Transformation Optics Method Research Articles

Metamaterial-Based LTCC Compressed Luneburg Lens Antenna at 60 GHz for Wireless Communications

In this study, a metamaterial-based LTCC compressed Luneburg lens was designed, manufactured and measured. The lens was designed at 60 GHz to utilize the unlicensed mm-wave spectrum available for short-range high-capacity wireless communication networks. The transformation optics method was applied to ensure the compression of the Luneburg lens antenna and thus maintain a low-profile structure. The two different types of unit cells for low and high permittivity regions were considered. The parametric study of the effect of compression on lens performance was presented. The antenna is implemented with a standard high-permittivity LTCC process, and details of the manufacturing process for the metamaterial lens are discussed. The low-profile lens is thinner than 2 mm and measures 19 mm in diameter. A size reduction of 63.6% in comparison with a spherical lens was achieved. The near-field to far-field mm-wave measurement technique is presented, and the measurement results show a peak antenna gain of 16 dBi at 60 GHz and a beam-scanning capacity with 1 dB scan loss within a 50° field of view.

Effects of Metal Boundary Stretching and Sample Translational Motion on Microwave Heating

For the purpose of improving uniformity and efficiency of microwave heating, moving components are widely used. In this paper, a kind of stretching microwave oven with a conveyor belt is designed. The conveyor belt and the stretching motion of the upper surface of the microwave oven make the electric field in the microwave cavity continuously change during heating, so that the absorption pattern of materials does not remain constant. The transformation optics method is used to simulate the stretching motion of the upper wall of the microwave oven, and the implicit function method is used to simulate the translational motion of the sample on the conveyor belt. The correctness of the simulation model is validated by experiments. The heating effects for the cases of fixed heating, stretching and translational motion are compared. Finally, the heating effects for the proposed model with different heated materials are also discussed.

Experimental Verification of Shape‐Independent Surface Cloak Enabled by Nihility Transformation Optics

AbstractThe manipulation of electromagnetic surface waves on interfaces that have arbitrary shape geometrical disorders has gained a lot of interest owing to its wealth of advantages in modern photonics and plasmonics devices. In this paper, based on the well‐established method of transformation optics, a reconfigurable cloak that is competent in guiding the propagation of surface plasmon polaritons (SPPs) at a distorted metal‐dielectric interface is introduced. The presented cloak consists of a homogeneous anisotropic material, dubbed as optic null medium (ONM) that is independent of the cloak geometrical shape. Whilst the homogeneity property of ONMs leads to the feasibility in fabrication, the shape independency feature yields an on‐demand control over the flow of SPPs, simply by changing the shape of the cloak while leaving its constituent material unchanged. In order to authenticate the idea, a triangular‐shape SPP cloak is designed and fabricated experimentally at microwave frequency. It is observed that by utilizing the designed cloak almost total transmission can be obtained from the distorted interface. The unique designing approach introduced here may open a new horizon to nano‐optics and downscaling of photonic circuits.

Nondispersive acoustic manipulation based on gradient curved waveguides

We propose that the propagation of acoustic waves in two-dimensional space can be controlled by the design of gradient embedded paths, which is inspired by a combination of metasurfaces and the inverse method of transformation optics. The phase variation along each gradient embedded path is smooth so that there is almost no reflection below a critical frequency. More importantly, the functionalities of devices designed by our approach are frequency-independent over a pretty broad band. In our work, gradient embedded paths are realized with gradient curved waveguides. Various potential applications such as acoustic directional emitter and focus lens without chromatic aberration are demonstrated.

Axis-symmetric ellipsoidal lens antenna design with independent E and H radiation pattern beamwidth

In this paper, a 3D ellipsoidal lens is designed based on the transformation optics (TO) method. The conventional Luneburg and Gutman lenses are mapped to an ellipsoidal lens to make a fan-beam radiation pattern. The Beamwidth of vertical and horizontal planes can be designed separately. The lens structure and its refractive index are axis-symmetric. So the fan-beam scanning can be resulted by rotating a source around the symmetric axis of the Lens in the horizontal plane. This structure can be used in search radar to estimate the azimuth angle of the targets. Full-wave simulations are performed using the software of CST.

