Showing papers on "Transformation optics published in 2021"
TL;DR: In this paper, the authors discuss the historical context, current progresses and possible future outcomes of metamaterials and highlight the interesting phenomena observed in optics/electromagnetic metammaterials with acoustic and elastic counterparts.
Abstract: The advancement in electromagnetic metamaterials, which commenced three decades ago, experienced a rapid transformation into acoustic and elastic systems in the forms of phononic crystals and acoustic/elastic metamaterials. Since its early discovery, numerous wave phenomena alongside the possible engineering applications have been highlighted. The existing and emerging fields of metamaterials are far more extensive, ranging from optics to acoustic, and all the way to the elastic systems. Numerous fantastic dynamic properties in optics and acoustic/elastic systems have been reported to date, which cannot be found in naturally occurring materials. The present review tends to discuss the historical context, current progresses and possible future outcomes of metamaterials. The fascinating phenomena observed in optics/electromagnetic metamaterials have been explained and linked with acoustic and elastic counterparts. The idea of perfect lens that is governed by negative permittivity and negative permeability via left-handed materials with negative refractive index properties and the transformation optics for invisibility cloaks and optical rainbow effect alongside the hyperbolic metamaterials are reviewed and discussed. Furthermore, the associated transformation into acoustic and elastic focusing effects via graded index metamaterials, acoustic/elastic invisibility cloaks, transformational acoustics, and seismology and metawedges resembling optical rainbow effects and the likes are explained. The present state of the art has been examined and the physics involved in the governing of those peculiar wave mechanisms has been highlighted. Starting from photonic crystals, phononic crystals and acoustic metamaterials, the present state of the art research in some subfields of acoustic metamaterials has been outlined, such as metasurfaces, topological phononic crystals and seismic metamaterials, the three exciting and emerging research topics. The substantial challenges involved in these realms are characterised and the possible future outcome is further evaluated. This review article may assist researchers and engineers to grasp the idea of metamaterials in not only photonic and phononic crystal systems, but also the counterpart subfields.
61 citations
TL;DR: In this article, a tandem neural network (T-NN) was proposed to efficiently streamline the inverse design of the transceivers of two planar metasurfaces to hide an object inside.
Abstract: Being invisible at will has been a long-standing dream for centuries, epitomized by numerous legends; humans have never stopped their exploration steps to realize this dream. Recent years have witnessed a breakthrough in this search due to the advent of transformation optics, metamaterials, and metasurfaces. However, the previous metasurface cloaks typically work in a reflection manner that relies on a high-reflection background, thus limiting the applications. Here, we propose an easy yet viable approach to realize the transmitted metasurface cloak, just composed of two planar metasurfaces to hide an object inside, such as a cat. To tackle the hard-to-converge issue caused by the nonuniqueness phenomenon, we deploy a tandem neural network (T-NN) to efficiently streamline the inverse design. Once pretrained, the T-NN can work for a customer-desired electromagnetic response in one single forward computation, saving a great amount of time. Our work opens a new avenue to realize a transparent invisibility cloak, and the tandem-NN can also inspire the inverse design of other metamaterials and photonics.
56 citations
TL;DR: In this article, a metadevice that fully resolves arbitrary beams on a higher-order Poincare sphere (HOPS) via a single-layer all-silicon metasurface was proposed.
Abstract: Characterizing the amplitude, phase profile, and polarization of optical beams is critical in modern optics. With a series of cascaded optical components, one can accurately resolve the optical singularity and polarization state in traditional polarimetry systems. However, complicated optical setups and bulky configurations inevitably hinder future applications for integration. Here, we demonstrate a metadevice that fully resolves arbitrary beams on a higher-order Poincare sphere (HOPS) via a single-layer all-silicon metasurface. The device is compact and capable of detecting optical singularities and higher-order Stokes parameters simultaneously through a single intensity measurement. To verify the validity of the proposed metadevice, different beams on HOPS0,0 and HOPS1,−1 are illuminated on the metadevices. The beams are fully resolved, and the reconstructed higher-order Stokes parameters show good agreement with the original ones. Taking the signal-to-noise ratio into account, the numerical simulations indicate that the design strategy can be extended to fully resolve arbitrary beams on HOPS with order up to 4. Because of the advantages of compact configuration and compatibility with current semiconductor technology, the metadevice will facilitate potential applications in information processing and optical communications.
