by a normal incident wave and are localized along the continuous line defect, where translational symmetry is broken. The nanoplasmonic crystal is formed by a triangular lattice of SiO2 cylinders on top of SiO2 and Ag thin films. Numerical simulations demonstrated that edge modes that are concentrated around a defect in a nanophotonic crystal may result in 12 field enhancements of the electric field for the incident plane wave of wavelength 545 nm. This effect can be employed for improving the sensitivity of surface-enhanced Raman scattering (SERS) spectroscopy, increasing the Purcell factor of quantum systems and improving the efficiency of higher harmonic generation.

Broadband noise attenuation in the flow duct using metamaterial-based acoustic liners

An acoustic liner design based on metamaterial is proposed for broadband duct noise reduction. The material is constructed by four periodically arranged partitions-embedded units covered by a perforated plate, forming a linear reflected phase-shifting within 0 to 2π in a wide target frequency range. The sound absorption performance under normal incident waves is firstly examined by both numerical simulation and impedance tube measurement, leading to close agreement to the target design. Then, the meta-liner is installed in a flow tube to assess its capability of reducing broadband noise in a duct with aerodynamic flows. The experiments investigations are performed in an advanced grazing flow tube developed at the Hong Kong University of Science and Technology. The effect of sound levels (ranging from 120dB to 140dB), flow speeds with the Mach number up to 0.3, and the position of sound source (at upstream and downstream sides of meta-liner) are investigated. Results show that a nearly flat transmission loss is achieved in the target frequency range by the metamaterial-based acoustic liner in various conditions, showing its potential for practical industrial applications.

Broad-angle coherent perfect absorption-lasing and super-collimation in two-dimensional non-Hermitian photonic crystals.

Coherent perfect absorption-lasing (CPAL) and collimation have been intensively studied for normal and small angle wave incidence. Here, we report a two-dimensional non-Hermitian photonic crystal for broad-angle CPAL and super-collimation. The synergy of a nonsymmorphic glide symmetry of the lattice, gain-loss modulation and an optimization of unit cell induces a parity-time phase transition in the band structure along the Brillouin zone boundary. The transition points, i.e., the exceptional points, form a slab-like contour, with nearly zero dispersion in both real and imaginary parts of the band structure. Such dispersionless band structure significantly enhances the range of incident angle for CPAL and collimation.

Numerical and Experimental Studies of Acoustic Processes in Model Channels for the Development of Wideband Single-Layer Sound-Absorbing Structures

In this paper, the results of numerical and experimental studies of the acoustic efficiency of a group of prismatic resonators of different volumes at an acoustic pressure level of 130 dB were presented. The prototype was developed, and representative samples of fragments of different-height sound-absorbing structures were manufactured by 3D-printing. Laboratory tests of the samples were carried out on the interferometer with normal incidence of sound waves. The acoustic efficiency of the developed sound-absorbing structures was experimentally confirmed. The different-height sound-absorbing structures allow to extend the broadband of the prismatic resonator group.

Low frequency sound absorption performance of large sized active micro-perforated panel absorber in free field

The active micro-perforated panel absorber has excellent low frequency sound absorption performance, which is expected to realize low-frequency noise reduction in large space of the cabin. Since its active sound absorption performance depends on the incident sound field environment, on the basis of the existing research conclusions in the duct, the active sound absorption performance of the large-sized active micro-perforated panel absorber under the excitation of a normal incident plane wave in typical free field environment is in depth investigated. First, the theoretical model of the active micro-perforated panel absorber is established by using the modal analysis approach, in which a reflection coefficient varying with position is introduced to represent the reflected sound wave on the surface of the active micro-perforated panel absorber in free field. Then, the physical mechanism of active control is thoroughly analyzed and the error sensing strategy is established. Finally, an experiment is carried out to validate the theoretical model and findings. Results obtained demonstrate that the greater the amplitude of the (0, 0, 0) cavity mode excited by the incident plane wave, the stronger the reflection effect on the incident sound wave is, and vice versa. The control source suppresses the (0, 0, 0) mode so that this mode will not reflect and absorb the incident plane wave substantially when its amplitude is reduced to an optimal value. This is main mechanism of the sound absorption improvement in the low frequency range. However, the excited high order cavity modes (except for (0, 0, 0) mode) greatly reflect the incident sound energy in free field and exert a negative effect on sound absorption improvement. Thus, the control performance of the active micro-perforated panel absorber weakens in free field in comparison with that in the duct. The pressure-release and impedance-matching strategies are still applicable to free field as long as such a situation holds, i.e. the (0, 0, 0) cavity mode can be substantially suppressed by the control source and at the same time the high order cavity modes cannot be highly excited.

