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This paper presents the design of a single-phase solid phononic crystal (PnC) structure featuring a regular hexagonal perforation pattern. The structure manifests three negative refraction bands, encompassing one for transverse waves and two for longitudinal waves, thereby enabling simultaneous control of shear and longitudinal waves. Due to the high symmetry of the triangular lattice, the equal frequency curves corresponding to the negative refraction band approach circular shapes, suggesting a nearly isotropic negative refraction effect. This negative refraction effect is achieved through specific mass resonance modes closely related to the porous structure designed in this paper. Initially, we analyze the band structure of the PnC, followed by designing the PnC plate structure to achieve negative refraction control for transverse waves at a frequency of 32.4 kHz, with a negative refraction index of −1. Additionally, negative refraction control for longitudinal waves is attained at frequencies of 44 and 64.54 kHz. Subsequently, we scrutinize the influence of various conditions on negative refraction, including different structural parameters, incident angles, and operating frequencies, while verifying the robustness of the designed phonon crystal structure. Leveraging the negative refraction characteristics of the structure, we construct an elastic wave lens to achieve perfect imaging of shear and longitudinal waves. Finally, employing finite element simulation and analyzing focusing imaging characteristics with different source positions, we validate that the results closely align with theoretical expectations. The solid PnC structure designed in this study holds significant potential for applications in the fields of elastic wave imaging. [ABSTRACT FROM AUTHOR]

Elastic metamaterials with double negativity can manipulate the propagation of elastic waves at sub-wavelength scales by inducing multiple resonances to achieve different negative effective parameters. Numerous efforts have been made to control the dynamic behavior by directly tuning the frequency range of double negativity in elastic metamaterials. This study examines the impact of tensile loads on double negativity in relation to ligament inclination angle. The findings will aid in the direct tunability of double negativity in single-phase chiral metamaterials. The study examines a simple single-phase four-ligament chiral unit with low-order double negativity. The presence of double negativity was verified through band structure analysis and calculation of the four effective dynamic parameters. Additionally, the effects of two geometrical factors on the frequency ranges of negative parameters were investigated through parametric scanning. The results indicate that the frequency range of double negativity reaches its maximum at a ligament inclination angle of around 45° and disappears as the angle approaches 65°. Given that the ligament inclination angle of the unit cells can be easily altered by external tension, this intriguing outcome is leveraged to achieve the emergence and vanishing of double negativity. This characteristic is confirmed through the examination of negative refraction phenomena via simulation examples. [ABSTRACT FROM AUTHOR]

The negatively refracted flexural wave is explored for phononic thin plates consisting of periodic holes and inclusions based on the effect of backward waves. Dispersion curves of these two types of phononic thin plates are calculated by using the multipole method, and the backward wave effect has been clearly found in their band diagrams. By using multiple scattering simulations, the birefraction phenomenon has been observed in a prism-shaped hole array, whereas single negative refraction has been realized in a prism-shaped inclusion array. The explanations using Snell's law are presented for the occurrence of birefraction and sole negative refraction. Experiment verifications of birefraction phenomenon show good agreements with both numerical and analytical results. Numerical results also indicate the multiple scattering simulation is an alternative method for observing negative refraction of flexural waves. [ABSTRACT FROM AUTHOR]

Nanostructured metamaterials can offer optical properties beyond what is achievable in conventional media, such as negative refraction or sub‐wavelength imaging. Due to their structural anisotropy, the class of high aspect ratio metamaterials is of interest for the possibility of achieving hyperbolic optical properties, i.e., both metallic and dielectric behavior based on the excitation direction. Although investigated numerically, the fabrication of tailor‐made metamaterials is complex or often beyond the reach of current technology. For wire metamaterials composed of aligned metallic nanowires in a dielectric matrix, since the free carrier concentration in metals is fixed, light‐matter interaction cannot be adjusted after fabrication. Here, metamaterials based on plasmonic ITO nanotubes with controllable hyperbolic response are introduced. The synthesis is achieved by a template‐based liquid‐phase technique. The tuning mechanism is based on controlling the carrier density in ITO via oxygen vacancy concentration. The process is reversible, the photonic features are activated by creating oxygen vacancies and can be switched off by filling them up again. Further, it is shown that the carrier concentration can also be controlled via a static electric field. Optical simulations support the experimental findings and highlight the parameters that determine the optical response of the metamaterial. [ABSTRACT FROM AUTHOR]

