Showing papers on "Transformation optics published in 2022"

Optical manipulation with metamaterial structures

Abstract: Optical tweezers employing forces produced by light underpin important manipulation tools employed in numerous areas of applied and biological physics. Conventional optical tweezers are widely based on refractive optics, and they require excessive auxiliary optical elements to reshape both amplitude and phase, as well as wavevector and angular momentum of light, and thus impose limitations on the overall cost and integration of optical systems. Metamaterials can provide both electric and optically induced magnetic responses in subwavelength optical structures, and they are highly beneficial to achieve unprecedented control of light required for many applications and can open new opportunities for optical manipulation. Here, we review the recent advances in the field of optical manipulation employing the physics and concepts of metamaterials and demonstrate that metamaterial structures could not only help to advance classical operations such as trapping, transporting, and sorting of particles, but they can uncover exotic optical forces such as pulling and lateral forces. In addition, apart from optical manipulation of particles (that can also be called “meta-tweezers”), metamaterials can be powered dynamically by light to realize ingenious “meta-robots.” This review culminates with an outlook discussing future novel opportunities in this recently emerged field ranging from enhanced particle manipulation to meta-robot actuation.

Experimental Realization of a Superdispersion‐Enabled Ultrabroadband Terahertz Cloak

Abstract: Invisibility has been a topic of long‐standing interest owing to the advent of metamaterials and transformation optics, but still faces open challenges after its tremendous development in recent decades. One of the big challenges is the narrow bandwidth, as the realization of an invisibility cloak is usually based on a metamaterial—an artificial composite material composed of subwavelength resonator structures that are always associated with dispersion. Different from previous works that have tried to eliminate the material dispersion to enhance the bandwidth of an invisibility cloak, here, it is found that by judiciously harnessing the material dispersion, the bandwidth of the cloak can still be significantly increased. Interestingly, the material dispersion does not violate the law of causality. As a proof of concept, an ultrabroadband terahertz (THz) carpet cloak is experimentally demonstrated through an array of superdispersive microparticles, rendering the target object invisible to detection by both time‐ and frequency‐domain wideband systems. The work presents a feasible invisibility strategy that is closer to practical applications and may pave a brand‐new way for the development of dispersion‐dominated ultrabroadband metadevices.

Active and tunable nanophotonic metamaterials

Abstract: Abstract Metamaterials enable subwavelength tailoring of light–matter interactions, driving fundamental discoveries which fuel novel applications in areas ranging from compressed sensing to quantum engineering. Importantly, the metallic and dielectric resonators from which static metamaterials are comprised present an open architecture amenable to materials integration. Thus, incorporating responsive materials such as semiconductors, liquid crystals, phase-change materials, or quantum materials (e.g., superconductors, 2D materials, etc.) imbue metamaterials with dynamic properties, facilitating the development of active and tunable devices harboring enhanced or even entirely novel electromagnetic functionality. Ultimately, active control derives from the ability to craft the local electromagnetic fields; accomplished using a host of external stimuli to modify the electronic or optical properties of the responsive materials embedded into the active regions of the subwavelength resonators. We provide a broad overview of this frontier area of metamaterials research, introducing fundamental concepts and presenting control strategies that include electronic, optical, mechanical, thermal, and magnetic stimuli. The examples presented range from microwave to visible wavelengths, utilizing a wide range of materials to realize spatial light modulators, effective nonlinear media, on-demand optics, and polarimetric imaging as but a few examples. Often, active and tunable nanophotonic metamaterials yield an emergent electromagnetic response that is more than the sum of the parts, providing reconfigurable or real-time control of the amplitude, phase, wavevector, polarization, and frequency of light. The examples to date are impressive, setting the stage for future advances that are likely to impact holography, beyond 5G communications, imaging, and quantum sensing and transduction.

