Normal Incident Wave Research Articles

Virtual nondestructive evaluation for anisotropic plates using Symmetry Informed Sequential Mapping of Anisotropic Green’s function (SISMAG)

In this article, a generalized computational method to simulate virtual nondestructive evaluation (NDE) of anisotropic composite plates is presented. The ultrasonic wave fields were computed using a modified and generalized version of Distributed Point Source Method (DPSM), a semi-analytical mesh-free technique. The anisotropic Green’s functions required for DPSM implementation were calculated using Fourier transform method and Radon transform method and compared. It is established that the Green’s functions obtained from two different methods are identical. Applying generalized mathematical formulations, NDE of different degrees of anisotropic: transversely isotropic, orthotropic and monoclinic material are simulated and reported in this article. To boost the computational efficiency, a Symmetry Informed Sequential Mapping of Anisotropic Green’s function (SISMAG) is introduced with DPSM and discussed in detail. To prove the above claims, virtual NDE experiments of anisotropic plates with normal and angle incidence of the ultrasonic wave are simulated using a circular transducer of central frequency ∼1 MHz. Wave fields inside both the fluid and the solid media were calculated.

Polarization-independent electromagnetically induced transparency-like metasurface

A classical electromagnetically induced transparency-like (EIT-like) metasurface is numerically simulated. This metasurface is composed of two identical and orthogonal double-end semitoroidals (DESTs) metal resonators. Under the excitation of the normal incidence waves, each of the two DESTs structure exhibits electromagnetic dipole responses at different frequencies, which leads to the polarization-independent EIT-like effect. The features of the EIT-like effect are qualitatively analyzed based on the surface current and magnetic field distribution. In addition, the large index is extracted to verify the slow-light property within the transmission window. The EIT-like metasurface structure with the above-mentioned characteristics may have potential applications in some areas, such as sensing, slow light, and filtering devices.

Manufacturing sound-absorbing structures by 3d-printing

In this research we developed the methods and technologies for manufacturing sound-absorbing structures (SAS) samples using Dimension SST 1200 and Envisiontec Perfactory XEDE units. While testing the technique, the shortcomings of both units were identified. To eliminate the identified shortcomings, we formulated some recommendations and adjusted the main stages of manufacturing SAS with the application of these methods. Experimental acoustic studies of the produced reference samples of the SAS on an interferometer with normal incidence of waves were carried out. Acoustic tests of samples of SAS manufactured with consideration of the developed technological solutions showed good sustainability of the obtained results, we revealed strong correlation with the samples from polymer composite materials manufactured using standard plant technologies.

Engineered Metasurface of Gold Funnels for Terahertz Wave Filtering

Surface plasmon polariton (SPP) excitation of the coupled light at small contact area of chromium pillars as the interface of metastructured gold funnel layer and silica medium can be enhanced locally in the gold meta-funnel-structured filter. In the present investigation, the filter is comprised of three layers, namely gold meta-funnels, nano-sized chromium pillars, and silica as the substrate. The incoming infrared (IR) waves, coupled with the excited plasmons at the first and second layers, form an excitation, known as deformed plasmon polariton. Asymmetric distribution of localized SPPs takes place owing to the inherent converging plasmonic feature of the gold funnel structure. The formation of reflection peaks with different magnitudes at different incidence angles of the polarized wave in the spectral characteristics makes the structure prominent for filtering the IR waves. Moreover, the gold meta-funnel-structured filter possesses the additional feature of distinguishing the type of polarized incidence wave. It was found that the transmission remains maximum corresponding to the normal incidence of the TE-polarized waves, whereas the TM-polarized waves over the same wavelength range are almost blocked for any value of incidence angle. The existence of transmission peaks corresponding to the TE waves demonstrates another application of this device as metastructured polarizer filter.