A study of linear-to-linear (LTL) cross-polarization conversion metasurface (CPCM) in microwave regime

An efficient new polarization convertor was considered in this paper. It contains semi-circular cutting head coupling, V-shaped head, and diagonal metal strip with the same picture rotation 180⁰ based on the unit cell’s electromagnetic resonant features. This article shows a linear polarization convertor that correctly simulates a metasurface which can be converted to an ultra-wideband frequency with a wide-angle of the incident. To achieve the broad-band polarization conversion, it operates x-and y-linear polarization waves in the specular reflection mode. Four resonances lead to an increase in bandwidth and cross-polarization reflection. The simulated results indicate that it is possible to convert a bandwidth of 92 % from 6.1 to 16.5 GHz with a conversion ratio of 78 % to the reflected waves of x or y. The cross-polarized reflection phase and magnitude can be controlled separately by the geometry and rotation angle. The proposed metasurface polarization capability shows wide-angle incidents from 0⁰ to 45⁰. We designed a model to simulate the ultra-broadband results for x-and y-polarized normal incident waves. The usual metasurface can be discovered promising applications in remote sensing, imaging technology, and reflector antennas operating at microwave frequency.

Ultrasonic interference and critical attenuation in metal-plastic bilayer laminates

The resonance of ultrasonic waves in a metal-plastic bilayer laminate placed in an infinite medium is theoretically investigated, and critical attenuation is shown to occur under specific conditions. The bilayer laminate is subjected to normal wave incidence from the side of the plastic layer. Based on a linear viscoelastic model, the amplitude of the reflection spectrum for the laminate is formulated to obtain the exact results for wave resonance and critical attenuation by numerical calculations. Furthermore, the approximate formulae for the conditions of the resonance and critical attenuation are derived explicitly by Taylor expansions with respect to the loss factor of the plastic. As a result, the exact solution shows that the resonance frequencies of the plastic layer slightly increase with increasing loss factor. Their variation with the loss factor agrees well with the second-order approximation result but is insignificant in the cases of common plastics with relatively low loss factors. The exact results also show that the amplitude at each order resonance depends on the loss factor and has a minimum at a critical loss factor. The obtained critical loss factor decreases with increasing resonance order and is well reproduced by the zeroth-order approximation. Experiments are performed on two types of metal-plastic bonded laminates, demonstrating that the measured reflection spectra are in good agreement with the theoretical predictions. In particular, the amplitude of the measured reflection spectrum decreases significantly at a resonance frequency, which is theoretically predicted to satisfy the critical attenuation condition.

Optical Trapping of a Single Molecule of Length Sub-1 nm in Solution

Optical Trapping of a Single Molecule of Length Sub-1 nm in Solution

Investigation of the Effect of Normal Incidence of RF Wave on Human Head Tissues Employing Cu and Ni Grid PET Films

The rising number of frequency bands and the demand for wireless communication devices has become a growing concern regarding health and safety. The human head is a vulnerable body part when exposed to mobile phones. To ensure a high level of protection of the head from undesirable Electromagnetic Field (EMF) emissions, a shield is incorporated in this paper between the head and the mobile smartphone. The shielding material used to protect the head from the RF emissions is Copper (Cu) grid transparent Polyethylene Terephthalate (PET) film and Copper (Cu) grid transparent PET film with Nickel (Ni) coating forming a laminated mesh. The RF emission metric from the smartphone is determined to evaluate the Specific Absorption Rate (SAR) numerically with a variation in frequencies ranging from 850MHz to 5.47GHz at normal wave incidence by the Transmission Line Method. The variation in frequency is observed in two head models, one of an adult and one of a child. Compared with the no shield condition, a significant SAR reduction is observed when PET-Cu or PET-Cu-Ni conductive coating transparent shielded mesh is embodied on the front part of the mobile phone between the phone and the head. In the child 7-layered head model at 5.47GHz, a significant reduction in SAR is observed from 10.5W/kg to 0.00001W/kg using the Cu grid PET film and to 0.0000032W/kg using Cu and Ni grid PET film.