Soft‐Matter‐Based Hybrid and Active Metamaterials

Abstract: Metamaterials are artificial electromagnetic media structured on the subwavelength scale for controlling the propagation of waves by means of transformation optics. The research activity is now focusing on attaining active metamaterial functionalities, including tunability, and the shaping and modulation of electromagnetic waves. Among all the different architectures, soft‐matter‐based active metamaterials, or hybrid composites, have gained special importance as they allow a variety of tuning strategies to be used, including those based on temperature, application of external electric or magnetic fields, and all‐optical methods that benefit of the strongly nonlinear responses of materials. This review aims to summarize recent and ongoing progress on active metamaterials exploiting resonances in plasmonic and dielectric materials hybridized with soft‐matter assemblies including liquid crystals, colloids, polymers, and granular matter. The theoretical background of these hybrid systems is outlined, experimental realizations are overviewed, and state‐of‐the‐art applications in multifunctional platforms for light–matter interactions are presented. Finally, up to date challenges in the field that still remain open for further research are discussed.

Nonlocal effects in temporal metamaterials

Abstract: Abstract Nonlocality is a fundamental concept in photonics. For instance, nonlocal wave-matter interactions in spatially modulated metamaterials enable novel effects, such as giant electromagnetic chirality, artificial magnetism, and negative refraction. Here, we investigate the effects induced by spatial nonlocality in temporal metamaterials, i.e., media with a dielectric permittivity rapidly modulated in time. Via a rigorous multiscale approach, we introduce a general and compact formalism for the nonlocal effective medium theory of temporally periodic metamaterials. In particular, we study two scenarios: (i) a periodic temporal modulation, and (ii) a temporal boundary where the permittivity is abruptly changed in time and subject to periodic modulation. We show that these configurations can give rise to peculiar nonlocal effects, and we highlight the similarities and differences with respect to the spatial-metamaterial counterparts. Interestingly, by tailoring the effective boundary wave-matter interactions, we also identify an intriguing configuration for which a temporal metamaterial can perform the first-order derivative of an incident wavepacket. Our theoretical results, backed by full-wave numerical simulations, introduce key physical ingredients that may pave the way for novel applications. By fully exploiting the time-reversal symmetry breaking, nonlocal temporal metamaterials promise a great potential for efficient, tunable optical computing devices.

Electromagnetic-acoustic biphysical cloak designed through topology optimization.

Abstract: Various strategies have been proposed to achieve invisibility cloaking, but usually only one phenomenon is controlled by each device. Cloaking an object from two different waves, such as electromagnetic and acoustic waves, is a challenging problem, if not impossible, to be achieved using transformation theory and metamaterials, which are the major approaches in physics. Here, by developing topology optimization for controlling both electromagnetic and acoustic waves, we present a multidisciplinary attempt for designing biphysical cloaks with triple-wave cloaking capabilities, specifically for Ez- and Hz-polarized waves and acoustic wave. The topology-optimized biphysical cloak cancels the scattering of the three waves and reproduces the original propagating waves as if nothing is present, thus instilling the desired cloaking capability. In addition, we describe cloaking structures for multiple incident directions of the three waves and structures that work for both electromagnetic waves and sound waves of different wavelengths.

Tunable liquid-solid hybrid thermal metamaterials with a topology transition.

Abstract: Thermal metamaterials provide rich control of heat transport which is becoming the foundation of cutting-edge applications ranging from chip cooling to biomedical. However, due to the fundamental laws of physics, the manipulation of heat is much more constrained in conventional thermal metamaterials where effective heat conduction with Onsager reciprocity dominates. Here, through the inclusion of thermal convection and breaking the Onsager reciprocity, we unveil a regime in thermal metamaterials and transformation thermotics that goes beyond effective heat conduction. By designing a liquid-solid hybrid thermal metamaterial, we demonstrate a continuous switch from thermal cloaking to thermal concentration in one device with external tuning. Underlying such a switch is a topology transition in the virtual space of the thermotic transformation which is achieved by tuning the liquid flow via external control. These findings illustrate the extraordinary heat transport in complex multicomponent thermal metamaterials and pave the way toward an unprecedented regime of heat manipulation.

