I. Introduction
In the past few years, there is a high demand for terahertz devices such as attenuators, filters, antennas and couplers [1], [2], [4], [5], [6]. The tunable filter is one of the most important components in terahertz applications. Until now, several terahertz filter structures have been reported. Some theoretical investigations on these filters have been performed using traditional material such as metallic hole arrays, photonic crystals and matematerials. However, most of these structures are difficult to manufacture and require important machining; also, these structures are not compatible with planar circuits. These disadvantages have limited the practical use of these structures.