High gain, wide-angle QCTO-enabled modified Luneburg lens antenna with broadband anti-reflective layer

The gradient-index (GRIN) Luneburg lens antenna offers significant benefits, e.g. high aperture efficiency, low-power, minimal cost, wide beam scanning angle and broad bandwidth, over phased array antennas and reflector antennas. However, the spherical shape of the Luneburg lens geometry complicates the integration of standard planar feed sources and poses significant implementation challenge. To eliminate the feed mismatch problem, the quasi-conformal transformation optics (QCTO) method can be adopted to modify the lens’ spherical feed surface into a planar one. However, Luneburg lenses designed with QCTO method are limited to poor performance due to the presence of the reflections and beam broadening arising from the quasi-conformal mapping. In this paper, we present a new method of implementing QCTO-enabled modified Luneburg lens antenna by designing a broadband anti-reflective layer along with the modified lens’s planar excitation surface. The proposed anti-reflector layer is inherently broadband in nature, has a continuously tapered inhomogeneous dielectric permittivity profile along its thickness, and ensures broadband impedance matching. To show the new QCTO modified Luneburg lens antenna, an example lens antenna was designed at Ka-band (26–40 GHz) and fabricated using fused deposition modeling (FDM) based additive manufacturing technique. Electromagnetic performance of the lens antenna was experimentally demonstrated.

Accordion microwave oven for uniformity and efficiency heating

Conventional microwave oven heating has the problem of poor heating uniformity and low heating efficiency. A novel microwave oven heating model has been proposed for improving heating uniformity by making the electric field in the microwave oven drastically change by changing the geometry of the microwave oven during the heating process. The transformation optics method is used to solve the computational difficulties caused by the continuous change of the mesh during the motion. Finally, the experiment verified the correctness of the simulation. In addition, important factors affecting heating efficiency were discussed.

Conformal optical devices based on geodesic lenses.

Conformal transformation optics provides a simple scheme for manipulating light rays with inhomogeneous isotropic dielectrics. However, there is usually discontinuity for refractive index profile at branch cuts of different virtual Riemann sheets, hence compromising the functionalities. To deal with that, we present a special method for conformal transformation optics based on geodesic lenses with special closed surfaces. The requirement is a continuous refractive index profile of dielectrics, which shows the almost perfect performance of designed devices. We demonstrate such a proposal by achieving conformal transparency (invisibility without cloaking region) and reflection. We can further achieve conformal invisible cloaks by two methods with perfect conductors. The conformal transformation optics method based on geodesic lenses may also find applications in other waves that obey the Helmholtz wave equation in two dimensions.

Smoothed Particle Method for Full-Wave Simulation of Microwave Applicators With Translational-Rotary Elements

Elements in both translation and rotation such as the screw propeller are widely applied to improve microwave heating uniformity. However, it is challenging to numerically model this heating process due to the difficulties in dynamic meshing for the moving elements. Our previous transformation optics method will be also burdensome to use in this case because it is very difficult to get the permittivity and permeability tensors. In this letter, a mesh-free numerical calculation based on a smoothed particle method is proposed to solve this complicated case. The proposed method uses smoothed particles to replace the meshes. Thus, the motion of the elements can be replaced by the motion of the particles and the dynamic meshing process can be avoided. A 2-D model is computed and the results are compared with those calculated by the finite element method. Good agreements are achieved.

Transformation devices with optical nihility media and reduced realizations

Starting from optical nihility media (ONM), we design several intriguing devices with transformation optics method in two dimensions, such as a wave splitter, a concave lens, a field rotator, a concentrator and an invisibility cloak. The extreme anisotropic property of ONM hinders the fabrication of these devices, which could be effectively realized by simplified materials with Fabry-P\'erot resonances (FPs) at discrete frequencies. Moreover, we propose a reduced version of simplified materials with FPs to construct a concentrator and a rotator, which is feasible in experimental fabrications. The simulations of total scattering cross sections confirm their functionalities.