Microwave Metasurface Cloaking for Freestanding Objects

Abstract: Since the introduction of transformation optics and experimental demonstration using microwave metamaterials in 2006, a variety of electromagnetic invisibility cloaking techniques and physical realizations based on coordinate transformations, scattering cancellation, and control of reflected and transmitted waves have been reported from microwaves to visible. However, existing cloaking methodologies face challenges in reducing cloak thicknesses, concealing large volumes, and cloaking free-standing objects. Here, we design, fabricate, and experimentally validate a unidirectional, single-layer printed metasurface cloak for free-standing cylindrical objects in the microwave regime. Based on a spatially modulated reactance surface, the printed metasurface converts an incident plane wave on the lit side into a surface wave, which carries power to the shadow side before reconstructing the incident wave behind the object. Using a subwavelength-thin printed circuit prototype, the cloaking function is experimentally confirmed, demonstrating a thin, passive cloak that conceals a large, free-standing object.

Radially Periodic Metasurface Lenses for Magnetic Field Collimation in Resonant Wireless Power Transfer Applications

Abstract: In this paper, a new procedure and topology of magnetic metasurface lens are proposed for improving the performance of Resonant Wireless Power Transfer systems. Firstly, three subwave-length unit cells are optimized in order to get negative magnetic permeability in the same frequency, each one with a different refractive index, and they are experimentally characterized. Then, the Transformation Optics technique is applied in order to find the unit cell arrangements which lead to the magnetic field focusing in a given direction. For this purpose, two coordinate transformations are proposed, and the effective electric permittivity and permeability that generate these profiles are calculated. It is shown from simulations and measurements that the refractive index gradient produced by the radial disposition of the unit cells can lead to a magnetic field manipulation similar to the optical converging and diverging lenses. Both metamaterials lenses are built, and the calculation of the magnetic flux density as a function of the measured induced voltage in a probe coil verifies their effects on the magnetic field. Finally, their performance in a resonant wireless power transfer system is tested, and improvements in terms of efficiency and range are presented. The proposed design method and the lenses that were developed demonstrate that metasurface lenses can improve efficiency without reducing the range, once these lenses are positioned close to the transmitter coil. Besides that, this method can reduce the losses due to misalignment between coils once the field can be collimated in a specific direction.

Multiobjective Optimization of Bespoke Gradient-Index Lenses: A Powerful Tool for Overcoming the Limitations of Transformation Optics

Abstract: Gradient-index (GRIN) lenses have paved the way for electromagnetic wave tailoring due to their spatially varying permittivities, which enable continuous manipulation of the wave as it passes through the lens volume. However, traditional GRIN design methodologies, such as transformation optics or geometrical optics, often generate lens profiles that are limited to single-band performance. Here, we propose a multiobjective optimization strategy customized for designing dual-band GRIN lenses. Utilizing this powerful inverse-design tool, in conjunction with an advanced manufacturing technique, a GRIN lens with compact size and nonintuitive permittivity distribution is designed, fabricated, and characterized. The lens is capable of realizing two unique gain enhancement objectives at the L and C bands simultaneously, where different feeding sources are employed at the two bands.

3-D Metamaterials: Trends on Applied Designs, Computational Methods and Fabrication Techniques

Abstract: Metamaterials are artificially engineered devices that go beyond the properties of conventional materials in nature. Metamaterials allow the creation of negative refractive indexes, light trapping with epsilon-near-zero compounds, bandgap selection, superconductivity phenomena, non-Hermitian responses and, more generally, to manipulate the propagation of electromagnetic and acoustic waves. In the past, low computational resources and the lack of proper manufacturing techniques have limited the attention to 1-D and 2-D metamaterials. However, the true potential of metamaterials will be ultimately reached in 3-D configurations, when the degrees of freedom associated to the propagating direction are finally exploited in design. This is expected to lead to a new era in metamaterial field, from which future high-speed and low-latency communication networks can benefit. Here, a comprehensive overview of the past, present and future trends related to 3-D metamaterial devices is presented, focusing on efficient computational methods, innovative designs and functional manufacturing techniques.

