257 research outputs found

    Photonic Metasurfaces for Spatiotemporal and Ultrafast Light Control

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    The emergence of photonic metasurfaces - planar arrays of nano-antennas - has enabled a new paradigm of light control through wave-front engineering. Space-gradient metasurfaces induce spatially varying phase and/or polarization to propagating light. As a consequence, photons propagating through space-gradient metasurfaces can be engineered to undergo a change to their momentum, angular momentum and/or spin states

    Waveplates based on metasurfaces in the THz range

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    Los platos de onda basados en metasuperficies son componentes clave en electromagnetismo, ya que permiten un control total de la polarización de las ondas electromagnéticas con la ventaja de presentar estructuras más compactas que los platos de onda convencionales. Además, la aplicación del principio de Pancharatnam Berry (PB) a los platos de media onda (HWP) basados en metasuperficies, permite la manipulación de frentes de onda junto con la conversión de la polarización de las ondas incidentes circularmente polarizadas, simplemente girando los meta-átomos que componen la metasuperficie. Para lograr altos niveles de eficiencia de transmisión con platos de onda basados en metasuperficies, generalmente se requieren diseños multicapa. Esto implica estructuras voluminosas y complica el proceso de fabricación, restando importancia a la ventaja de utilizar metasuperficies. El propósito de esta tesis realizada en la Universidad Pública de Navarra y en L’École Polytechnique Fédérale de Lausanne, Suiza (EPFL) es ofrecer una ventaja tecnológica tanto para el control de polarización como para la manipulación del frente de onda y contribuir al desarrollo de dispositivos basados en metamasuperficies, incluyendo su fabricación y verificación experimental. Los resultados incluyen lo siguiente: Un plato de media onda operando en transmisión, ultradelgado y basado en una metasuperficie en zigzag de dos capas que opera en la parte baja del espectro del THz con un 90% de eficiencia de transmisión, que se demuestra numérica y experimentalmente. Se lleva a cabo un análisis detallado de la robustez del dispositivo con respecto a los desalineamientos de las capas mediante el diseño y la fabricación de dos dispositivos adicionales con el máximo desalineamiento entre capas en ambas direcciones transversales. Una metalente ultradelgada y compacta basada en el principio Pancharatnam Berry con solo dos capas alcanzando un 90% de eficiencia de transmisión, enfocando el frente de onda de una onda incidente polarizada circularmente y convirtiendo su polarización. La estructura es estudiada semi-analítica y numéricamente y medida experimentalmente, comprobándose un excelente comportamiento como HWP PB metalente a 87 GHz. Una aplicación de ingeniería de frentes de onda para la manipulación de los mismos se demuestra numéricamente en el rango de ondas milimétricas mediante la integración de la metalente en un sistema de antena-metalente, que se estudia semi-analíticamente y se corrobora experimentalmente. El sistema convierte la polarización de las ondas polarizadas circularmente, logrando un incremento de la directividad de antena de 17 dB a ⁓35 dB a 87 GHz con un AR inferior a 0.5 dB. Finalmente, se presentan dos configuraciones extra del sistema para trabajar entre los extremos del rango de frecuencia comprendido entre 75 GHz y 105 GHz, con directividades ⁓32 dB y AR < 3 dB.Transmissive waveplates based on metasurfaces are key components in electromagnetism, as they allow for a full control of the electromagnetic wave polarization with the advantage of presenting structures more compact than conventional waveplates. Moreover, applying the Pancharatnam Berry (PB) principle to half-wave plate (HWP) metasurfaces allows the manipulation of wavefronts along with the conversion of the handedness of circularly polarized incident waves by simply rotating the meta-atoms that compose the metasurface. For achieving high levels of transmission efficiency with transmissive waveplates based on metasurfaces, multiple layer designs are usually required. It implies bulky structures and complicates the fabrication process, downplaying the aim of the use of metasurfaces. The purpose of this thesis performed at the Public University of Navarre and at École Polytechnique Fédérale de Lausanne (EPFL) is to offer a technological advantage both for polarization controlling as wavefront manipulation and contribute to the development of metasurface-based devices, including their fabrication and experimental verification. The results include the following: An ultrathin transmissive half-wave plate based on a bi-layered zigzag metasurface operating at the lower-frequency edge of the THz spectrum with a 90% of transmission efficiency, which is numerically and experimentally demonstrated. A detailed analysis of the device robustness with respect to layer misalignments is carried out by designing and fabricating two additional devices with the maximum possible shift between layers along both transverse directions. A compact ultrathin metalens based on the Pancharatnam Berry principle with only two layers with a 90% of transmission efficiency, focusing the wavefront of a circularly polarized incident wave and converting its handedness. The structure is semi-analytically and numerically studied and experimentally measured, verifying an excellent behavior as HWP PB metalens at 87 GHz. A wavefront engineering application for wavefront manipulation is numerically demonstrated in the millimeter-wave range by the integration of the metalens in an antenna-metalens system, which is semi-analytically studied and experimentally corroborated. The system converts the handedness of circular polarized waves, achieving an increment of the antenna directivity from17 dB to ⁓35 dB at 87 GHz with an AR lesser than 0.5 dB. Finally, two extra system configurations are presented to work around the frequency range extremes comprised between 75 GHz and 105 GHz, with directivities ⁓32 dB and AR < 3 dB.Programa de Doctorado en Tecnologías de las Comunicaciones, Bioingeniería y de las Energías Renovables (RD 99/2011)Bioingeniaritzako eta Komunikazioen eta Energia Berriztagarrien Teknologietako Doktoretza Programa (ED 99/2011