Intersubband absorption of p-type wurtzite GaN/AlN quantum well for fiber-optics telecommunication

The intersubband transition of wurtzite (WZ) p-type GaN/AlN quantum well (QW) structures grown on GaN substrate was investigated theoretically using the multiband effective-mass theory. The peak value of the TE-polarization absorption spectrum is found to be similar to that of the TM-polarization absorption spectrum. The absorption coefficients for TE- and TM-polarizations are mainly attributed to the absorption from the ground state (m1 = 1) because holes are mainly confined in ground states near the band-edge in an investigated range of the carrier density. We observe that a transition wavelength of 1.55 μm can be obtained for the QW structure with a relatively thin (∼16 Å) well width. Thus, we expect that a p-type WZ AlN/GaN heterostructure is applicable for a photodetector application for fiber-optic communications with normal incidence of wave.

Narrow stop band microwave filters by using hybrid generalized quasi-periodic photonic crystals

Using the transfer matrix method approach (TMM), the microwave transmission spectra in hybrid quasi-periodic multilayer photonic structures are studied. These structures are formed by concatenating several quasi-periodic sequences such as the generalized Cantor (GC (a, b, c)), generalized Fibonacci (GF (m, n)) and generalized Thue–Morse (GTM (m, n)). The results are presented for a normal incident wave with transverse electric (TE) polarization. The optimization of the sequences’ parameters (a, b, c, m and n) permits us to obtain polychromatic filters that cover the frequency range of Global Systems for Mobile Communication (GSM). The manipulation of these parameters fixes the number of photonic band gaps (PBGs) and the position of the transmission peaks.

Beam‐Editing Coding Metasurfaces Based on Polarization Bit and Orbital‐Angular‐Momentum‐Mode Bit

AbstractCoding metasurfaces are aimed at representing digital information of the metasurface, usually by programing digital unit cells to control electromagnetic waves. However, some information sequences cannot be recognized by the receiver, because of nonorthogonality of the usual phase codes. Here, new coding method is proposed to encode information with orthogonal parameters in the emitting beam, which reduces information loss in the system. A vector beam modulator is proposed by combining orthogonal polarizations and orbital angular momentum (OAM) modes. A normal incident wave can be modulated by OAM‐mode bit and polarization bit, which are regarded as specific information by the receiver. A polarization converter is used to realize the polarization selection (polarization bit) and phase control, independently. The phase patterns on the coding metasurfaces can be programed to realize the designed OAM modes (OAM bits) in the microwave frequency. Three schemes are presented to emit multiple OAM modes in dual polarizations, one of which is manufactured and measured for near and far fields. The simulations and experiments are in outstanding agreement, verifying the excellent performance of the proposed schemes. This work has great potential in communication applications of coding metasurfaces.

Sound transmission of a spherical sound wave through a finite plate

For an incident plane wave on an infinite plate, a doubling of mass or frequency adds 6dB to the sound transmission loss (TL), but for an incident spherical wave on an infinite plate, a doubling of mass or frequency adds only 3dB to the TL. In reality, the discrepancies of the sound transmission due to plane wave and spherical wave incidence might not be so huge, since the influences resulted from the plate size and the distance between the source and the plate cannot be ignored. In this article, the sound transmission of a spherical wave through a finite plate is theoretically analyzed through the modal expansion method. The transmission losses for typical plates are illustrated and as well are compared with that of the mass laws due to normal and spherical wave incidence, respectively. The effects of parameters such as the size of the plate, the distance between the source and the plate, and the horizontal shift of the plate are investigated. An indicator for the estimation of the TL through a finite plate due to a point source is given for the potential of practical applications.

Anomalous reflection focusing metasurface based on a dendritic structure

In this study, we designed a metasurface based on dendritic model structures. This metasurface consists of three branches, which rotate symmetrically around the z-axis. In accordance with the principle of optical resonator, the dendritic metasurface achieved distinct anomalous reflection and refraction in the infrared spectrum at an incidence angle range of 0–90°. On the basis of this phenomenon, the structure of an asymmetric dendritic metasurface unit was designed, with each unit consisting of two branches. This metasurface unit could achieve abnormal reflection and refraction for a normal incident plane wave. An opposite phenomenon would occur when the metasurface was rotated 180° around the z-axis. We used this feature to design a metasurface that could achieve a prominent reflective plane focusing effect at the mid-wave infrared band as a result of anomalous reflection. The dendritic metasurface had a uniform structure that could be adjusted flexibly. The anomalous phenomenon could be realized over a wide range of wavebands and angles at the mid-wave infrared band. In addition, the focusing effect was evident, and the reflection efficiency was higher than those of other structures.