Abstract\nHIGHLIGHTSAs a special branch of metamaterials characterized by phase discontinuity, elastic metasurfaces have presented extraordinary capacity for elastic wave manipulation. This class of metamaterials has received extensive attention in recent years. In this paper, a subwavelength elastic metasurface with locally resonant prisms is employed to steer the propagation of the A0 Lamb wave. Based on the dispersion curve and transmission ratio, we establish that the resonant feature of prisms is influenced by the diagonal ratio of cross section. By gradually tuning the diagonal ratio of prism cross section, the pattern of resonance modes and the mechanism of resonance superposition are clarified. In this regard, two different resonant modes can be superimposed at a certain frequency. A complete 2π phase shift that is crucial for redirecting wave propagation can therefore be realized. To have a broader working frequency range, the influence of prism height on the resonance superposed frequency and the effect of plate thickness on the incident flexural wave are explored. Based on the generalized Snell’s law, negative refraction is verified numerically through a metasurface assembled from unit cells with constant phase change. In addition, other wavefront modulations such as eccentric focusing, nonparaxial propagation are also demonstrated. This novel design introduces new degrees of freedom in many engineering applications such as seismic wave protection, structural health monitoring and energy harvesting.Proposing a novel subwavelength prismatic metasurface at mm-scale capable of operating in the kHz frequency range.Introducing an approach for altering the resonant mode frequency of building block.Establishing mechanism of resonant mode frequency variation and superposition.Realizing full phase span from 0 to 2π by superimposing of compression and bending modes.Demonstrating abnormal wavefront control for flexural wave by novel prismatic metasurfaces, such as negative refraction, eccentric focusing, parabolic and curvilinear wave trajectories. [ABSTRACT FROM AUTHOR]

This paper presents spectral element based approach for determining free planner wave propagation in two-dimensional periodic square-frame-grid-lattices (SqL). Implementing the newly developed nonlinear eigenvalue solver in conjunction with Bloch–Floquet periodic boundary condition, band structures for various configurations of SqL with attached mass or spring-mass resonator are analysed. Variation of the attenuation bandwidth is studied for different mass and frequencies of the attachments to the SqL. At the natural frequency of the resonator, an attenuation bandgap can be perceived; therefore, by tuning the natural frequency of the resonator, sub-wavelength band-gap can be obtained at the desired frequency for enhanced vibration or noise isolation. However, in few cases, mostly while the resonators are attached at the center point of the SqL, the attenuation bandgap near the natural frequency of the resonator may disappear as the center point lying in the node for that free wave frequency. Additionally, existence of dirac cone, eigenfrequency-loci veering are observed in the band-structures. The iso-frequency contours are also investigated to develop an insight comprehension about the underlying physics of energy transmission and mechanics of wave propagation. Various salient wave propagation features, namely self-collimation, lensing, and negative refraction of group velocity are identified and elucidated in this paper. [ABSTRACT FROM AUTHOR]

The compound polyaniline (Poly-ANI) with different concentrations of (H2SO4) sulfuric acid has been synthesized by the chemical polymerization method. The prepared compounds have been characterized using number of techniques including FTIR, FE-SEM, EDS and DSC. Additionally, UV–Vis spectroscopy employed for studying the linear optical properties of polymer with different acid concentrations. Third order optical nonlinearity was characterized using Z-scan at 532 nm. The results showed that the nonlinear refractive index has a negative sign. It was observed that the nonlinear refractive index changes in different ratios of H2SO4. The high value of nonlinear refractive index ( n 2 ) obtained along Z-axis is 74.62 × 10 - 7 cm2/W, and the corresponding χ 3 is 21.5 × 10−5 esu. Also, the Poly-ANI film shows the response to NH3 gas sensing in the range 20 ppm-250 ppm and can be used for NH3 sensing application. [ABSTRACT FROM AUTHOR]

A parameter study of ultrashort pulse laser‐induced modifications in the bulk of ZnS crystals is reported. Experimental results on these modifications and their dependence on the pulse energy, writing speed, and depth are presented, with an emphasis on cross‐sectional morphology and induced refractive index changes. Localized permanent material modifications have been inscribed in the bulk of the crystal using a laser with a center emission wavelength of 2.09 μm and a pulse duration of ≈4 ps. The morphology strongly depends on the laser and optical focusing parameters, in particular, on the pulse energy and processing speed, with a significant shift in depth dependence for short pulse‐to‐pulse separations. Depending on the applied pulse energy, distortions of the lateral profile of the refractive index changes appear in the form of oscillatory features in the transverse plane relative to the inducing laser beam. The true extent of the modified material is revealed by the alternating lateral profile with largest induced negative refractive index change, Δn, of −3.88 × 10−2 ± 0.18. Such parametric insight is of critical importance for the understanding and optimization of the fabrication process and for realizing compact 3D photonic devices in the bulk of materials in a reproducible manner. [ABSTRACT FROM AUTHOR]

In this paper, a novel negative refractive index metamaterial (NIM) is developed and characterized. The proposed metamaterial exhibits negative effective permittivity (εeffe) and negative effective permeability (µeffe) at millimeter wave frequency of 28 GHz. This attractive feature is utilized to enhance the gain of a microstrip patch antenna (MPA). Two thin layers of 5 × 5 subwavelength unit cell array of NIM are placed above a single MPA to enhance the gain of the antenna. Each unit cell has an area of 3.4 × 3.4 mm2. A gain increase of 7.9 dBi has been observed when using the proposed NIM as a superstrate. Furthermore, the NIM array is placed over a 2 × 2 array of MPAs with four ports to demonstrate versatility of the metamaterial. The total size of the 2 × 2 antenna array system with N-MTM is about 61.1 × 34 × 16mm3 (5.71λ × 3.18λ × 1.5λ, where λ is the free-space wavelength at 28 GHz). The measurement result indicate that the maximum gain of the antenna array is 13.5dBi. A gain enhancement of 7.55 dB in E-Plane and 7.25 dB in H-Plane at the resonant frequency of 28 GHz is obtained. The proposed antenna structure is suitable for 5G millimeter wave communications, in particular, for possible implementation in future millimeter wave access points and cellular base stations. [ABSTRACT FROM AUTHOR]

