Tunable 2D Electromagnetic Band-Gap (EBG) Structures based on Micro-Electro-Mechanical Systems (MEMS) for THz frequencies

摘要

Artificial periodic structures, consisting of an assembly of identical or alike elements in one, two or three dimensions, can be engineered in such a way that they exhibit interesting features when interacting with electromagnetic (EM) waves throughout the spectrum. In optics, photonic band-gap (PBG) crystals exhibit band-gaps where the propagation of EM waves is prohibited. Previous research at lower frequencies resulted in the emergence of frequency-selective surfaces (FSS), consisting of resonant metallo-dielectric periodic arrays with pass/stop band characteristics. Other interesting EM properties, such as in-phase reflection and negative index of refraction, have been demonstrated by means of periodic arrays, such as artificial magnetic conductors (AMC) and negative index materials (NIM). Recently, these and other designs have been categorized under the broad term electromagnetic band gap (EBG) structures [1]. Generally, passive EBG structures exhibit static single-frequency response due to their fixed geometry and materials properties. Static multi-frequency operation devices are achievable by appropriately tailoring single-layer as well as multilayered configurations, where various band gaps or resonant modes are obtained [2-3]. However, in many applications it would be desirable to control dynamically the frequency response. Reconfigurable EBG structures, where the frequency response can be adjusted to the application's specific needs at certain moment and situation, without adding new modules, would greatly benefit communication systems (e.g. aerospace, mobile or wireless applications) in terms of flexibility and efficiency. The tunability mechanisms in periodic arrays, such as EBG structures, can be classified into two major groups, namely (1) geometry-tunable and (2) material-tunable devices. The former reconfigures the shape or arrangement of the elements of the array, whereas the latter exploits the tunable properties of certain materials. The first group includes PiN and varactor diodes, as well as micro-electro-mechanical systems (MEMS) [4-7]. In the second group, we can find plasma elements, as well as ferrite, semiconductor and liquid crystals substrates [8-9]. Every tunability method exhibits unique characteristics, has advantages and drawbacks with respect to the other techniques, and therefore, its usefulness depends on the final application. MEMS structures have shown promising performance in switchable [4-6] and tunable [7] periodic arrays.