1639 Propagation Features of Metamaterial NRD Waveguides P. Baccarelli, P. Burghignoli, F. Frezza, A. Galli, P. Lampariello, and S. Paulotto "La Sapienza" University of Rome, Rome, Italy - Department of Electronic Engineering Abstract Nonradiative dielectric (NRD) waveguides comprised of time-dispersive isotropic double-negative or single- negative slabs sandwiched between metal plates are studied. Dispersion features of bound (surface) modes are considered, showing how to achieve ranges of unimodal backward propagation by means of epsilon-negative media. Such ranges allow us to relax the symmetry constraints, typical of ordinary double-positive NRD guides, aimed at avoiding the excitation of the undesired LSEo1 dominant mode. Complex proper leaky modes supported by double-negative NRD guides are also illustrated, which show physical ranges of radiation into the first higher-order TE1 mode of the background parallel-plate waveguide. Index Terms Nonradiative dielectric waveguide, metamaterials, surface waves, leaky waves. I. INTRODUCTION Since its introduction in 1981 by Yoneyama and Nishida [1], the nonradiative dielectric (NRD) waveguide has received much attention, thanks to its attractive features of versatility, reduced radiation and low losses, which make it particularly suitable for the higher microwave and millimeter-wave ranges. The original NRD guide consists of a dielectric strip of relative permittivity , and width b sandwiched between metal plates spaced a distance a apart, with a being less than half a free-space wavelength /0 (see Fig. 1). Such a spacing prevents radiation to occur if symmetry is properly maintained, since higher-order parallel-plate waveguide modes are below cutoff. The operating mode is the LSMo, mode, which is an odd mode, i.e., has a short-circuit bisection at the symmetry plane x = 0; its electric field is mainly parallel to the metal plates, thus reducing the ohmic losses at higher frequencies. Under the nominal operating conditions, the LSEoB mode can also propagate, which is an even mode, i.e., has an open-circuit bisection at x = 0, with the electric modal field mainly orthogonal to the metal plates; this is a high-loss undesired mode, which should not be excited (by maintaining the proper symmetry) or eliminated by insertion of suitable mode-suppressing elements. Modified NRD guides employing complex materials have been studied in recent years by different research groups [2]- [10]. Nonreciprocal effects arising from the use of transversely magnetized ferrite slabs have been reported for both surface [2] and leaky modes [3]. In [4], pseudochiral omega slabs are employed, which allow us to enhance the bandwidth for single-mode operation for LSM modes. More recently, metamaterial-based NRD guides have been considered (see [5]-[10]). In [5]-[7], results for isotropic double-negative (DNG) metamaterials with fixed (i.e., frequency independent) and negative values of the constitutive parameters are reported, including field distributions [5], backward propagation, mode bifurcation, and super-slow waves [6], [7]. Coupling effects between double-positive (DPS) and DNG NRD guides are studied in [7] and [8]. The use of a single-negative (SNG) or DNG metamaterial as the embedding medium for a DPS NRD guide is studied in [9]. Anisotropic metamaterials are considered in [10], where conditions for having a TE/TM modal decomposition with respect to the longitudinal direction are derived. In this paper, propagation features of surface and leaky modes are shown for a metamaterial NRD guide based on DNG or SNG media. Time-dispersive constitutive parameters are assumed, with typical plasma-like and Lorentz-like models for the permittivity and permeability, respectively. With reference to surface waves, in Sec. II conditions are sought to suppress the propagation of undesired modes on the basis of recently-investigated properties of TE and TM waves propagating along metamaterial slabs [11]. The employment of epsilon-negative (ENG) slabs with suitable slab width and metal-plate spacing allows us to achieve the sought unimodal condition. With reference to leaky waves, in Sec. III the existence of transition regions from a proper surface regime to a proper leaky regime (both backward) is reported, showing the possibility to obtain frequency ranges in which the leaky mode is physical and radiates into the first higher-order mode of the background parallel-plate waveguide. a IVK < > x b Fig. 1. Cross section of the metamaterial NRD guide considered here, with the relevant physical and geometrical parameters. II. SURFACE-MODE PROPAGATION One goal of the present investigation is to find conditions for having propagation of the operating LSMo, mode along a 0-7803-9542-5/06/$20.00 C2006 IEEE y T