Fixed points on band structures of non-Hermitian models: Extended states in the bandgap and ideal superluminal tunneling

Abstract: Space-time reflection symmetry (PT symmetry) in non-Hermitian electronic models has drawn much attention over the past decade mainly because it guarantees that the band structures calculated under open boundary conditions be the same as those calculated under periodic boundary conditions. PT symmetry in electromagnetic (EM) models, which are usually borrowed from electronic models, has also been of immense interest mainly because it leads to 'exceptional' parameter values below which non-Hermitian operators have real eigenvalues, although PT symmetry is not the sole symmetry which allows such exceptional parameter values. In this article, we examine one-dimensional PT-symmetric non-Hermitian EM models to introduce novel concepts and phenomena. We introduce the band-structure concept of 'fixed points', which leads to 'bidirectional' reflection zeros in the corresponding finite structures, contrary to a common belief about the EM structures with PT symmetry (and without P and T symmetries). Some of the fixed points manifest themselves as what we name 'extended states in the bandgap' on the band structure while some other fixed points are the 'turning points' of the band structure. The extended states in the band gap are in fact the dual of the well-known 'bound states in the continuum' while the turning points allow us to observe 'ideal' superluminal tunneling in the corresponding finite structures. By 'ideal' superluminal tunneling we mean the case where not only the transmission coefficient has an almost uniform phase over a broad bandwidth but also the magnitudes of the transmission and reflection coefficients are almost equal to unity and zero, respectively, over the bandwidth.