Ana Díaz-Rubio

Perfect control of reflection and refraction using spatially dispersive metasurfaces

Nonuniform metasurfaces (electrically thin composite layers) can be used for shaping refracted and reflected electromagnetic waves. However, known design approaches based on the generalized refraction and reflection laws do not allow realization of perfectly performing devices: there are always some parasitic reflections into undesired directions. In this paper we introduce and discuss a general approach to the synthesis of metasurfaces for full control of transmitted and reflected plane waves and show that perfect performance can be realized. The method is based on the use of an equivalent impedance matrix model which connects the tangential field components at the two sides on the metasurface. With this approach we are able to understand what physical properties of the metasurface are needed in order to perfectly realize the desired response. Furthermore, we determine the required polarizabilities of the metasurface unit cells and discuss suitable cell structures. It appears that only spatially dispersive metasurfaces allow realization of perfect refraction and reflection of incident plane waves into arbitrary directions. In particular, ideal refraction is possible only if the metasurface is bianisotropic (weak spatial dispersion), and ideal reflection without polarization transformation requires spatial dispersion with a specific, strongly nonlocal response to the fields.

From the generalized reflection law to the realization of perfect anomalous reflectors

Nonlocal metasurface for perfect anomalous reflection demonstrates a new possibility for controlling electromagnetic energy flow. The use of the generalized Snell’s law opens wide possibilities for the manipulation of transmitted and reflected wavefronts. However, known structures designed to shape reflection wavefronts suffer from significant parasitic reflections in undesired directions. We explore the limitations of the existing solutions for the design of passive planar reflectors and demonstrate that strongly nonlocal response is required for perfect performance. A new paradigm for the design of perfect reflectors based on energy surface channeling is introduced. We realize and experimentally verify a perfect design of an anomalously reflective surface using an array of rectangular metal patches backed by a metallic plate. This conceptually new mechanism for wavefront manipulation allows the design of thin perfect reflectors, offering a versatile design method applicable to other scenarios, such as focusing reflectors, surface wave manipulations, or metasurface holograms, extendable to other frequencies.

Multidisciplinary approach to cylindrical anisotropic metamaterials

Anisotropic characteristics of cylindrically corrugated microstruc- tures are analyzed in terms of their acoustic and electromagnetic (EM) behavior paying special attention to their differences and similarities. A simple analyti- cal model has been developed using effective medium theory to understand the anisotropic features of both types of waves in terms of radial and angular compo- nents of the wave propagation velocity. The anisotropic constituent parameters have been obtained by measuring the resonances of cylindrical cavities, as well as from numerical simulations. This permits one to characterize propagation of acoustic and EM waves and to compare the fundamental anisotropic features generated by the corrugated effective medium. Anisotropic coefficients match closely in both physics fields but other relevant parameters show significant dif- ferences in the behavior of both types of waves.

Flat Engineered Multichannel Reflectors

Recent advances in engineered gradient metasurfaces have enabled unprecedented opportunities for light manipulation using optically thin sheets, such as anomalous refraction, reflection, or focusing of an incident beam. Here we introduce a concept of multi-channel functional metasurfaces, which are able to control incoming and outgoing waves in a number of propagation directions simultaneously. In particular, we reveal a possibility to engineer multi-channel reflectors. Under the assumption of reciprocity and energy conservation, we find that there exist three basic functionalities of such reflectors: Specular, anomalous, and retro reflections. Multi-channel response of a general flat reflector can be described by a combination of these functionalities. To demonstrate the potential of the introduced concept, we design and experimentally test three different multi-channel reflectors: Three- and five-channel retro-reflectors and a three-channel power splitter. Furthermore, by extending the concept to reflectors supporting higher-order Floquet harmonics, we forecast the emergence of other multiple-channel flat devices, such as isolating mirrors, complex splitters, and multi-functional gratings.

