Carolina Mateo-Segura

Flat Luneburg Lens via Transformation Optics for Directive Antenna Applications

The great flexibility offered by transformation optics for controlling electromagnetic radiation by virtually re-shaping the electromagnetic space has inspired a myriad of dream-tailored electromagnetic devices. Here we show a 3D-transformed microwave Luneburg lens antenna which demonstrates high directivity, low side-lobe level, broadband response and steerable capabilities. A conventional Luneburg lens is redesigned accounting for dielectric materials that implement a coordinate transformation, modifying the lens geometry to accommodate its size and shape for easy integration with planar microwave antenna applications. An all dielectric lens is manufactured following a thorough holistic analysis of ceramic materials with different volume fractions of bi-modal distributed titanate fillers. Fabrication and measurements of a 3-D flat Luneburg lens antenna validate the design and confirm a high-directivity performance. A directivity of 17.96 dBi, low side-lobe levels for both main planes ~ -26 dB, excellent directivity performance within the X-band and beam-steering up to 34 ° were achieved.

Sub-Wavelength Profile 2-D Leaky-Wave Antennas With Two Periodic Layers

A fast and accurate analysis and synthesis technique for high-gain sub-wavelength 2-D Fabry-Perot leaky-wave antennas (LWA) consisting of two periodic metallodielectric arrays over a ground plane is presented. Full-wave method of moments (MoM) together with reciprocity is employed for the estimation of the near fields upon plane wave illumination and the extraction of the radiation patterns of the LWA. This yields a fast and rigorous tool for the characterisation of this type of antennas. A thorough convergence study for different antenna designs is presented and the operation principles of these antennas as well as the radiation characteristics are discussed. Moreover, design guidelines to tailor the antenna profile, the dimensions of the arrays as well as the antenna directivity and bandwidth are provided. A study on the radiation efficiency for antennas with different profiles is also presented and the trade off between directivity and radiation bandwidth is discussed. Numerical examples are given throughout to demonstrate the technique. A finite size antenna model is simulated using commercial software (CST Microstripes 2009) which validates the technique.

Bandwidth Enhancement of 2-D Leaky-Wave Antennas With Double-Layer Periodic Surfaces

Broadband high-gain 2-D Fabry-Perot (FP) leaky-wave antennas (LWAs) consisting of two periodic metallodielectric arrays over a ground plane are presented. Full-wave method of moments (MoM) is employed for the estimation of the near fields upon plane wave illumination and the extraction of the far field directivity and radiation patterns of the LWA. This yields a fast and rigorous tool for the characterization of this type of antennas. Qualitative design guidelines to tailor the antenna directivity bandwidth are provided for the first time based on a detailed analysis of the excited modes. Numerical examples are given to demonstrate the technique and prove the improvement in the antenna bandwidth. The proposed antenna exhibits a six-fold bandwidth improvement compared with the single array LWA with the same directivity. Simulated and experimental results from a finite size antenna prototype are presented.

Resonant Effects and Near-Field Enhancement in Perturbed Arrays of Metal Dipoles

The equivalent self impedance of a perturbed array of metal dipoles is derived and the resonant effects upon plane wave illumination are studied. It is revealed that as a result of the perturbation, the scattering within a frequency range is dominated by the excitation of the odd mode. This corresponds to significant deviation compared to the unperturbed case. It is demonstrated that within this frequency range, very strong near- fields are excited in the vicinity of the array. Following a careful calculation of the near-fields using the periodic method of moments, the near-fields for a number of perturbed array designs are calculated and an increase in the near-field strength of more than 70 times compared with the incidence is demonstrated. The results are corroborated with HFSS.

Power Stored and Quality Factors in Frequency Selective Surfaces at THz Frequencies

A study of the external, loaded and unloaded quality factors for frequency selective surfaces (FSSs) is presented. The study is focused on THz frequencies between 5 and 30 THz, where ohmic losses arising from the conductors become important. The influence of material properties, such as metal thickness, conductivity dispersion and surface roughness, is investigated. An equivalent circuit that models the FSS in the presence of ohmic losses is introduced and validated by means of full-wave results. Using both full-wave methods as well as a circuit model, the reactive energy stored in the vicinity of the FSS at resonance upon plane-wave incidence is presented. By studying a doubly periodic array of aluminium strips, it is revealed that the reactive power stored at resonance increases rapidly with increasing periodicity. Moreover, it is demonstrated that arrays with larger periodicity-and therefore less metallisation per unit area-exhibit stronger thermal absorption. Despite this absorption, arrays with higher periodicities produce higher unloaded quality factors. Finally, experimental results of a fabricated prototype operating at 14 THz are presented.

