Resonance-induced transmissions through waveguides below cut-off frequencies: An effective-medium model for waveguide

It is well known that electromagnetic (EM) waves can not transmit through a metallic waveguide at frequencies well below its cutoff frequency, since only evanescent waves are allowed inside the waveguides under such conditions. However, through inserting local resonance structures of either electric or magnetic type into the waveguide, we find extraordinary transmissions of EM waves with different polarizations through the waveguide at frequencies well below the waveguide´s cut-off value [1] [2]. We have identified two different mechanisms for such unusual transparencies, in which the medium at transparency can be characterized either by doubly positive epsiv_eff and mu_eff [see Fig. l(left)] or by doubly negative epsiv_eff and mu_eff [see Fig. 1 (right)] [see also Ref. 2 for such a mechanism]. We also demonstrated that while a hollow metallic waveguide can be viewed as an effective medium with epsiv_wg = 1 - f_p ²/f², mu_wg = 1 for the transverse-electric (TE) polarized waves, it should be viewed as a different effective medium with epsiv_wg = 1, mu_wg = 1 - f_p ²/f² for the transverse-magnetic (TM) polarized waves, where f_p is the cut-off frequency of the waveguide. We have designed realistic electric/magnetic resonance structures, and have performed accurate finite-different-time-domain (FDTD) simulations on such realistic structures to successfully demonstrate all theoretical predictions based on the model calculations.