21 citations
TL;DR: In this article, a novel method for designing transformation optical devices based on electrostatics is presented, where wavefronts and energy flux lines correspond to equipotential surfaces and electrostatic flux lines, respectively.
Abstract: We present a novel method for designing transformation optical devices based on electrostatics. An arbitrary transformation of electrostatic field can lead to a new refractive index distribution, where wavefronts and energy flux lines correspond to equipotential surfaces and electrostatic flux lines, respectively. Owing to scalar wave propagating exactly following an eikonal equation, wave optics and geometric optics share the same solutions in the devices. The method is utilized to design multipole lenses derived from multipoles in electrostatics. The source and drain in optics are considered as corresponding to positive charge and negative charge in the static field. By defining winding numbers in virtual and physical spaces, we explain the reason for some multipole lenses with illusion effects. Besides, we introduce an equipotential absorber to replace the drain to correspond to a negative charge with a grounded conductor. Therefore, it is a very general platform to design intriguing devices based on the combination of electrostatics and transformation optics.
11 citations
TL;DR: In this article, a series of Eaton lenses with a singularity for flexural waves can be obtained by approaching a near-zero thickness of the plate precisely at the location of the singularity.
Abstract: Transformation optics, which is generically applicable to other classical waves such as acoustic and elastic waves, provides an emerging design paradigm to manipulate waves. However, some lenses and optical-transformation devices require a singular refractive index; meeting this requirement is a significant challenge. A method called transmutation can relax some types of index singularity into finite anisotropy around the singularity. Here, we show that such lenses with a singularity for flexural waves can be obtained by approaching a near-zero thickness of the plate precisely at the location of the singularity. As examples, we demonstrate a series of Eaton lenses theoretically and experimentally by projecting the refractive index in space onto the thickness in plates and by working in a broad frequency range in which impedance mismatch is negligible. This framework offers an insight into feasible methods that can be used to develop singular devices such as cloaking devices on thin flexible curved plates and can be further extended to a general methodology for shaping elastic waves. We hope that this elastic platform can also be a test bed to indirectly study unprecedented phenomena enabled by gravitational and quantum fields in terms of analog models.
11 citations
TL;DR: In this article, a graded-index all-dielectric lens was proposed to enhance the directivity of an H-plane horn antenna by modifying the physical space shape of the lens to eliminate singular index values and rescaling the optical path inside the lens.
Abstract: Conformal transformation optics is employed to enhance an H-plane horn’s directivity by designing a graded-index all-dielectric lens. The transformation is applied so that the phase error at the aperture is gradually eliminated inside the lens, leading to a low-profile high-gain lens antenna. The physical space shape is modified such that singular index values are avoided, and the optical path inside the lens is rescaled to eliminate superluminal regions. A prototype of the lens is fabricated using three-dimensional printing. The measurement results show that the realized gain of an H-plane horn antenna can be improved by 1.5–2.4 dB compared to a reference H-plane horn.
10 citations
TL;DR: In this article, a bilayer meta-device for binary masses manipulation (O2 and N2) was proposed by directly solving static Fick's law, and only homogeneous media were employed in such design owing to the independence of spatial transformations.
9 citations
TL;DR: In this paper, a gradient index lens was used to mimic the Schwarzschild precession in the orbit of the star S2 near the Galactic Center massive black hole, which was recently first detected by European Southern Observatory.
Abstract: General relativity establishes the equality between matter-energy density and the Riemann curvature of spacetime. Therefore, light or matter will be bent or trapped when passing near the massive celestial objects, and Newton’s second law fails to explain it. The gravitational effect is not only extensively studied in astronomy but also attracts a great deal of interest in the field of optics. People have mimicked black holes, Einstein’s ring, and other fascinating effects in diverse optical systems. Here, with a gradient index lens, in the geometrical optics regime, we mimic the Schwarzschild precession in the orbit of the star S2 near the Galactic Center massive black hole, which was recently first detected by European Southern Observatory. We also find other series of gradient index lenses that can be used to mimic the possible Reissner–Nordstrom metric of Einstein’s field equation and dark matter particle motion. Light rays in such gradient lenses will be closed in some cases, while in other cases it would be trapped by the center or keep dancing around the center. Our work presents an efficient toy model to help investigate some complex celestial behaviors, which may require long period detection by using high-precision astronomical tools. The induced gradient lenses enlightened by the gravitational effect also enrich the family of absolute optical instruments for their selective closed trajectories.