Metamaterial absorber for frequency selective thermal radiation

Here, a polarization-independent frequency-selective thermal emitter (FSTE) in the mid-infrared range was proposed and fabricated based on metamaterial absorber. FSTE can significantly improve the capability of infrared stealth and heat radiation. The simulated results show that the proposed FSTE has high reflectivity in the two atmospheric windows (3.0–5.0μm and 8.0–14.0μm) for normal incidence waves, and the emissivity is suppressed completely under 0.06. In the atmosphere absorption band of 5.5–7.6μm, the FSTE exhibits high absorption and high emissivity, which can passively cool themselves through radiative emission of heat to outer space. E-beam lithography was used to fabricate the FSTE and the infrared emissivity characteristics were analyzed. The experimental results have consistency with the simulation results.

A novel approach for optimal design of multilayer wideband microwave absorber using wind driven optimization technique

In general, cost function for optimal design of multilayer microwave absorbers found in literature had not taken account of normal and oblique incidence (TE & TM polarization) of wave in optimization problem which leads to sub-optimal design limited only for particular incidence of wave, either a normal or oblique incidence of wave at a time. To overcome the above problem, this paper focuses on optimal design of multilayer absorbers through a proposed cost function which can work for both normal and oblique incidence of wave up to wide angle of incidence (0–60°) with TE & TM polarization simultaneously. The objective of this optimization process is to reduce maximum reflection coefficient of the absorber by selecting suitable layers of materials from the available database in literature and optimize the total thickness to possible minimum level. Five models of four layer multilayer absorbers are proposed using WDO algorithm for different frequency bands (ranging from 2–8, 8–12, 12–18, 2–12 and 2–18GHz). A detailed analysis are done and it shows that the application of WDO yields improved numerical results in terms of thickness and oblique incidence of wave as compared with the earlier reported results.

Polarization independent and tunable plasmonic structure for mimicking electromagnetically induced transparency in the reflectance spectrum

This paper proposes a plasmonic metamaterial that is able to mimic electromagnetically induced transparency in the reflectance spectrum within the GHz frequency range. Each meta-atom consists of a cross-slot structure as the bright resonator positioned on one side of the FR-4 substrate, and four spiral structures as the dark resonator located on the opposite side. Free space experimental results demonstrate that at normal incidence of plane wave, the metamaterial possesses the properties of tunability and polarization independence. In addition, based on simulation results the metamaterial also possesses slow wave property, with group refractive index of 56; and refractive-index-based sensing capability, with figure of merit of 6.1. In the strong coupling configuration, the plasma frequency and coupling constant of the metamaterial were calculated to be approximately 5.4 × 1010 rad s−1 and 9.8 × 109 rad s−1 respectively. While the respective damping constants of the bright resonator and dark resonator were calculated to be approximately 4.6 × 1010 rad s−1 and 1.9 × 1010 rad s−1.

Liquid crystal control of the plasmon resonances at terahertz frequencies in graphene microribbon gratings.

We theoretically study the influence of the liquid crystal (LC) orientational state on the absorption, reflection, and transmission spectra of a graphene microribbon grating placed between a nematic LC and an isotropic dielectric substrate. We calculate the absorption, reflection, and transmission coefficients at normal incidence of a far-infrared transverse magnetic wave (THz) and show that control of the orientational state of the LC layer enables the manipulation of the magnitude of the absorption and reflection maxima. The influence the LC orientational state on the plasmonic resonance increases with increasing the isotropic substrate dielectric constant and the graphene microribbon width to grating spacing ratio.