The main objective of this research work is focused on the acoustic response analysis of functionally graded material (FGM) sandwich cylindrical shell structure immersed in convected fluids. In the analysis, two common types of FGM sandwich cylindrical shells are considered, which include one with FGM face shell and homogeneous ceramic or metal core, and the other with FGM core and homogeneous face shell. Given the structure is immersed in a mean flowing fluid of velocity tangential to the acoustically deformed boundary and it is excited by an incidence oblique plane wave. Fluid–structure coupling is considered by imposing velocity continuity condition at fluid–structure interfaces. By comparing the numerical results with the existing solutions in open literature, the validity of the proposed theoretical model is verified. Finally, based on the developed theoretical mode, the influences of the gradient index, sandwich type, external mean flow, and skin-core-skin ratios on the structural sound transmission performance of FGM sandwich cylindrical shell have been investigated, wherein the first resonance dip shift noticeably to high frequency as the Mach number is increased, and case II has the largest amplitude fluctuation and the lowest STL curve magnitudes, in addition, when M 1 = M 2 > 1.2 , two branches, including positive refraction branch and the negative refraction branch, are found in the contour plots under opposite flow direction conditions, and the large skin-core-skin ratio is beneficial to improve the sound insulation characteristics of structures. [ABSTRACT FROM AUTHOR]

Herein, we report the extraction of natural pigment curcumin from curcuma longa and their linear and third-order nonlinear optical (NLO) characteristics. The characterization techniques viz., UV–Visible absorption, FT-IR, Micro Raman and Gas Chromatography Mass Spectrum (GC–MS) are used to study the spectral characteristics of curcumin. Third-order NLO features of curcumin are studied using Z‒scan technique with a semiconductor diode laser working at 405 nm wavelength. The natural pigment exhibits negative nonlinear index of refraction resulting from self-defocusing and positive coefficient of absorption is the consequence of reverse saturable absorption (RSA). The order of nonlinear index of refraction (n2) and nonlinear coefficient of absorption (β) is measured to be 10−7 cm2/W and 10−2 cm/W, respectively. Third-order NLO susceptibility (χ(3)) and second-order hyperpolarizability (γ) of curcumin is measured to be 2.73 × 10‒7 esu and 1.67 × 10‒31 esu, respectively. A low optical limiting (OL) threshold of 0.71 mW is observed in the extracted pigment. The experimental results are supplemented by quantum mechanical calculations of the NLO parameters. The overall result finding is that curcumin extracted from curcuma longa has the potential to be novel optical candidates for photonics and optoelectronics applications. [ABSTRACT FROM AUTHOR]

Left-handed materials are known to exhibit exotic properties in controlling electromagnetic fields, with direct applications in negative reflection and refraction, conformal optical mapping, and electromagnetic cloaking. While typical left-handed materials are constructed periodic metal-dielectric structures, the same effect can be obtained in composite guest–host systems with no periodicity or structural order. Such systems are typically described by the effective-medium approach, in which the components of the electric permittivity tensor are determined as a function of individual material properties and doping concentration. In this paper, we extend the discussion on the mixing rules to include left-handed composite systems and highlight the exotic properties arising from the effective-medium approach in this framework in terms of effective values and dispersion properties. [ABSTRACT FROM AUTHOR]

Circular dichroism (CD) is a significant optical response of chiral plasmonic nanostructures, with extensive applications in chiral sensing, biological detection, and negative refractive index media. In this study, a substantial enhancement of CD signal is achieved by embedding gold nanorod into planar gold L-shaped nanohole arrays. The structural surface charge distribution revealed that electric and magnetic resonance modes are generated under different circularly polarized lights at the same resonance wavelength. The enhanced CD signals originate from stronger magnetic resonance on L-shaped films and embedded nanorod under right circularly polarized light. Moreover, the CD effect of the nanostructure could be controlled by adjusting the relative positioning, aspect ratio of the embedded nanorod, and the period of the array. To further extend the applications of the nanostructure, the relationship between wavelength and refractive index is explored. These results offer novel insights into the design and manipulation of planar chiral materials with substantial CD and also expand the application of chiral structures in refractive index sensors. [ABSTRACT FROM AUTHOR]