9 citations
TL;DR: In this paper, the authors derived a mode-matching technique based on the generalized Floquet theorem for glide-symmetric structures, which benefits from a lower computational cost since it takes advantage of the glide symmetry in the structure.
Abstract: In this paper, we study the wave propagation in a metallic parallel-plate structure with glide-symmetric elliptical holes. To perform this study, we derived a mode-matching technique based on the generalized Floquet theorem for glide-symmetric structures. This mode-matching technique benefits from a lower computational cost since it takes advantage of the glide symmetry in the structure. It also provides physical insight on the specific properties of Floquet modes propagating in these specific structures. With our analysis, we demonstrate that glide-symmetric structures with periodic elliptical holes exhibit an anisotropic refractive index over a wide range of frequencies. The equivalent refractive index can be controlled by tuning the dimensions of the holes. Finally, by combining the anisotropy related to the elliptical holes and transformation optics, a Maxwell fish-eye lens with a 33.33% size compression is designed. This lens operates in a wideband frequency range from 2.5 GHz to 10 GHz.
9 citations
TL;DR: In this article, fundamental bounds on the performance of monochromatic scattering cancellation and field zeroing cloaks made of prescribed linear passive materials occupying a predefined design region are formulated by projecting field quantities onto a sub-sectional basis and applying quadratically constrained quadratic programming.
Abstract: Fundamental bounds on the performance of monochromatic scattering-cancellation and field-zeroing cloaks made of prescribed linear passive materials occupying a predefined design region are formulated by projecting field quantities onto a sub-sectional basis and applying quadratically constrained quadratic programming. Formulations are numerically tested revealing key physical trends as well as advantages and disadvantages between the two classes of cloaks. Results show that the use of low-loss materials with high dielectric contrast affords the highest potential for effective cloaking.
9 citations
TL;DR: In this paper, a transformation optics formalism is used to rigorously model a wide range of twisted anisotropic fibers, which could only be analyzed using perturbative methods.
Abstract: In this study, we show that transformation optics formalism can be used to rigorously model a wide range of twisted anisotropic fibers, which could only be analyzed using perturbative methods. If the material anisotropy of fibers has an intrinsic origin or is induced by axially or helically symmetric physical factors, then they can be transformed into a form usable in rigorous two-dimensional (2D) modeling. We demonstrate the effectiveness of the proposed approach in 2D modeling of the propagation characteristics of first-order eigenmodes in twisted and spun fibers with high linear birefringence. We derive the equivalent electric permittivity tensors for such fibers in the helical coordinate system and study the evolution of the first-order modes toward vortex modes with increasing twist rate. The obtained results confirm that the proposed method can reveal phenomena that cannot be predicted by analytical approaches.
TL;DR: In this article, the authors investigate the behavior of temporal boundaries and show that traditional approaches that assume constant dielectric properties, with loss incorporated as an imaginary part, necessarily lead to unphysical solutions.
Abstract: Temporally modulated optical media are important in both abstract and applied applications, such as spacetime transformation optics, relativistic laser-plasma interactions, and dynamic metamaterials. Here we investigate the behaviour of temporal boundaries, and show that traditional approaches that assume constant dielectric properties, with loss incorporated as an imaginary part, necessarily lead to unphysical solutions. Further, although physically reasonable predictions can be recovered with a narrowband approximation, we show that appropriate models should use materials with a temporal response and dispersive behaviour.
TL;DR: In this paper, a transformation-optical method is presented to enhance the directivity of a cylindrical wire antenna by using an all-dielectric graded index medium.
Abstract: A transformation-optical method is presented to enhance the directivity of a cylindrical wire antenna by using an all-dielectric graded index medium. The strictly conformal mapping between two doubly connected virtual and physical domains is established numerically. Multiple directive beams are produced, providing directive emission. The state-of-the-art optical path rescaling method is employed to mitigate the superluminal regions. The resulting transformation medium is all-dielectric and nondispersive, which can provide broadband functionality and facilitate the realization of the device using available fabrication technologies. The realization of the device is demonstrated by dielectric perforation based on the effective medium theory. The device's functionality is verified by carrying out both ray-tracing and full-wave simulations using finite-element-based software COMSOL Multiphysics.
