Frequency selective surfaces for Terahertz applications
Sanz Fernandez, Juan Jose
Fernandez, Juan Jose Sanz
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This thesis presents both theoretical and experimental investigations of the performance and capabilities of frequency selective surfaces (FSS) applied at THz frequencies. The aim is to explore and extend the use of FSS, traditionally limited to microwave frequencies, towards the THz regime of the spectrum, where interesting applications such as imaging, sensing and communications exist. The contribution of this work lies in three main areas within the scope of THz FSS, namely, performance, prototyping and applications. Unlike microwave FSS where extensive research has been performed to evaluate the performance of different FSS designs, particular problems arise at THz frequencies, significantly, the ohmic losses. While a few notable studies can be found on the issue of ohmic losses, part of this thesis investigates, for the first time, the power dissipation due to the presence of both ohmic and dielectric losses, in relation to the power stored in the vicinity of the FSS, the currents induced in the elements of the array and the array’s terminal impedance. By doing so, a better understanding of the performance of THz FSS has been given in terms of their quality factor, allowing for design guidelines previously unavailable. In order to demonstrate multiband operation experimentally, a novel fabrication process has been designed and developed to manufacture capacitive or dipole-based THz FSS on a dielectric layer. Dry deep-reactive ion etching has been employed in order to avoid the use of wet etching to provide better control of etch characteristics. Various FSS operating around 15THz have been demonstrated experimentally. In addition, THz FSS have been investigated theoretically in the realm of three different applications, namely, multiband operation, sensing capability and reconfigurability. Multiband characteristics using single-screen FSS have been achieved by perturbed dipole FSS exhibiting up to four resonances due to the excitation of even and odd current modes. After studying the near-fields in perturbed FSS, it has been found that this type of FSS represent a very attractive candidate for sensing applications due to the revealed near-field enhancement phenomena related to the excitation of the odd mode, where currents flow in opposite directions. Finally, a novel tunability approach to reach frequency reconfigurability by varying the near-field coupling between two closely spaced layers in a dual-layer configuration has been proposed. A MEMS movable four-arm membrane has been suggested to vary the distance between the two layers mechanically, leading to the frequency tuning effect. This approach has been shown to be particularly suitable for THz frequencies, and has been applied to demonstrate theoretically tunable FSS and other periodic structures, such as artificial magnetic conductors and dielectric gratings.