Metasurfaces suitable for the terahertz gap are alternatives to naturally occurring materials and could accelerate the development of terahertz flat optics integrated with terahertz continuous-wave sources. However, metasurfaces have yet to be commonly adopted in terahertz devices that require a number of specific material properties because of the many choices in meta-atom design. In this paper, we demonstrate that simple dimension control of a single kind of meta-atom enables the design of a wide range of refractive indices from large positive to negative values in the 0.3-THz band. Measurements by terahertz time-domain spectroscopy verify three kinds of metasurfaces with (1) an extremely high refractive index of 12.3 + j0.88 and reflectance of 5.1% at 0.31 THz, (2) a zero refractive index of − 0.44 + j0.12 and reflectance of 2.6% at 0.34 THz, and (3) a negative refractive index of − 5.4 + j0.32 and reflectance of 22.7% at 0.31 THz. The 0.3-THz band is a frequency band for candidates to 6G wireless communications. Our results offer an accessible material platform for terahertz flat optics such as metalenses, antennas, and phase plates. Terahertz flat optics based on our presented metasurfaces would be a welcome contribution to the development of terahertz industries, including 6G wireless communications. [ABSTRACT FROM AUTHOR]

In this study, an ultra-thin multi-polygon structure is employed to develop an ultra-thin double-negative (DNG) metamaterial (MM). The developed structure showcases an ultra-wideband Single-Negative (SNG) response, characterised by a minimal thickness of 0.2 mm, within the frequency range of 2–12 GHz. MM parameters reveal distinctive characteristics, including consistently negative permeability identifying it as a Mue Negative (MNG) material. The permittivity displays Epsilon Negative (ENG) traits within specific frequency bands, giving rise to a DNG refractive index within these negative regions. This remarkable adaptability underscores its immense potential for controlling electromagnetic waves, catering to an array of applications. Experimental validation aligns exceptionally well with simulated results, affirming the practical efficacy of this design. The results clearly show the potential of the adopted approach for various MM practical applications. [ABSTRACT FROM AUTHOR]

Optical metamaterials have attracted intensive attention in recent years for their broad applications in superlenses, electromagnetic cloaking, and bio-sensing. Negative refractive index (NRI) metal–dielectric–metal fishnet metamaterials (MMs) are typically used for beyond-diffraction-limit imaging. However, there are few reports about the substrate-thickness dependence of NRI, which strongly affects the practical application. In our study, it is demonstrated that the membrane-based NRI MMs with a more negative index work better than the bulk substrate-based counterparts. In addition, a regular periodic vibration of NRI with the thickness of the membrane substrate was theoretically studied. The destructive interference of the thin film can explain this phenomenon. Furthermore, the proposed explanation was further proved by substituting the dielectric spacer with a larger permittivity. Therefore, an NRI structure on a membrane substrate with constructive interference can be a good choice in ultra-compact photoelectronic devices. This study can be a guide to the practical application of ultracompact NRI devices. [ABSTRACT FROM AUTHOR]

In this article, a joint topology optimization procedure is executed to design phononic crystals (PnCs) with a maximized band gap. First, the band gap of two-dimensional PnCs is calculated using a finite element model under the plane strain assumption for easy and efficient optimization. In the numerical computation of the band gaps, the band structures of in-plane and out-of-plane modes are analyzed and verified using COMSOL. Then, the density-based and feature-driven methods are sequentially used to design the unit cell of the PnCs with single-phase material. A stiffness constraint is added to ensure the continuity of the material inside the unit cell, and a boundary constraint guarantees continuity amongst the unit cells. The optimization problem is solved through the Method of Moving Asymptotes (MMA), and reconfigurable manufacturing PnC is obtained via joint topology optimization. Lastly, the band structures and vibration modes of the reconstructed model are analyzed. Research results show that the optimal PnCs generate broadband vibrations by localized resonance and have negative refraction characteristics. This research also confirms that the joint topology optimization is an efficient approach for designing novel elastic metamaterials. [ABSTRACT FROM AUTHOR]

A new quad T-shaped triple band metamaterial is presented in this paper containing three resonance frequencies at 5.7 GHz, 9.6 GHz and 12.1 GHz which are covered C, X, and Ku-band, respectively. These three resonance frequencies are observed by high-frequency electromagnetic solver CST (Computer simulation technology) microwave studio at a frequency range of 4–16 GHz with the electric, magnetic and surface current distributions, and the effective parameters are carried out using the robust method from the transmission and reflection coefficient. The junction of four T-shaped conducting layers is introduced wider bandwidth of transmission coefficient from 4.7 to 6.2 GHz, 8.4 to 10.4 GHz, and 11.3 to 13.5 GHz and the symmetric orientation of the split rings exhibits stable responses of EM properties. The proposed metamaterial unit cell has shown a strong accumulation of electric and magnetic field components, where the negative permittivity, negative permeability and negative refractive index are shown at the resonance frequencies of the transmission coefficient. The different angular orientations of the proposed metamaterial with different dielectric constants are analyzed for parametric study. [ABSTRACT FROM AUTHOR]