TL;DR: In this article, a superscatterer with a double negative metamaterial and an invisible gateway was demonstrated to stop electromagnetic waves in an air channel with a width much larger than the cutoff width of the corresponding rectangular waveguide.
Abstract: Illusion devices, such as superscatterer and invisible gateway, have been theoretically studied under the theory of transformation optics and folded geometry transformations. The realization of these devices needs building blocks of metamaterials with negative permittivities and permeabilities. However, superscattering effects, such as stopping wave propagation in an air channel, have not been verified from illusion devices physically because of the challenge of metamaterial design, fabrication, and material loss. In this Letter, we implement a big metamaterial superscatterer, and experimentally demonstrate its superscattering effect at microwave frequencies by field-mapping technology. We confirm that superscattering is originated from the excitation of surface plasmons. Integrated with superscatterer, we experimentally display that an invisible gateway could stop electromagnetic waves in an air channel with a width much larger than the cutoff width of the corresponding rectangular waveguide. Our results provide a first direct observation of superscattering effect of double negative metamaterials and invisible gateway for electromagnetic waves. It builds up an ideal platform for future designs of other illusion devices.
TL;DR: In this paper, a plasmonic right-angled waveguide bend and divider are proposed using the Transformation Optics (TO) approach to obtain the transformation media of a bend and a T-shaped divider.
Abstract: In this work, a plasmonic right-angled waveguide bend and divider are proposed. Using the Transformation Optics (TO) approach the transformation media of a bend and a T-shaped divider are obtained. Such media with continuous refractive index are realized with the help of graphene in the terahertz frequency range, key to effectively guiding the surface plasmon polariton (SPP) propagation on the 90 degree curves. Components with such capability are promising for THz device applications.
Posted Content•
TL;DR: In this article, the authors show how non-conformal distortions of optical space are intimately connected to the complex refractive index distribution of an isotropic non-Hermitian medium.
Abstract: Coordinate transformations are a versatile tool to mould the flow of light, enabling a host of astonishing phenomena such as optical cloaking with metamaterials. Moving away from the usual restriction that links isotropic materials with conformal transformations, we show how non-conformal distortions of optical space are intimately connected to the complex refractive index distribution of an isotropic non-Hermitian medium. Remarkably, this insight can be used to circumvent the material requirement of working with refractive indices below unity, which limits the applications of transformation optics. We apply our approach to design a two-dimensional broadband dielectric cloak, which relies on non-conformal coordinate transformations to tailor the non-Hermitian refractive index profile around a cloaked object. Our insights bridge the fields of two-dimensional transformation optics and non-Hermitian photonics.
TL;DR: In this article, the authors exploit a tapered fiber to simulate the accelerated motion of effective particles and obtain an effective Unruh temperature, where the shape of the tapered waveguide is controlled by controlling the shape and speed of light propagating in the fiber.
Abstract: One of the most fascinating aspects of quantum fields in curved spacetime is the Unruh effect. The direct experimental detection of Unruh temperature has remained an elusive challenge up to now. Gradient optical waveguides manipulating the dispersion of photons are assumed to realize the great acceleration of effective particles, leading to a high effective Unruh temperature. However, experimentally achieving this optical waveguide has not yet been reported. In this work, we exploit a tapered fiber to simulate the accelerated motion of effective particles and obtain an effective Unruh temperature. When light propagating in a tapered fiber is affected by the external high refractive index medium, a leaky phenomenon akin to bremsstrahlung will be observed, and the pattern of leaky radiation is dependent on the acceleration of photons. During the experiments, different accelerations corresponding to different Unruh temperatures are achieved by controlling the shape of the tapered waveguide.
TL;DR: In this paper, the electromagnetic detection of an ideal gyrating cloak, the effect of rotational speed on the electromagnetic wave interaction with a perfect non-inertial cloak was investigated.
Abstract: For the first time, electromagnetic wave interaction with a perfect non-inertial cloak is investigated. The electromagnetic detection of an ideal gyrating cloak, the effect of rotational speed on t...
TL;DR: In this article, a plasmonic right-angled waveguide bend and divider are proposed using the Transformation Optics (TO) approach to obtain the transformation media of a bend and a T-shaped divider.