Transforming Space with Non-Hermitian Dielectrics

Abstract: Coordinate transformations are a versatile tool to mold 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 nonconformal 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 broadband unidirectional dielectric cloak, which relies on nonconformal 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.

Transformation Theory for Spatiotemporal Metamaterials

Abstract: The transformation theory provides a distinct method for designing parameters in spatial dimensions, facilitating intriguing functions such as cloaking, concentrating, and rotating. However, with the introduction of temporal dimension, the transformation theory becomes particularly elusive because coordinate transformations apply only to static parameters. Here, we develop the transformation thermotics for designing spatiotemporal metamaterials. Specifically, we consider the transient heat-conduction equation with dynamic thermal parameters, whose transformation principles are theoretically derived and numer-ically confirmed. We further uncover spatiotemporal thermal cloaking, concentrating, and rotating with transformation thermotics as model applications. In contrast to conventional static parameters, dynamic parameters may provide unique opportunities for achieving thermal functions with the additional asymmetric feature. Our spatiotemporal scheme has remarkable advantages in dynamic heat regulation and provides insights into particle or plasma diffusion and wave propagation.

Transformation of Equivalent Graphene Plasmonics for Metagratings and Hyperbolic Metamaterials

Abstract: Recent advances in graphene plasmonics offer numerous opportunities for enabling the design and manufacture of a variety of nanoscale optical devices. Here, a method of designing metagratings and hyperbolic metamaterials based on the geometrical transformation of the proposed equivalent graphene is reported. The physical mechanism underlying this method is the strongly enhanced light–matter interaction of equivalent graphene plasmonics, which can be characterized by effective conductance and remains constant in geometrical transformation. As proof of the method, we design and demonstrate a compact retroreflector that can anomalously reflect the incident wave along its original path utilizing the roll-up plasmonic structure. In addition, an air-medium hyperbolic metamaterial and hyperlens consisting of periodic plasmonic structures are also theoretically predicted and experimentally validated by anomalous diffraction. Compared with the existing approaches of designing these metadevices, the proposed method significantly lowers the requirement on their compositional materials. The concept of equivalent graphene effectively links optical and microwave plasmonics on the basis of the effective conductance model. In this case, the methodology of geometrical transformation can function in both the microwave and nanofields and serves as a platform for designing nanoscale and microwave focusing lenses and diverse on-chip optical wave control devices.

Research on the reflection-type ELC-based optomechanical metamaterial

Abstract: In this paper, we propose a new kind of optomechanical metamaterial based on a planar ELC-type absorbing structure fabricated on the low-loss flexible substrate. The nonlinear coupling mechanism and nonlinear response phenomenon of the proposed optomechanical metamaterial driven by electromagnetic induced force are analyzed theoretically. The mechanical deformation/displacement and the mechanical resonance frequency shift of the metamaterial unit deposed on the flexible substrate are also numerically and experimentally demonstrated to reveal the coupling phenomenon of electromagnetic field and mechanical field. These results will help researchers to further understand the multi-physics interactions of optomechanical metamaterials and will promote the developments of new type of metasurface for high-efficiency dynamic electromagnetic wave controlling and formatting.

Broadband continuous supersymmetric transformation: a new paradigm for transformation optics

Abstract: Abstract Transformation optics has formulated a versatile framework to mold the flow of light and tailor its spatial characteristics at will. Despite its huge success in bringing scientific fiction (such as invisibility cloaking) into reality, the coordinate transformation often yields extreme material parameters unfeasible even with metamaterials. Here, we demonstrate a new transformation paradigm based upon the invariance of the eigenspectra of the Hamiltonian of a physical system, enabled by supersymmetry. By creating a gradient-index metamaterial to control the local index variation in a family of isospectral optical potentials, we demonstrate broadband continuous supersymmetric transformation in optics, on a silicon chip, to simultaneously transform the transverse spatial characteristics of multiple optical states for arbitrary steering and switching of light flows. Through a novel synergy of symmetry physics and metamaterials, our work provides an adaptable strategy to conveniently tame the flow of light with full exploitation of its spatial degree of freedom.