Acoustic hyperbolic metamaterials are artificial materials with hyperbolic dispersion characteristics and strong anisotropy. Their negative refractive properties are the theoretical basis for studying the implementation of high-resolution focused superlenses. In response to the problem that the recognition of far-field noise sources is limited by the resolution of 0. 5 times wavelength acoustic Rayleigh diffraction recognition, combined with the excellent control effect of acoustic metamaterials on sound waves, a hyperbolic metamaterial that can achieve sub wavelength super-resolution imaging is introduced, and its negative refractive characteristics are used to design an acoustic hyperbolic structure for working at a frequency of 2 271. 5 Hz. The dispersion and negative refraction characteristics of the hyperbolic structure of this configuration were analyzed, and the results show that, the group velocity direction of sound waves propagating in this hyperbolic metamaterial is perpendicular to the wave vector and follows the normal direction of the dispersion curve. The research in this paper provides some design references for realizing arbitrary regulation of sound wave and elastic wave, as well as focusing, locating, identifying and amplifying noise sources. [ABSTRACT FROM AUTHOR]

Two dimensional all dielectric grating metamaterial based on polydimethylsiloxane and TiO2 materials were prepared via sputtering method in two different configurations as PDMS/TiO2 (50 nm)) and 2D grating (etalon) TiO2 (50 nm)/PDMS/TiO2 (50 nm). As it was measured by a charged coupled device (CCD) camera, the hotspots confirm four points symmetry of the structure and also the refractive index falls in the negative part to show meta state. In addition, the absorption spectra show the clear separation between electric dipole (ED) and Magnetic dipoles (MD) by enhancing the incidence angle onto the sample. We noted the gradual increase in the angle begins to appear and became more noticeable from 52o to 60o and the reason for this separation at these angles may be the fall of the beam at an oblique angle, and it is clear that inclination at an oblique angle leads to a more pronounced metamaterial behavior. [ABSTRACT FROM AUTHOR]

This article presents the unique 3D metamaterial split ring resonator (3D‐MTM SRR) with a high negative refractive index at 2.45 GHz frequency. It is integrated with two layers: one square bracket structure printed on the backside of the substrate and another dual split ring with an air gap placed above the substrate. Both structures are coupled with metallic vias, which serve as connecting vias to create the 3D‐MTM SRR. Simulation and experiment results show good agreement for a high negative refractive index and the effective medium ratio of −30 and 6.16, respectively. To fully comprehend the resonance phenomena of the proposed 3D‐MTM, an analogous circuit analysis is carried out. Thus, the unique properties of 3D‐MTM SRR allow development of the high‐performance radio frequency devices for IoT applications. [ABSTRACT FROM AUTHOR]

Using a free, open‐source ray‐tracing program, photorealistic spectral images have been created to show how several prisms would appear in the real world if they were made with materials that have a negative index of refraction (metamaterials). The aim of this work is to provide students with a visual interpretation of the atypical behavior of negative‐index materials and a look at dispersion in the visible range in such prisms, resulting in educationally valuable outcomes. [ABSTRACT FROM AUTHOR]

Double‐negative refractive index materials have attracted sustained experimental and theoretical interest because they can display a range of surprising optical phenomena, including negative Doppler shifts and perfect lensing. Double‐negative indexes have been achieved experimentally in engineered metamaterials; however, these materials become increasingly challenging to fabricate at shorter wavelengths, and at optical wavelengths only 2D negative index materials have been achieved. Here, it is shown that a double‐negative index can occur in a natural material, near narrow optical transitions of dielectric crystals stoichiometric in a rare‐earth ion. Optical measurements of two candidate materials, the magnetically‐ordered erbium crystals, ErCl3 ·6H2O and 7LiErF4, which have ultra‐narrow optical linewidths of 3 GHz and 250 MHz, respectively, in the telecom band are presented. It is shown that the spectral density of 7LiErF4 is sufficient to achieve a negative index at 1530 nm. This material can enable the exploration of negative refractive index effects at optical wavelengths in a truly 3D, natural medium. [ABSTRACT FROM AUTHOR]

This document provides an overview of the design and applications of metamaterials, which are composites with engineered architectures that have unique properties. The article discusses different design strategies for metamaterials, including nonlinear properties and bio-inspired approaches. It also explores the expansion of metamaterial functionalities, such as tunable architectures and topological properties. The article highlights recent breakthroughs in metamaterial functionalities, including plasmon-mediated optical transmission and negative refraction. It discusses various applications of metamaterials, such as photolithography, sensors, and wave control devices. The authors acknowledge the contributions of the authors and editors involved in the Special Topic. [Extracted from the article]