    Antennas using left handed transmission lines

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    The research described in this thesis is concerned with the analysis and design of conventional wire antenna types, dipoles and loops, based on the left-handed transmission line approach. The left handed antennas have a unique feature that the wavelength of the induced current becomes shorter with decreasing frequency. The left handed transmission line concept can be extended to construct reduced-size dipole or loop antennas in the VHF frequency band. The use of higher order modes allows orthogonal polarisation to be obtained, which is thought to be a feature unique to these antennas. Efficiency is a key parameter of left handed antennas as the heavy left handed loading increases the resistive loss. A study of the efficiency of small dipole antennas loaded with a left-handed transmission line is specially described, and the comparison with conventional inductive loading dipoles. In a low order mode, the efficiency of L-loading dipole is better with low number of unit cell. If the number of cell increases, CL-loading presents comparable and even better performance. In a high mode the meandered left handed dipole gives the best efficiency due to the phase distribution, presenting orthogonal polarization as well. The optimized dipole loaded with parallel plate capacitors and spiral inductors presents the best performance in impedance and efficiency, even better than the conventional inductive loading. A planar loop antenna using a ladder network of left handed loading is also presented. Various modes can be obtained in the left handed loop antenna. The zero order mode gives rise to omnidirectional patterns in the plane of the loop, with good efficiency. By loading the loop with active components, varactors, a tunable left handed loop antenna with a switchable radiation pattern is implemented. The loop gives an omnidirectional pattern with a null to z axis while working in an n = 0 mode and can switch to a pattern with a null at phi = 45° in the plane of the loop in an n = 2 mode

    Objects of maximum electromagnetic chirality

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    We introduce a definition of the electromagnetic chirality of an object and show that it has an upper bound. Reciprocal objects attain the upper bound if and only if they are transparent for all the fields of one polarization handedness (helicity). Additionally, electromagnetic duality symmetry, i.e., helicity preservation upon interaction, turns out to be a necessary condition for reciprocal objects to attain the upper bound. We use these results to provide requirements for the design of such extremal objects. The requirements can be formulated as constraints on the polarizability tensors for dipolar objects or on the material constitutive relations for continuous media. We also outline two applications for objects of maximum electromagnetic chirality: a twofold resonantly enhanced and background-free circular dichroism measurement setup, and angle-independent helicity filtering glasses. Finally, we use the theoretically obtained requirements to guide the design of a specific structure, which we then analyze numerically and discuss its performance with respect to maximal electromagnetic chirality.Comment: This version contains an example of how to use the theoretically derived constraints for designing realistic structures. It also contains a discussion related to the optical chirality densit

    Microstrip-fed Wideband Circularly Polarized Printed Antenna

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    A wideband circularly-polarized printed antenna is proposed, which employs an asymmetrical dipole and a slit in the ground plane which are fed by an L-shaped microstrip feedline using a via. The proposed antenna geometry is arranged so that the orthogonal surface currents, which are generated in the dipole, feedline and ground plane, have the appropriate phase to provide circular polarization. A parametric study of the key parameters is made and the mechanism for circular polarization is described. The measured results show that the impedance bandwidth is approximately 1.34 GHz (2.45 GHz to 3.79 GHz) and the 3 dB axial ratio bandwidth is approximately 770 MHz (2.88 GHz to 3.65 GHz) which represent fractional bandwidths of approximately 41% and 23%, respectively, with respect to a centre frequency of 3.3 GHz
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