A Hemispherical Wide-Angle Beamsteering Near-Surface Focal-Plane Metamaterial Luneburg Lens Antenna Using Transformation-Optics

Abstract: A hemispherical Luneburg lens (LL) with the focal plane near lens surface is proposed using a transformation optics (TO)-based metamaterial (MTM) lens (metalens) concept for wide-angle beamsteering applications. The TO technique has long been implemented to reduce the longitudinal dimension of an LL. However, the transformed lenses, usually having a long focal length, suffer from limited volume reduction, severe reflection, narrow beamscanning range (BSR), and low capture efficiency. The proposed TO mapping in this article, engendered by transforming the upper and lower parts of the original LL separately, preserves the original beamsteering property of an LL in half a space with the focus retained at its flat surface before exerting any index approximations. The effects of the approximations and the discretization on the performance of the transformed LL are then investigated and analyzed comprehensively. After that, a hemispherical lens with a radius of 60 mm is exemplified at X-band and fabricated using a ten-layered nonresonant gradient index MTM. The designed TO-based LL is fed by a stacked patch antenna linearly shifted at a distance of 4 mm (a focal-to-diameter ratio of 0.03) beneath the lens to achieve a wide beamsteering range of ±41° with a maximum gain of 15.7 dBi and a gain tolerance of 1.7 dB. The proposed TO-based metalens antenna features compactness with a near-surface focal plane, wide BSR, and easy fabrication, which shows potential applications in far-field imaging, radar, and 5G wireless communication.

Adjoint Sensitivity Optimization of Three-Dimensional Directivity-Enhancing, Size-Reducing GRIN Lenses

Abstract: Gradient Index (GRIN) lenses have the potential to tailor the radiation behavior of existing antennas for improved gain performance. Conventional methods such as transformation optics have mainly focused on two-dimensional (2-D) problems (e.g., axisymmetric or sectoral horn lenses), and often produce designs with unrealistic material parameters or poor impedance matching. In this letter, we propose a new method of designing freeform GRIN lenses using adjoint sensitivities to rapidly find optimal solutions within feasible material bounds that achieve desired performance goals. A proof-of-concept 3-D GRIN lens that achieves comparable performance to a larger 20 dB standard gain horn albeit with considerable size reduction is optimized, fabricated, and tested in order to highlight the effectiveness of the proposed approach.

Considerations on boundary conditions in transformation optics for complete field computation in generic waveguides

Abstract: Transformation optics is a powerful tool allowing remapping a generic domain into another yet maintaining a correspondence of the solution of Maxwell's equations within the two domains. Transformation optics has been used in the past to conveniently solve for modes in a generic waveguide by transforming its cross‐section into a circular one. Such a transformation needs particular care in enforcing boundary conditions in the transformed, circular, domain. In this paper, a theoretical treatment of boundary conditions is presented, allowing for an accurate computation of all field components in a generic waveguide.

Strictly conformal transformation optics for directivity enhancement and unidirectional cloaking of a cylindrical wire antenna

Abstract: Abstract Using conformal transformation optics, a cylindrical shell made of an isotropic refractive index material is designed to improve the directivity of a wire antenna while making it unidirectionally invisible. If the incident wave comes from a specific direction, it is guided around the wire. Furthermore, when an electrical current is used to excite the wire, the dielectric shell transforms the radiated wave into two lateral beams, improving directivity. The refractive index of the dielectric shell is calculated using the transformation optics recipe after establishing a closed-form conformal mapping between an annulus and a circle with a slit. The refractive index is then modified and discretized using a hexagonal lattice. Ray-tracing and full-wave simulations with COMSOL Multiphysics are used to validate the functionality of the proposed shell.