The copper oxide nanoparticles (CuO NPs) were synthesized via solution combustion approach with Bengal gram powder as a fuel. The XRD analysis confirms the monoclinic structure of CuO NPs. FT-IR spectrum reveals the formation of CuO NPs. SEM studies show that the NPs are nearly spherical, and the elemental composition was confirmed by EDAX. The absorption spectra revealed a direct optical band gap of 1.50 eV. DFT calculations were performed to determine the HOMO–LUMO energy bandgap and found to be 1.72 eV, which is in good agreement with the experimental value. The Z-scan technique was used to investigate the third-order nonlinear optical (NLO) characteristics using DPSS continuous wave laser (532 nm, 200 mW). A high reverse saturation absorption and negative nonlinear refractions were observed. Third-order NLO parameters β , n 2 and χ (3) were found to be 1 9. 6 × 1 0 − 5 cm/W, − 1. 1 2 × 1 0 − 8 cm2/W and 5. 9 7 × 1 0 − 7 esu, respectively. The CuO NPs also displayed strong optical limiting behavior with a limiting threshold of 2.11 kW/cm2. [ABSTRACT FROM AUTHOR]

We demonstrate a method to realize unidirectional negative refraction in an acoustic parity-time (PT)-symmetric system, which is composed of a pair of metasurfaces sandwiching an air gap. The pair of metasurfaces possesses loss and gain modulations. The unidirectional negative refraction, which is strictly limited to the case of incident wave imposing on the loss end of the metasurface, is demonstrated at the exception point (EP) in this PT -symmetric system, while the incidence from the other side leads to strong reflection. Based on rigorous calculations, we explicitly show the underlying mechanism of this model to achieve unidirectional wave scatterings around the EP in the parametric space. In addition, the perfect imaging of a point source in the three-dimensional space, as a signature of negative refraction, is simulated to provide a verification of our work. We envision that this work may sharpen the understanding of PT -symmetric structures and inspire more acoustic functional devices. [ABSTRACT FROM AUTHOR]

The introduction of ultratransparent medium has not yet solved the problem of transmission loss caused by reflection in negative refraction (NR) devices. In this study, theoretical analysis and simulation verification are carried out and an ultratransparent medium with wide-angle NR-index is proposed; the imaging with its flat lens is discussed. A photonic crystal (PC) is designed as an instance of the medium, and the rationality was analyzed. The PC can realize NR within 50° as well as positive refraction (PR) within 75° with transmittances over 99%. It is an ideal material for multifunctional NR/PR devices with ultra-low loss. [ABSTRACT FROM AUTHOR]

The Green's functions are compared for the Maxwell, Beltrami and Helmholtz equations as applied to effective infinite isotropic media, including electromagnetic metamaterials. The Dyadic and scalar Green's functions are calculated based on the 3D and 2D Fourier transforms and the residue theory without using the so-called concept of a negative refractive index. [ABSTRACT FROM AUTHOR]

We present an inertia-augmented relaxed micromorphic model that enriches the relaxed micromorphic model previously introduced by the authors via a term Curl P · in the kinetic energy density. This enriched model allows us to obtain a good overall fitting of the dispersion curves while introducing the new possibility of describing modes with negative group velocity that are known to trigger negative refraction effects. The inertia-augmented model also allows for more freedom on the values of the asymptotes corresponding to the cut-offs. In the previous version of the relaxed micromorphic model, the asymptote of one curve (pressure or shear) is always bounded by the cut-off of the following curve of the same type. This constraint does not hold anymore in the enhanced version of the model. While the obtained curves' fitting is of good quality overall, a perfect quantitative agreement must still be reached for very small wavelengths that are close to the size of the unit cell. [ABSTRACT FROM AUTHOR]

Purpose: To evaluate and compare the objective refractions obtained by autorefraction and aberrometry under different lighting conditions with an isofocal intraocular lens (Isopure, BVI medical, Liége, Belgium) compared to a monofocal control lens (Micropure, BVI medical, Liége, Belgium) with the same platform and material. Methods: Prospective, comparative and randomized study on patients undergoing cataract surgery and bilateral isofocal or monofocal IOL implantation. A total of 44 subjects were randomly assigned to either the isofocal group (n = 22) or the Micropure (n = 22). Manifest refraction (MR) was always performed under the same lighting conditions for all the patients. For objective refraction the autorefractor KR8800 and the aberrometer OPD-Scan III (Nidek Inc., Tokyo, Japan.) were used. For each eye included in the study, six result sets were collected: MR, AR (autorefraction measured with the autorefractor), WF-P and WF-M (Zernike-coefficients-based objective refraction, photopic and mesopic pupil size), OPD-C and OPD-M (autorefraction measured with the aberrometer in photopic and mesopic conditions). Results: The mean sphere for MR was 0.03 ± 0.32D for the Isopure group and 0.24 ± 0.22D for the monofocal group (p = 0.013). For the Isopure group, Friedman analysis showed statistically significant differences for sphere measured with WF-P (p = 0.035), WF-M (p = 0.018) and OPD-M (p = 0.000), and SE measured with OPD-M (p = 0.004). In the Micropure lens group, the Friedman analysis showed differences for all values studied (p < 0.05). Correlation coefficients showed that AR is the objective method with the strongest correlation values for all components of refraction for both groups. Conclusion: The modification of the surfaces of the isofocal lens does not have a negative impact on the refraction obtained by AR compared to a standard monofocal intraocular lens. [ABSTRACT FROM AUTHOR]