Single-Polarized Low-Profile High-Order Bandpass Frequency Selective Surfaces Based on Aperture-Coupled Patch Resonators Under TM and TM Modes

This communication presents a methodology to construct the aperture-coupled dual-mode patch resonators (AC-DMPRs)-based periodic element to realize single- and dual-band high-order bandpass frequency selective surfaces (FSSs). Initially, a single patch resonator (PR) loaded with two sets of shorting pins is theoretically studied by using the characteristic mode analysis (CMA) method and by applying periodic boundary conditions, respectively. The results demonstrate that the radiation pattern and resonant frequency of TM00 mode in the PR can be effectively reshaped and controlled by the shorting pins, so that the TM00 and TM01 modes can be simultaneously excited under the normal incidence of a plane wave. As such, the single-mode PR is transformed into a DMPR. Moreover, such DMPRs are arranged in a back-to-back manner with coupling apertures etched on the common middle metallic layer to form an AC-DMPR-based bandpass FSS element. By reasonably reallocating the resonant frequencies of these two resonant modes and etching different apertures to provide magnetic and/or electric couplings in the AC-DMPR, single- and dual-band high-order bandpass FSSs can be conveniently and flexibly constructed, respectively. Finally, the conceptual designs are validated by measurements.

Direct estimation of porosity from seismic data using rock- and wave-physics-informed neural networks

Petrophysical inversion is an important aspect of reservoir modeling. However, due to the lack of a unique and straightforward relationship between seismic traces and rock properties, predicting petrophysical properties directly from seismic data is a complex task. Many studies have attempted to identify the direct end-to-end link using supervised machine learning techniques, but they face challenges such as lack of a large petrophysical training data set or estimates that may not conform with physics or depositional history of the rocks. We present a rock- and wave-physics-informed neural network (RW-PINN) model that can estimate porosity directly from seismic image traces with no wells or with a limited number of wells and with predictions that are consistent with rock physics and geologic knowledge of deposition. The RW-PINN takes advantage of auto-differentiation to compute the gradients across the rock- and wave-physics models. As an example, we use the uncemented-sand rock-physics model and normal-incidence wave physics to guide the learning of the RW-PINN to eventually get good estimates of porosities from normal-incidence seismic traces and limited well data. Training the RW-PINN with few wells (weakly supervised scenario) helps in tackling the problem of nonuniqueness as different porosity logs can give similar seismic traces. We use a weighted normalized root mean square error loss function to train the weakly supervised network and demonstrate the impact of different weights on porosity predictions. The RW-PINN's estimated porosities and seismic traces are compared to predictions from a completely supervised model, which gives slightly better porosity estimates but matches the seismic traces poorly and requires a large amount of labeled training data. We demonstrate the complete workflow for executing petrophysical inversion of seismic data using self-supervised or weakly supervised RW-PINNs.

Wideband and high-efficiency spin-locked achromatic meta-device.

Achromatic devices present unique capabilities in efficient manipulation of waves and have wide applications in imaging and communication systems. However, the research of achromatic devices is limited by the narrow bandwidth, low efficiency as well as large configurations. In this paper, we propose a general strategy to design spin-locked achromatic metasurface with broadband and high efficiency properties in microwave region. A multi-resonant model is used to control the dispersion within a wide bandwidth by tuning its resonant intensity, resonance numbers as well as resonant frequency. As a proof of the concept, two achromatic meta-devices with ultra-thin profile at microwave frequency are experimentally investigated. The achromatic deflector can reflect the normal incident waves to the same angle within 9.5 to 11.5GHz, while the other achromatic lens can focus the excitations at the same focal points. The experimentally working efficiency of the meta-devices fluctuates around 71-82% and 57-65% within the target working bandwidth, respectively. Moreover, our meta-devices can preserve the charity of the excitations. The scheme of this research shows great advances in the design of broadband and high-efficiency achromatic devices which can also be applied to other frequency ranges and inspires the realization of ultrabroadband and high-efficiency metadevices.