Abstract: In this work, a plasmonic right-angled waveguide bend and divider are proposed. Using the Transformation Optics (TO) approach the transformation media of a bend and a T-shaped divider are obtained. Such media with continuous refractive index are realized with the help of graphene in the terahertz frequency range, key to effectively guiding the surface plasmon polariton (SPP) propagation on the 90 degree curves. Components with such capability are promising for THz device applications.
27 Sep 2021
TL;DR: In this article, the effective line element of a trajectory of light propagation conforms to ultrahyperbolic geometry and all conformal spaces with Minkowski space-time.
Abstract: Hyperbolic metamaterials have attracted considerable interest in the research community for their peculiar ability to enhance control of electromagnetic waves' propagation. Although conformal transformation optics provides a unique platform for metamaterials design, this method has not been used for hyperbolic metamaterials yet. This comes from the lack of a well-defined mathematical structure. We extend conformal transformation optics to hyperbolic metamaterials, by applying Clifford algebra to analyze light propagation. We will show that the effective line element of a trajectory of light propagation conforms to ultrahyperbolic---not Euclidean---geometry. We also, by using conformal hyperbolic mapping, obtain all conformal spaces with Minkowski space-time. Finally, we employ this theory to study the electric-field pattern of dipoles.
TL;DR: In this article, two novel perturbation solutions for the electromagnetic analysis of eccentric coaxial waveguides are described. But the proposed approach, instead, employs transformation optics to map the original problem into a problem consisting of a concentric coaxial wguide filled with an anisotropic and inhomogeneous medium.
Abstract: This article describes two novel perturbation solutions for the electromagnetic analysis of eccentric coaxial waveguides. Different techniques have been used for solving this type of problem in the past. Most of them rely on solving a complicated transcendental equation obtained from the Graf’s addition theorem or on brute-force numerical methods such as finite differences and finite elements. The proposed approach, instead, employs transformation optics to map the original problem into a problem consisting of a concentric coaxial waveguide filled with an anisotropic and inhomogeneous medium. The corresponding (transformed) wave equation is then solved via a regular perturbation series. This method allows to compute field and wavenumber corrections of the transverse magnetic (TM) and transverse electric (TE) modes for coaxial waveguides with small eccentricities. In addition, another solution is obtained by using the cavity-material perturbation approach for providing the cutoff wavenumbers of TM and TE fields. The proposed perturbation solutions are validated against full-wave finite-element and finite-volume results in several examples.
TL;DR: In this article, a conformal transformation scheme is proposed to solve the problem of lateral shift of the reflected light ray, which can hide an object on a reflecting plane, can operate over a broadband frequency range.
Abstract: With the advent and rapid development of the transformation optics and metamaterials, invisibility cloaks have captivated much attention in recent years. While most cloaking schemes suffer from limited bandwidth, the carpet cloak, which can hide an object on a reflecting plane, can operate over a broadband frequency range. However, the carpet cloaks experimentally realized thus far still have several limitations. For example, the quasi-conformal mapping carpet cloak leads to a lateral shift of the reflected light ray, while the birefringent carpet cloak only works for a specific polarization. In this work, we propose a conformal transformation scheme to tackle these two problems simultaneously. As an example, we design a mid-infrared carpet cloak in a silicon platform and demonstrate its polarization-insensitive property as well as the minimized lateral shift over a broad frequency band from 24 to 28.3 THz.
TL;DR: In this paper, an innovative method to produce omnidirectional OAM beams based on spatial transformation is proposed at microwave frequencies for potential applications in wireless communications, which can be potentially implemented with an all-dielectric medium showing a gradient permittivity distribution.
Abstract: Vortex electromagnetic waves carrying orbital angular momentum (OAM) have been widely discussed for potential applications in wireless communications. Belonging to the Laguerre–Gaussian beams family, such type of waves present a hollow conical shape and divergence characteristics along with a directional radiation. In this paper, an innovative method to produce omnidirectional OAM beams based on spatial transformation is proposed at microwave frequencies. As a proof-of-concept demonstration, a lens with omnidirectional radiation in the horizontal plane is designed and simulated with an incident vortex beam carrying the OAM mode l = +2. The designed lens can be potentially implemented with an all-dielectric medium showing a gradient permittivity distribution. Furthermore, the proposed lens presents good performances over a wide operational bandwidth spanning from 8 to 17 GHz. By converting the directional beam to an omnidirectional one, the proposed method opens the door to the potential development of microwave vortex antenna systems.
