This research line covers five research topics:

Microwave and Photonic Metamaterials

The goal of this research line include new filters designs based on periodic or quasiperiodic substructures applied to wireless communications, and the research about the scalability of this designs to the near infrared spectrum.

Filter Design:

In the last years, the introduction of the Photonic/Electromagnetic Band-Gap (PBG/EBG) concept has renewed vigorously the interest on the field and has propitiated a fruitful transfer of knowledge between the microwave and optical communities. The aim of these PBG/EBG structures in planar microwave technology has been to improve the behaviour of circuits and antennas by introducing stopbands to forbid the propagation of electromagnetic waves in the unwanted frequency bands and directions. These novel devices have found very promising applications including the implementation of filters and resonators, the improvement of the efficiency and radiation pattern of antennas, harmonic tuning in power amplifiers, oscillators and mixers, and the suppression of spurious bands in filters.

However, the frequency response of a PBG/EBG structure features a wide and deep rejected band as intended, but it also includes characteristics that may be detrimental in many of the proposed applications. Fortunately, the coupled-mode theory has been successfully employed to model these structures, providing a valuable physical insight on the operation of the devices and an excellent link with the well established topic of Bragg Gratings in the optical regime. One concept of special relevance within the context of structure optimization is the relationship that can be established in terms of Fourier Transformation between the frequency response and the coupling coefficient resulting from the perturbation of the device. This relationship allowed us to propose several techniques to substantially improve the behaviour of PBG/EBG structures when used in microwave circuits. Specifically, windowing techniques were reported to eliminate the ripple in the passbands, while chirping techniques were used to increase the rejected bandwidth. Moreover, the use of a sinusoidal perturbation was proposed to suppress the spurious harmonic stopbands, and the addition of multiple sine patterns achieved multiple-frequency tuned structures.

Nevertheless, the Fourier Transform relationship used is only accurate for devices with very low reflectivity. Hence, the synthesis of a generic frequency response is only done in a first approximation. To surpass these difficulties, in this research line we want to obtain an exact relationship between the frequency response and the coupling coefficient of a generic device. The solution of this inverse scattering problem is studied by different methods, i.e. by using Gel’fand-Levitan-Marchenko (GLM) method. This allows for the synthesis of devices with arbitrary frequency response only constrained by the principles of causality, passivity and stability. This synthesis method could enormously improve the performances of microwave devices, as matched filter for Ultra-Wide Band applications.

The goal of this research line include new filters designs applied to wireless communications, and the research about the scalability of this designs to the near infrared spectrum.

Metamaterials:

The term “metamaterials” as used here denotes artificial material structures composed of metal, dielectric, and/or metal/dielectric-composite substructures that have feature sizes variously comparable to or smaller than the wavelengths of the electromagnetic radiation that they are intended to handle. Metamaterials that have periodic or quasiperiodic substructures can also be characterized as photonic crystals.

The next-generation metamaterials expected to emerge from this research line could enormously improve the performances of microwave devices (filters, radar subsystems, antennas,…).

Natural materials and most artificial materials exhibit positive refraction: the real parts of their indices of refraction are positive. In contrast, metamaterials can be designed to exhibit negative refraction at certain wavelengths: At those wavelengths, the real parts of their indices of refraction are negative, and for optical waves propagating through them, optical power flows antiparallel to the wave vectors. A metamaterial that exhibits negative refraction is also denoted a left-handed metamaterial because mathematically, the antiparallelarity of the Poynting vector (the electromagnetic-power-density vector) and the wave vector is a consequence of the fact that the electric, magnetic, and Poynting vectors constitute a left-handed triple instead of a right-handed triple as in a vacuum or an ordinary material.

Heretofore, negative refraction has not been widely exploited as a means of directing electromagnetic radiation. The instant research and development effort has been directed toward understanding and exploiting negative refraction for directing microwave radiation (antennas). 

The goal of this research line include new metamaterial designs applied to wireless communications, and the research about the scalability of this designs to the near infrared spectrum.

Contact: David Benito, Miguel Ángel Gómez Laso, Israel Arnedo