System‐by‐design method for efficient linear array miniaturisation through low‐complexity isotropic lenses

The design of miniaturised linear arrays by using isotropic lenses with reduced complexity is addressed. The problem at hand is formulated as a global optimisation one, then solved by means of a computationally-efficient customisation of the system-by-design (SbD) paradigm suitably integrated with the quasi-conformal transformation optics method. To yield arbitrary transformation geometries with a limited set of degrees-of-freedom and to reduce the lens complexity, while guaranteeing the same radiation performance of a larger reference aperture, a spline-based description of the lens shape is adopted. Representative numerical results are reported to assess the effectiveness as well as the computational efficiency of the proposed SbD-based approach.

Illusion Optics: Disguising with Ordinary Dielectric Materials.

Illusion devices are usually designed using transformation optics. Here, a new method is proposed to achieve optical illusions without external devices by elaborately manipulating the scattering potential of an object. In contrast to the conventional transformation optics method, which completely replaces one object by the image of another object using complementary\restoring media and a superlens, the method described here is more of a cosmetic operation for an object, which modifies the scattering pattern of the object to mimic another object by exchanging their scattering potentials in two symmetrical areas in the wave vector domain. Only positive isotropic nonmagnetic materials are introduced in the present method, which is impossible using the conventional method because superlenses require negative-index materials. Both numerical simulations and experimental demonstrations are used to verify the performance of the illusion devices of this method.

A Flat Dual-Polarized Transformation-Optics Beamscanning Luneburg Lens Antenna Using PCB-Stacked Gradient Index Metamaterials

Based on a transformation optics method, a flat compact dual-polarized Luneburg lens antenna is proposed and implemented using the printed-circuit-board-stacked gradient-index metamaterials for beamscanning and multibeam applications at X-bands. The transformed material properties of the planar Luneburg lens are designed with 17-layered permittivity distribution of polynomials. Each layer is discretized into $41 \times 41$ pixels made of broadband and less polarization-dependent unit cells responsible for desired index distributions. The effects of transformation, approximation, and discretization on the lens performance are analyzed comprehensively. Also, to validate the implementation method, a flat Luneburg lens with a thickness of 14.1 mm, a focal length of 28 mm, and an aperture size of $98.9 \times 98.9$ mm2 is designed and tested. A stacked aperture-coupled patch antenna operating at 10 GHz is applied as a feeder. The measured results show that the proposed antenna can operate over a bandwidth of ~20% with an antenna efficiency of 32%, a cross-polarization level of <−17.1 dB, as well as the maximum gain of 15.9/16.35 dBi and a scanning angle of ±32°/±35° for two orthogonal polarizations, respectively. The presented flat Luneburg lens antenna featuring broad bandwidth, high gain, wide scanning angle, and easy fabrication has a high potential in 5G wireless communication, imaging, and remote sensing applications.

Design of a homogeneous-material cloak and illusion devices for active and passive scatterers with multi-folded transformation optics

By adopting the multi-folded linear transformation optics method and its corresponding numerical techniques, we introduce a method to design an open cloak that has windows to exchange matter with the outer world. Compared with conventional remote cloaks based on inhomogeneous materials, in this paper, the inhomogeneity of materials is removed and cloaking functionality can be achieved by anisotropic properties. Based on this, open-illusion devices for both active and passive scatterers have been investigated to make any emitter or object look like another pre-defined emitter or object, respectively. The proposed devices have perfect performance with an object-independent feature for moving objects in particular. The proposed devices will open up possibilities and be useful for practical implementations of the cloak and illusion optics technologies that were recently developed.

Electromagnetic Amplification of Microwave Phonons in Nonlinear Resonant Microcavities

In this paper, we report on the numerical simulation of a multiphysic electromagnetic/mechanical problem, as a further step toward the development of microwave circuits based on phonon propagation at micro and nanoscale. In the future, this kind of circuits is likely to integrate optomechanically pumped phonon sources and detectors, as well as phonon processing components (waveguides, splitters, and memories) to process information by means of phonons. In particular, we propose a rigorous approach for the solution of the electromagnetic/mechanical system of equations governing light behavior in optomechanical cavities, with the help of the transformation optics method. Such a kind of calculation allows the development of an efficient generation of microwave coherent phonon sources, by engineering their propagation or coupling with phonon waveguides.