Second-harmonic generation in a multilayered structure with nonlinear spring-type interfaces embedded between two semi-infinite media

The acoustic second-harmonic generation behavior in a multilayered structure with nonlinear spring-type interlayer interfaces is analyzed theoretically to investigate the frequency dependence of second-harmonic amplitudes in the reflected and transmitted fields when the structure is subjected to the normal incidence of a monochromatic longitudinal wave. The multilayered structure consists of identical linear elastic layers and is embedded between two identical linear elastic semi-infinite media. The layers are bonded to each other by spring-type interfaces possessing identical linear stiffness but different quadratic nonlinear parameters. By combining a perturbation analysis with the transfer-matrix method, analytical expressions are derived for the second-harmonic amplitudes of the reflected and transmitted waves. The second-harmonic amplitudes due to a single nonlinear interface are shown to vary remarkably with the fundamental frequency, reflecting the pass and stop band characteristics of the Bloch wave in the corresponding infinitely extended layered structure. By calculating the spatial distribution of second-harmonic amplitude inside the multilayered structure, the influence of the position of the nonlinear interface as well as the number of layers on the frequency dependence of second-harmonic amplitudes of the reflected and transmitted waves is elucidated. When all interlayer interfaces possess the identical nonlinearity, the second-harmonic amplitudes on both sides of the structure are shown to increase monotonically with the number of layers in the frequency ranges where both fundamental and double frequencies are within the pass bands of Bloch wave. The influence of two non-dimensional parameters, i.e., the relative linear compliance of the interlayer interfaces and the acoustic impedance ratio between the layer and the surrounding semi-infinite medium, on the second-harmonic amplitudes is elucidated.

A 3D unstructured grid nearshore hydrodynamic model based on the vortex force formalism

A new three-dimensional nearshore hydrodynamic model system is developed based on the unstructured-grid version of the third generation spectral wave model SWAN (Un-SWAN) coupled with the three-dimensional ocean circulation model FVCOM to enable the full representation of the wave-current interaction in the nearshore region. A new wave–current coupling scheme is developed by adopting the vortex-force (VF) scheme to represent the wave–current interaction. The GLS turbulence model is also modified to better reproduce wave-breaking enhanced turbulence, together with a roller transport model to account for the effect of surface wave roller. This new model system is validated first against a theoretical case of obliquely incident waves on a planar beach, and then applied to three test cases: a laboratory scale experiment of normal waves on a beach with a fixed breaker bar, a field experiment of oblique incident waves on a natural, sandy barred beach (Duck’94 experiment), and a laboratory study of normal-incident waves propagating around a shore-parallel breakwater. Overall, the model predictions agree well with the available measurements in these tests, illustrating the robustness and efficiency of the present model for very different spatial scales and hydrodynamic conditions. Sensitivity tests indicate the importance of roller effects and wave energy dissipation on the mean flow (undertow) profile over the depth. These tests further suggest to adopt a spatially varying value for roller effects across the beach. In addition, the parameter values in the GLS turbulence model should be spatially inhomogeneous, which leads to better prediction of the turbulent kinetic energy and an improved prediction of the undertow velocity profile.

Acoustic radiation force on a fluid cylindrical particle immersed in water near an impedance boundary.

This work presents a theoretical model to calculate the acoustic radiation force of a fluid cylindrical particle immersed in water near a boundary. A solution of the acoustic radiation force function, which is the radiation force per unit energy density and unit cross-sectional surface area, is derived for a cylinder near a boundary in normal incident plane wave by applying the translation addition theorem of cylindrical function. The effects of impedance boundary on acoustic radiation force of a fluid oleic acid cylinder and a mixture fluid cylinder immersed in water are analyzed with particular emphasis on the radius of fluid cylinder and the distance from its center to the impedance boundary. The results reveal that the existence of particle trapping behavior depends on the choice of the nondimensional frequency ka as well as the offset distance from the impedance boundary. This study provides a theoretical basis for acoustic manipulation, which may be of benefit to the improvement and development of the acoustic control technology.