The phenomenon of selective diffraction is extended to in-plane elastic waves, and we design surface corrugated periodic laminates that incorporate crystal momentum transfer, which, due to the rich physics embedded within the vector elastic system, results in frequency, angle, and wave-type selective diffraction. The resulting devices are elastic grating couplers, with additional capabilities as compared to analogous scalar electromagnetic couplers, in that the elastic couplers possess the ability to split and independently redirect, through selective negative refraction, the two body waves present in the vector elastic system: P- (compressional) and SV- (shear-vertical) elastic waves. The design paradigm, and interpretation, is aided by obtaining isofrequency contours via a non-dimensionalized transfer matrix method. [ABSTRACT FROM AUTHOR]

We study theoretically the in-plane electromagnetic response and the exciton–plasmon interactions for an experimentally feasible carbon nanotube (CN) film system composed of parallel aligned periodic semiconducting CN arrays embedded in an ultrathin finite-thickness dielectric. For homogeneous single-CN films, the intertube coupling and thermal broadening bring the exciton and interband plasmon resonances closer together. They can even overlap due to the inhomogeneous broadening for films composed of array mixtures with a slight CN diameter distribution. In such systems, the real part of the response function is negative for a broad range of energies (negative refraction band), and the CN film behaves as a hyperbolic metamaterial. We also show that for a properly fabricated two-component CN film, by varying the relative weights of the two constituent CN array components, one can tune the optical absorption profile to make the film transmit or absorb light in the neighborhood of an exciton absorption resonance on-demand. [ABSTRACT FROM AUTHOR]

We propose a simple method to design a wave trapping mechanism, referred to as an elastic open cavity, for shear horizontal waves using the concept of metasurfaces. Conventional open cavity designs have utilized bulk metamaterial media with negative refractive indices at certain frequencies. Here, we show that wave trapping can be attained depending on the incident angle with a suitable design and arrangement of several metasurfaces. This design strategy can be applied to waves with various incident angles and frequencies. We show that the incident angles and the number of metasurfaces are closely related. In addition, connections between the characteristic length of the metasurfaces and the resonance frequency are discussed. As an illustration, we theoretically derive the wave fields for an elastic open cavity with six metasurfaces. Numerical simulations based on the finite element calculations are performed to verify the correctness and consistency of the theory. Instead of using reflecting walls to localize energy, this wave trapping mechanism stores mechanical energy via constructive interference by guiding wave fronts that travel around closed contours. The demonstration of this concept could be a potential guide to design new energy harvesting devices. [ABSTRACT FROM AUTHOR]

In this work, we present numerical calculations of the acoustic scattering properties of inclusions characterized by fractional order behavior. Fundamental quantities, such as the differential and the total scattering cross sections, are calculated for a wide range of forcing frequencies, and their characteristics are analyzed in the perspective of remote sensing and material characterization applications. The numerical simulations show the occurrence of resonance frequencies related to the fractional order mismatch and suggest the ability of fractional inclusion to induce lensing effects similar to those observed in materials with a negative index of refraction. The tools developed in this framework offer a novel approach for modeling and predicting the transport of field quantities throughout complex inhomogeneous and highly scattering media. [ABSTRACT FROM AUTHOR]

The realization of multifunction by simply combining acoustic devices together is always attractive, but it is never an easy task due to the complexity of most acoustic metamaterials. In this paper, a kind of unbalanced gain–loss acoustic metasurface is designed, which can work as an energy amplifier, suggesting that it may be applied to an acoustic signal detector or a sensor. In addition, an acoustic negative refraction system is established by simply putting two of these metasurfaces together, which can produce negative refraction for the acoustic waves from different directions and different incident angles, even focusing and imaging with precisely selected parameters. These functions are quite rare for materials without the double-negative-parameter. This work provides a new route to use the gain/loss materials and achieve multifunctions without complex design. [ABSTRACT FROM AUTHOR]

An elliptical chiral split‐ring‐resonator‐based bilayer thin metasurface is introduced in this paper. The proposed metasurface realizes strong electromagnetic (EM) activity with asymmetric transmission in the K‐band of the microwave regime. The efficiency is more than 80% over a range of frequencies from 18.6 to 20.8 GHz. Angular stability up to 60° makes the structure robust for practical applications. The structure also exhibits a negative refractive index for circularly polarized wave due to the strong chirality factor. Polarization azimuth rotation angle of 90° and ellipticity of 0° reveal the pure EM/optical activity. Furthermore, the simulation and measured results also affirmed orthogonal pure linear polarization transformation. The proposed chiral metasurface finds applications in radar and satellite belonging to K‐band. [ABSTRACT FROM AUTHOR]

We numerically and experimentally demonstrate super-resolution focusing of the lowest anti-symmetric (A0) mode Lamb waves in a thin aluminum plate. The subwavelength focusing/imaging is achieved by exploiting the anisotropy in phononic crystal (PC) lattices and amplification of evanescent waves. To this end, we embedded a PC flat lens in the aluminum plate, consisting of holes arranged in a square lattice formation. We revealed that the bound slab phonon modes amplify evanescent waves, as previously observed for electromagnetic and acoustic waves. Hence, the slab mode helps propagate subwavelength information through the PC lens to reach the near-field image formed due to negative refraction and result in the high resolution image. [ABSTRACT FROM AUTHOR]