Transformation optics approach to mesoscopic plasmonics

Abstract: The development of nanofabrication brings the nanostructure to a scale of a few nanometers or even subnanometer, where the quantum effects of electrons become essential in the optical response. This work establishes an analytical method that combines transformation optics and Feibelman $d$ parameters to investigate the nonclassical effects of plasmonic systems. The introduction of the $d$ parameter incorporates most of the quantum effects, while transformation optics vastly simplifies the problem by converting the complex nanostructure in physical space into a simple slab geometry in virtual space.

A bi-dimensional compressed Luneburg lens antenna for miniaturization based on transformation optics

Abstract: Transformed Luneburg lens has been widely employed to provide aberration-free imaging and high-gain antenna system, but whose focal plane and beam scanning range decrease correspondingly. In this paper, a two-dimensional compressed elliptical cylindrical Luneburg lens is presented based on transformation optics (TO) to achieve miniaturization and wide-angle beam steering. The Jacobian matrix and the permittivity tensor are calculated after supposing formulas to compress the focal plane, while maintaining the lens’ inherent performance. The gradient permittivity is achieved by two ring-type periodic unit cells on the basis of the Equivalent Medium Theory. The lens is then attached between a pair of parallel metal plates to further improve its gain and lower the side lobe level (SLL). To demonstrate this assumption, a prototype of this Luneburg lens is manufactured by isotropic material and 3D printing technique. The antenna operates at 3.3–5 GHz with a peak gain of 16.1/15.9 dBi. A 2D beam scanning range of ±50° and ± 20° can be implemented by merely five feeds, the side lobe level keeping less than -16.3/-16 dB. Measured results coincide well with theoretical predictions, offering a beneficial transformation mapping to both microwaves and optics.

Level-Set-Based Shape & Topology Optimization of Thermal Cloaks

Abstract: Thermal metamaterials are gaining increasing popularity, especially for heat flux manipulation purposes. However, due to the high anisotropy of the structures resulting from the transformation thermotics or scattering cancellation methods, researchers are resorting to topology optimization as an alternative to find the optimal distribution of constituent bulk materials to realize a specific thermal function. This paper proposes to design a thermal cloak using the level-set-based shape and topology optimization. The thermal cloak design is considered in the context of pure heat conduction. The cloaking effect is achieved by reproducing the reference temperature field through the optimal distribution of two thermally conductive materials. The structural boundary is evolved by solving the Hamilton-Jacobi equation. The feasibility and validity of the proposed method to design thermal meta-devices with cloaking functionality are demonstrated through two numerical examples. The optimized structures have clear boundaries between constituent materials and do not exhibit thermal anisotropy, making it easier for physical realization. The first example deals with a circular cloaking region as a benchmark design. The robustness of the proposed method against various cloaking regions is illustrated by the second example concerning a human-shaped cloaking area. This work can inspire a broader exploration of the thermal meta-device in the heat flux manipulation regime.

Metamaterial-enabled asymmetric negative refraction of GHz mechanical waves

Abstract: Wave refraction at an interface between different materials is a basic yet fundamental phenomenon, transversal to several scientific realms - electromagnetism, gas and fluid acoustics, solid mechanics, and possibly also matter waves. Under specific circumstances, mostly enabled by structuration below the wavelength scale, i.e., through the metamaterial approach, waves undergo negative refraction, eventually enabling superlensing and transformation optics. However, presently known negative refraction systems are symmetric, in that they cannot distinguish between positive and negative angles of incidence. Exploiting a metamaterial with an asymmetric unit cell, we demonstrate that the aforementioned symmetry can be broken, ultimately relying on the specific shape of the Bloch mode isofrequency curves. Our study specialized upon a mechanical metamaterial operating at GHz frequency, which is by itself a building block for advanced technologies such as chip-scale hybrid optomechanical and electromechanical devices. However, the phenomenon is based on general wave theory concepts, and it applies to any frequency and time scale for any kind of linear waves, provided that a suitable shaping of the isofrequency contours is implemented.