Photonic band-gap and defect modes of a one-dimensional photonic crystal under localized compression

The rupture of periodicity caused by one defect (defect layer) in a one-dimensional photonic crystal (1DPhC) results in a narrow transmission spectral line in the photonic band-gap, and the field distribution shows a strong confinement in the proximity of the defect layer. In this work, we present a theoretical model to calculate the frequency of defect modes caused by defect layers induced by localized mechanical stress. Two periodical arrangements were studied: one with layers of poly(methyl-methacrylate) (PMMA) and polystyrene (PS), PMMA-PS; the other with layers of PMMA and fused silica (SiO2), PMMA-SiO2. The defect layers were induced by localized compression (tension). The frequencies of the defect modes were calculated using elasto-optical theory and plane wave expansion and perturbation methods. Numerical results show that the frequency of the defect mode increases (decreases) when the compression (tension) increases. Based on the theoretical model developed, we show that compression of n layers of a 1DPhC induces n defect modes whose frequencies depend on the compression magnitude in the case of normal incidence of electromagnetic waves, in accordance with the results reported for other types of defect layers. The methodology shows the feasibility of the plane wave expansion and perturbation methods to study the frequency of the defect modes. Both periodical arrangements are suitable for designing mechanically tunable (1DPhC)-based narrow pass band filters and narrow reflectors in the (60, 65) THz range.

Toroidal Dipolar Excitation in Metamaterials Consisting of Metal nanodisks and a Dielectrc Spacer on Metal Substrate

We have investigated numerically toroidal dipolar excitation at optical frequency in metamaterials whose unit cell consists of three identical Ag nanodisks and a SiO2 spacer on Ag substrate. The near-field plasmon hybridization between individual Ag nanodisks and substrate forms three magnetic dipolar resonances, at normal incidence of plane electromagnetic waves. The strong coupling among three magnetic dipolar resonances leads to the toroidal dipolar excitation, when space-inversion symmetry is broke along the polarization direction of incident light. The influences of some geometrical parameters on the resonance frequency and the excitation strength of toroidal dipolar mode are studied in detail. The radiated power from toroidal dipole is also compared with that from conventional electric and magnetic multipoles.

Dark Mode Driven Extra-narrow and Multiband Absorber

Recently, metamaterial absorbers have received tremendous amount of interest because of their remarkable ability to manipulate the amplitude, phase, and polarization of light. However, most absorbers rely on the direct coupling of electric or magnetic field with external excitation, which lead to inevitable energy leakage to the surrounding environment and depress the quality factor of the structure. In this work, we investigate the multiband absorption property by exciting dark plasmonic modes in reflective symmetric and asymmetric metamaterials. Theoretically, the existence of dark plasmonic modes in asymmetric metamaterials is unambiguously illustrated by the improved eigen-mode theory. With the introduction of asymmetry, dark modes in metamaterials can be easily excited by normal incident plane wave. Moreover, we also directly excite the dark modes in symmetric absorber with oblique incidence. The dark modes splitting mechanism is also clarified with the excitation of designer surface plasmon. Dominated by magnetic dipole or higher-order multipole, these dark modes possess high quality factors (Q). Numerical results indicate that the metamaterial absorber maintains high absorbance within a wide-angle incidence (0~50°). The high Q asymmetric metamaterial absorber can be an excellent candidate for multiband plasmonic sensor.

Design of a wideband reflective linear polarization converter based on the ladder-shaped structure metasurface

In this paper, a ladder-shaped structure metasurface is proposed and investigated theoretically and experimentally, which can efficiently convert a linear polarized electromagnetic (EM) wave to its orthogonal component in a wideband range. The wideband reflective polarization convertibility is resulted from three resonance modes generated by the ladder-shaped structure MM for the normal incident waves. The mechanism of high-efficiency reflective polarization conversion is illustrated by surface current distribution and the destructive interference theory. The experimental measurement results show that the average polarization conversion ratio (PCR) is greater than 90% from 6.9 to 15.4GHz, which are in good agreement with the numerical simulations and theoretical predictions. The designed metasurface possesses the merits of wideband and high-efficiency, and thus has great application values in novel polarization-control devices.