In this paper, we study Multiple slit diffraction and n- array linear antennae in negative refractive index material. We show that the evanescent wave plays a vital role in the near-field term. The evanescent wave grows significantly, unlike in conventional materials, and satisfies a novel kind of convergence known as Cesàro convergence. We calculate the intensity of multiple slits and the antenna's amplification factor (AF) in terms of the Riemann zeta function. We further demonstrate that the Riemann zeta function gives rise to additional nulls. We deduce that all the diffraction scenarios in which the traveling wave satisfies the geometric series in the medium of the positive refractive index will enhance the evanescent wave, which satisfies Cesàro convergence in the medium of the negative refractive index. [ABSTRACT FROM AUTHOR]

Sudden stratospheric warmings (SSWs) can affect tropospheric weather and climate, while the factors affecting the surface response to SSWs are not fully understood. This study shows the important effects of preconditioned stratospheric state on the surface response to displacement and split SSWs using ERA5 reanalysis and historical simulations from 12 Coupled Model Intercomparison Project 6 (CMIP6) models. Both ERA5 and CMIP6 models show that preconditioned stratospheric signals (e.g., positive meridional potential vorticity gradient anomaly in the upper stratosphere) can promote the upward propagation of stratospheric planetary waves by decreasing the negative refractive index frequency before SSWs. The modulation of strong stratospheric preconditions causes larger upper‐stratospheric anomalies for subsequent SSWs and prominent barotropic downward impacts for split SSWs. Meanwhile, the strong preconditioned signals affect the preexisting tropospheric variability, masking the downward impacts of displacement SSWs. The surface response is thus weaker than that to displacement SSWs with weak preconditions. Plain Language Summary: Sudden stratospheric warmings (SSWs) feature the rapid warming of the polar stratosphere and the weakening of the polar night jet in winter. SSWs can be classified as displacement and split SSWs according to the vortex geometry. While SSWs have an overall impact on the troposphere, not all SSWs can exert a distinct downward impact on the troposphere. The factors affecting the surface response to SSWs are not fully understood. This study shows that preconditioned stratospheric state of SSWs has an important effect on the surface response to displacement and split SSWs. The preconditioned stratospheric signals (such as positive meridional potential vorticity gradient anomalies) can control the preexisting tropospheric variability or favor the enhancement of wavenumber‐2 waves in the stratosphere to affect the surface responses to displacement and split SSWs, respectively. As a result, the surface response between the events with strong and weak stratospheric preconditions is significantly different for both displacement and split SSWs. The results suggest that preconditioned stratospheric state can be an important factor for understanding the surface response to SSWs. Key Points: Preconditioned stratospheric state has an important effect on the surface response to displacement and split sudden stratospheric warmings (SSWs)Preconditioned stratospheric signal can affect the preexisting tropospheric variability, masking the downward impact of displacement SSWsPreconditioned stratospheric signal favors the growth of stratospheric wave‐2 activity, causing barotropic downward impacts of split SSWs [ABSTRACT FROM AUTHOR]

Broadband reverse saturable absorption is systematically investigated via Z-scan, transient absorption spectrum (TAS). The excited state absorption and negative refraction of Orange IV are observed in the Z-scan experiment at 532 nm. Meanwhile, two-photon-induced excited state absorption and pure two-photon absorption are observed at 600 nm and 700 nm with the pulse width of 190 fs, respectively. An ultrafast broadband absorption in the visible wavelength region is observed via TAS. The different nonlinear absorption mechanisms at multiple wavelengths are discussed and interpreted from the results of TAS. In addition, the ultrafast dynamics of negative refraction in the excited state of Orange IV are investigated via a degenerate phase object pump-probe, from which the weak long-lived excited state is extracted. All studies indicate that Orange IV has the potential to be further optimized into a superior broadband reverse saturable absorption material and also has certain reference significance for the study of optical nonlinearity in organic molecules containing azobenzene groups. [ABSTRACT FROM AUTHOR]

In this paper, a systematic approach is employed to design a photonic crystal with a boundary between positive and negative refraction to boost the refractive properties of the crystal. Mathematical techniques are employed to turn the process of equi-frequency contours' engineering into a nearly mechanical process free of trial-and-error steps. The designed demultiplexer's operational characteristics are then further improved by adding a second photonic crystal with an oblique boundary. The refracted beams in the first crystal impinge on an oblique interface of the second photonic crystal to experience a change in Bloch wavenumbers and even greater refraction angles so that a novel super-dispersive two-step optical demultiplexer is made. A beam divergence of 161 degrees is obtained for an input spectrum of λ = [1474 nm, 1550 nm] which is quite superior than the design's counterparts. [ABSTRACT FROM AUTHOR]