Low-profile and compact retroreflector enabled by a wide-angle and high-efficiency metalens

In this paper, a low-profile metasurface retroreflector based on an ultra-thin wide-angle and high-efficiency metalens is proposed and investigated. Owing to its stable high transmission magnitude and the insensitive phase shift of the proposed meta-atoms within a continuous wide-angle range, the designed metalens can efficiently focus both normal and various oblique incident angle waves to a pre-designed curved trajectory continuously. By placing a concave metal mirror which is exactly coincide with the determined focused spots, the cascaded structure thus can realize backscattering enhancement working at a continuous wide-angle range. A prototype is also fabricated, measured and characterized and the experimental results verify that the proposed retroreflector can perform an excellent backscattering enhancement within a stable 3 dB monostatic RCS for almost ±30° angle range. Moreover, the relationship between the focusing performances of wide-angle metalens (e.g., focused spot power intensity, focusing efficiency and positions) and the backscattering enhancement of cascaded wide-angle retroreflector is established and highlighted. Besides, its excellent backscattering enhancement (the absolute RCS level and angular range) can be compared to some traditional widely-used radar retroreflectors like trihedral corner reflector or Luneburg lens reflector for the similar physical size. The proposed metalens retroreflector also posses of a noticeable working bandwidth of about 0.2 GHz within X band. The presented retroreflective device is promising to develop into a low-profile, light-weight and cost-effective wide-angle retroreflector thus enabling various applications for modern microwave engineering.

Bifunctional Full-Space Metasurface Combining Pancharatnam–Berry and Propagation Phases

A bifunctional full-space metasurface (FS-MS) capable of generating reflective multiple vortex beams and focusing transmissive electromagnetic waves within different frequency bands is presented in this letter. The generation of vortex beams is based on the Pancharatnam–Berry (PB) phase, which exhibits the property of full phase control by rotating the orientation angle of an I-shaped patch. Meanwhile, this PB unit is backed by a metallic square loop frequency selective surface (FSS), which behaves as stopband and passband responses around 18 GHz and 12 GHz, respectively. By encoding PB phase to generate a certain phase response, four reflective vortex beams carrying orbital angular momentum (OAM) are achieved at a higher frequency band under normal incidence. On the other hand, the incident waves operating at a lower frequency band can pass through such an FSS-backed PB unit and be focused by a three-layer composite unit consisting of a polarization-conversion unit sandwiched by two orthogonal polarizers. Therefore, upon illumination of normal incident plane waves, the proposed FS-MS works as a multiple OAM beams generator and a focusing lens simultaneously. The proposed design can motivate the realizations of FS-MS and hold potential applications in intelligent housing systems.

Polarization-insensitive ultra-wideband metamaterial absorber comprising different forms of ZrN structures at the metasurface

Ultra-wideband metamaterial absorber comprising zirconium nitride (ZrN)-based metasurface was investigated. In particular, the metasurface had three different kinds of resonating element patterns (composed of ZrN), namely the concentric annular square, circular and triangular air gaps of different samples of ZrN, as distinguished by the nitrogen content of the medium (i.e., ZrN). Spectral performance of metamaterials was evaluated in terms of absorbance, and it was found that the metasurface structure with annular circular resonating elements provides the best performance owing to high absorbance with relatively stable wideband feature. The performance of the proposed structures was investigated under varying parametric situations considering the incidence excitations with the transverse electric (TE-) and transverse magnetic (TM-) conditions, and also, the normal and oblique incidence of waves. The nearly-perfect absorption along with the ultra-wideband property of the structure were shown. Apart from this, it also exhibits the polarization-insensitive characteristic regardless of the polarization state as well as polarization angle of the incidence waves. Such a kind of absorber would be useful for antireflection coating and solar energy harvesting applications in a wide spectral range from the visible to far-red regimes of wavelength.

All‐Dielectric Terahertz Metasurface with Giant Extrinsic Chirality for Dual‐Mode Sensing

Metasurface‐based sensors, with their planar compact structure and excellent sensing performance, have become one of the emerging technologies in metasurface and nanophotonics in recent years. In particular, chiral metasurfaces show unique advantages for chiral molecule detection and enantiomer identification. Herein, a scheme of dual‐mode sensing based on all‐dielectric terahertz metasurface via chiral and achiral spectroscopy is proposed. The proposed achiral meta‐atoms show great extrinsic chirality. The transmitted linearly polarized spectrum and the circular dichroism (CD) can be used as the sensing indicator under normal and oblique incidence of the terahertz waves, respectively. Among them, the working mode based on linear polarization spectrum only needs to measure the transmission spectrum once, which means wide applicability. The other mode using CD can eliminate the achiral terahertz absorption of the analyte through multiple measurements and has a higher signal‐to‐noise ratio. The results show that the maximum refractive index–frequency change rates of the two operating modes are −128.5 GHz/RIU and −124.7 GHz/RIU, respectively. In addition, there are two resonant peaks available in both working states, for both high‐precision and wide‐range refractive index detection. This new scheme is expected to be applied in industrial fields such as terahertz biochemical detection.