Subwavelength resolution for horizontal and vertical polarization by coupled arrays of oblate nanoellipsoids

A structure comprising a coupled pair of two-dimensional arrays of oblate plasmonic nanoellipsoids in a dielectric host medium is proposed as a superlens in the optical domain for both horizontal and vertical polarizations. By means of simulations it is demonstrated that a structure formed by silver nanoellipsoids is capable of restoring subwavelength features of the object for both polarizations at distances larger than half wavelength. The bandwidth of subwavelength resolution is in all cases very large (above 13%).

Analysis of broadband highly-directive Fabry-Perot cavity Leaky-Wave antennas with two periodic layers

Highly directive beams can be produced using Fabry-Pérot cavity Leaky-Wave antennas (LWA). These antennas have been typically implemented using a periodic partially-reflecting surface (PRS) forming a half-wavelength resonant cavity with a ground plane [1–3]. More recently, a planar artificial magnetic conductor (AMC) ground plane has been proposed as a means to reduce the profile of such antennas [4]. Although in all cases the cavity resonance increases the directivity of simple radiating sources, the strong directivity results in a very small antenna bandwidth. An efficient analysis technique of half-wavelength periodic 2-D LWAs has been presented based on the principle of reciprocity and full-wave spectral domain analysis [3]. An optical cavity ray model has also been used to find the radiation performance and resonance condition of this type of antennas [1, 2].

Highly directive Fabry-Perot leaky-wave nanoantennas based on optical partially reflective surfaces

Nanoantennas enhance the conversion between highly localized electromagnetic fields and far-field radiation. Here, we investigate the response of a nano-patch partially reflective surface backed with a silver mirror to an optical source embedded at the centre of the structure. Using full wave simulations, we demonstrate a two orders of magnitude increased directivity compared to the isotropic radiator, 50% power confinement to a 13.8° width beam and a ±16 nm bandwidth. Our antenna does not rely on plasmonic phenomena thus reducing non-radiative losses and conserving source coherence.

A Simple Technique for the Dispersion Analysis of Fabry-Perot Cavity Leaky-Wave Antennas

A simple analysis technique to extract the complex dispersion characteristics of thin periodic 2-D Fabry-Pérot leaky wave antennas (LWA) is presented. The analysis is based on a two-stage process that dispenses with the need for root-finding in the complex plane. Firstly, full-wave MoM together with reciprocity is employed for the estimation of the LWA radiation patterns at different frequencies from which the phase constant is calculated. Employing array theory the phase constant is subsequently used to estimate the radiation patterns for different values of the leakage rate. The correct value for the leakage rate is identified by matching the corresponding radiation pattern to that obtained using the full-wave method. To demonstrate this technique, we present results for half-wavelength and sub-wavelength profile LWAs. Unlike the transverse equivalent network method, the proposed technique maintains its accuracy even for antennas with low profile.

Near-field enhancement for infrared sensor applications

A detailed investigation on planar two dimensional metallodielectric dipole arrays with enhanced near-fields for sensing applications was carried out. Two approaches for enhanc- ing the near-fields and increasing the quality factor were studied. The reactive power stored in the vicinity of the array at resonance increases rapidly with increasing periodicity. Higher quality factors are produced as a result. The excitation of the odd mode in the presence of a perturbation gives rise to a sharp resonance with near-field enhanced by at least an order of magnitude compared to unperturbed arrays. The trade-off between near-field enhancement and thermal losses was also studied, and the effect of supporting dielectric layers on thermal losses and quality factors were examined. Secondary transmissions due to the dielectric alone were found to enhance and reduce cyclically the quality factor as a function of the thickness of the di- electric material. The performance of a perturbed frequency selective surface in sensing nearby materials was investigated. Finally, unperturbed and perturbed arrays working at infrared fre- quencies were demonstrated experimentally. C 2011 Society of Photo-Optical Instrumentation Engineers