Acoustic metasurfaces for scattering-free anomalous reflection and refraction

Manipulation of acoustic wavefronts by thin and planar devices, known as metasurfaces, has been extensively studied, in view of many important applications. Reflective and refractive metasurfaces are designed using the generalized reflection and Snells laws, which tell that local phase shifts at the metasurface supply extra momentum to the wave, presumably allowing arbitrary control of reflected or transmitted waves. However, as it has been recently shown for the electromagnetic counterpart, conventional metasurfaces based on the generalized laws of reflection and refraction have important drawbacks in terms of power efficiency. This work presents a new synthesis method of acoustic metasurfaces for anomalous reflection and transmission that overcomes the fundamental limitations of conventional designs, allowing full control of acoustic energy flow. The results show that different mechanisms are necessary in the reflection and transmission scenarios for ensuring perfect performance. Metasurfaces for anomalous reflection require non-local response, which allows energy channeling along the metasurface. On other hand, for perfect manipulation of anomalously transmitted waves, local and non-symmetric response is required. These conclusions are interpreted through appropriate surface impedance models which are used to find possible physical implementations of perfect metasurfaces in each scenario. We hope that this advance in the design of acoustic metasurfaces opens new avenues not only for perfect anomalous reflection and transmission but also for realizing more complex functionalities, such as focusing, self-bending or vortex generation.

Systematic design and experimental demonstration of bianisotropic metasurfaces for scattering-free manipulation of acoustic wavefronts

Recent advances in gradient metasurfaces have shown that by locally controlling the bianisotropic response of the cells one can ensure full control of refraction, that is, arbitrarily redirect the waves without scattering into unwanted directions. In this work, we propose and experimentally verify the use of an acoustic cell architecture that provides enough degrees of freedom to fully control the bianisotropic response and minimizes the losses. The versatility of the approach is shown through the design of three refractive metasurfaces capable of redirecting a normally incident plane wave to 60°, 70°, and 80° on transmission. The efficiency of the bianisotropic designs is over 90%, much higher than the corresponding generalized Snell’s law based designs (81%, 58%, and 35%). The proposed strategy opens a new way of designing practical and highly efficient bianisotropic metasurfaces for different functionalities, enabling nearly ideal control over the energy flow through thin metasurfaces.Acoustic bianisotropy does not exist in natural materials but can be designed with acoustic metamaterials. Here, Li et al. utilized acoustic bianisotropy and develop a practical metamaterial with improved transmission efficiency which outperforms the Generalized Snell’s Law.

Suitability of roll-to-roll reverse offset printing for mass production of millimeter-wave antennas: Progress report

In this work, we investigate different printing technologies suitable for mass production of millimeter-wave antennas and other devices, e.g., holograms and frequency selective absorbers, on flexible substrates. We concentrate especially on roll-to-roll reverse offset printing. The driving factors are low cost, high accuracy, high efficiency, and reliable performance. Therefore, we need to find and characterize suitable flexible substrates (permittivity and loss tangent at mm-wavelengths), conducting inks (viscosity, surface resistance of the resulting conducting layer), adhesion of the ink to the substrate, and feature size capable for printing mm-wave antennas and other passive devices in high volumes.

An Accurate Method for Measuring the Sheet Impedance of Thin Conductive Films at Microwave and Millimeter-Wave Frequencies

A simple and accurate method to measure the complex sheet impedance of thin conductive films on dielectric substrates is reported. This method allows for accurate extraction of the sheet impedance without characterizing the substrate (thickness and permittivity) beforehand, within a wide range of frequencies and sheet complex impedances. In this method, the sample is placed between two rectangular waveguide flanges, creating a discontinuity. The discontinuity is modeled by an equivalent

On the origin of pure optical rotation in twisted-cross metamaterials

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Experimental verification of total absorption by a low-loss thin dielectric layer

-circuit, and the sheet impedance is found from measured circuit parameters of the sample and bare substrate. We propose two retrieval approaches using different circuit parameters, examine their extraction accuracy with various substrate thicknesses and sheet impedances, and determine the most reliable extraction method. Uncertainty analysis under random perturbations of measured scattering parameters is also performed to investigate the robustness of the technique. Experimental studies are carried out to demonstrate the validity of the proposed approach for the impedance measurements of thin conductive ink films supported by both thin or thick (up to 0.28 times the wavelength in the dielectric) substrates.