TL;DR: In this paper, a cylindrical multilayer structure characterized by anisotropy and topological features was designed to tailor electromagnetic fields into desired patterns with orbital angular momentum in cylinear geometries as ring resonators and fibers.
Abstract: We design cylindrical multilayer structures characterized by anisotropy and topological features. This provides a new approach to tailor electromagnetic fields into desired patterns with orbital angular momentum in cylindrical geometries as ring resonators and fibers. We use transformation optics to deal with anisotropic circular structures, and rigorously define the edge states. The resulting topologically protected high-localized modes, at the core/cladding interface, with angular momentum may trigger the developments of new disorder-robust devices and high Q-microcavities for applications in light transmission, quantum technology, nonlinear optics, TeraHertz devices, and biophysical sensors.
TL;DR: This work developed the mathematical formalism and showed the successful operation of the cloaking system with the naked eye, by proposing a five-lens system producing at least three potential invisible regions with a large cloaked area.
Abstract: Electromagnetic cloaking has being continuously pursued using a large variety of approaches. In recent years, this effect has been observed using either complex devices based on the so-called Transformation Optics or simple systems based on conventional optics with proper characteristics. In the latter case, a simple arrangement of lenses working in the paraxial regime can provide broadband visible cloaking in a wide area. In this work, we analyzed and generalized this method by proposing a five-lens system producing at least three potential invisible regions with a large cloaked area (>90% of the visual field). In particular, we developed the mathematical formalism and show, both numerically and experimentally, the successful operation of the cloaking system with the naked eye.
TL;DR: In this paper, an optical funnel is designed by precisely filling subwavelength ceramic blocks with a gradient refractive index inside a tapered waveguide, which is isotropic and all above unit.
Abstract: An optical funnel, which performs as a passive electromagnetic compressor, can guide electromagnetic waves from a wide inlet to a narrow outlet without reflectance/scattering and squeeze electromagnetic fields uniformly to an air neck. In this study, an optical funnel is designed by precisely filling subwavelength ceramic blocks with a gradient refractive index inside a tapered waveguide. The gradient refractive index is designed by transformation optics, which is isotropic and all above unit, thus exhibiting a broadband feature. Due to the mechanism of impedance matching over the whole funnel, extremely low reflectance/scattering and stable enhancement of fields can be achieved. The field enhancement factor in different regions of the funnel (e.g., in the air neck) can be flexibly designed just by modifying the funnel-width ratios.
TL;DR: In this paper, the authors proposed the design of all-dielectric isotropic micro-or nanoparticles which, when used as structural elements of a metamaterial, can simultaneously provide negative effective electric permittivity and negative magnetic permeability.
Abstract: We propose the design of all-dielectric isotropic micro- or nanoparticles which, when used as structural elements of a metamaterial, can simultaneously provide negative effective electric permittivity and negative magnetic permeability. The particles comprise three dielectric layers with specially adjusted parameters that ensure simultaneous excitation of the electric and magnetic Mie dipole modes. The layer parameters were found by Mie scattering analysis and then verified by the simulations, which confirmed the double-negative behavior of the metamaterial at up to visible light frequencies. The isotropy of such particles suggests the possibility of using them in a double-negative liquid metamaterial.
TL;DR: In this article, the authors proposed a new design method for transformation cavities to realize a narrower width of the refractive index profile, a great advantage in experimental implementations, without losing the benefits of conformal mapping.
Abstract: Recently, gradient index cavities, or so-called transformation cavities, designed by conformal transformation optics have been studied to support resonant modes with both high Q-factors and emission directionality. We propose a new design method for transformation cavities to realize a narrower width of the refractive index profile, a great advantage in experimental implementations, without losing the benefits of conformal mapping. We study the resonant modes with both high Q-factor and directional emission in the newly designed transformation cavities, where the refractive index profile is 50 percent narrower than in previously proposed transformation cavities. By varying a system parameter with a fixed maximal value of the refractive index profile inside the cavity, the width of the refractive index profile narrows, the Q-factors become higher, and the near and far field patterns maintain their properties, namely conformal whispering gallery modes and bidirectional emission, respectively.