A novel EM concentrator with open-concentrator region based on multi-folded transformation optics

Conventional concentrators with inhomogeneous coating materials that fully enclose the destined region pose great challenges for fabrication. In this paper, we propose to design an EM concentrator with homogeneous materials. Distinguished from conventional ones, the elaborately designed EM concentrator features a concentrator region that is open to the outer-world, which is achieved with multi-folded transformation optics method by compressing and folding the coating materials to create window(s). Based on this concept, we also investigate open-rotator and open rotational-concentrator devices, which could simultaneously rotate and store the EM waves in the central destined region. Due to the open nature of our proposed designs, we believe they will find potential applications in remote controlling with impressive new functionalities.

Approximate isotropic cloak for the Maxwell equations

We construct a regular isotropic approximate cloak for the Maxwell system of equations. The method of transformation optics has enabled the design of electromagnetic parameters that cloak a region from external observation. However, these constructions are singular and anisotropic, making practical implementation difficult. Thus, regular approximations to these cloaks have been constructed that cloak a given region to any desired degree of accuracy. In this paper, we show how to construct isotropic approximations to these regularized cloaks using homogenization techniques so that one obtains cloaking of arbitrary accuracy with regular and isotropic parameters.

Three-dimensional transformation optics for arbitrary coordinate systems: transforming conductive materials and boundaries.

A three-dimensional transformation optics method, leading to homogeneous materials, applicable to any non-Cartesian coordinate systems or waveguides/objects of arbitrary cross-sections is presented. Both the conductive boundary and internal material of the desired device is determined by the proposed formulation. The method is applicable to a wide range of waveguide, radiation, and cloaking problems, and is demonstrated for circular waveguide couplers and an external cloak. An advantage of the present method is that the material properties are simplified by appropriately selecting the conductive boundaries. For instance, a right-angle circular waveguide bend is presented which uses only one homogenous material. Also, transformation of conductive materials and boundaries are studied. The conditions in which the transformed boundaries remain conductive are discussed. In addition, it is demonstrated that negative infinite conductivity can be replaced with positive conductivity, without affecting the field outside the conductive boundary. It is also observed that a negative finite conductivity can be replaced with a positive one, by accepting some small errors. The general mathematical procedure and formulation for calculating the parametric surface equations of the conductive peripheries are presented.

Plasmon-Exciton Coupling in Symmetry-Broken Nanocavities

We investigate the onset of strong coupling in the temporal dynamics of the exciton population at a single emitter interacting with symmetry-broken plasmonic nanocavities. These structures consist in pairs of metallodielectric elements separated by a nanometric gap, with different degrees of asymmetry imposed on their geometric or material characteristics. In order to describe the emergence of plasmon-exciton-polaritons in these systems, we extend and generalize a transformation optics method previously applied to dimers of identical particles. This approach provides a natural decomposition of the spectral density in terms of a well-defined set of plasmonic resonances, as well as an insightful description of the coupling strength dependence on the emitter position. On the one hand, we shed light into the low sensitivity of plasmon-exciton interactions to geometric asymmetry in cavities such as nanoparticle-on-a-mirror configurations. On the other hand, our findings reveal that a more complex spatial and s...

Non-contact method to freely control the radiation patterns of antenna with multi-folded transformation optics

In this paper, we propose to use multi-folded transformation optics method to design a non-contact illusion device that can distantly and freely manipulate the radiation behavior of antenna located at a certain distance and such manipulation is enabled by the use of mapped electromagnetic medium coated with the transformed medium. The proposed design aims to achieve the radiation pattern of our choice from the antenna that does not possess any electromagnetic medium. Based on this, the functionality of parabolic antenna is distantly achieved from the point source. We further extended our idea to array of antennas in which the proposed device distantly makes the linear array of antennas behave like a geometrically different array of antennas. Our work extends the concept of illusion optics for active scatterer that will be very helpful for future antenna design.