Robust topological edge modes may evolve into complex-frequency modes when a physical system becomes non-Hermitian. We show that, while having negligible forward optical extinction cross section, a conjugate pair of such complex topological edge modes in a non-Hermitian -symmetric system can give rise to an anomalous sideway scattering when they are simultaneously excited by a plane wave. We propose a realization of such scattering state in a linear array of subwavelength resonators coated with gain media. The prediction is based on an analytical two-band model and verified by rigorous numerical simulation using multiple-multipole scattering theory. The result suggests an extreme situation where leakage of classical information is unnoticeable to the transmitter and the receiver when such a -symmetric unit is inserted into the communication channel.

We demonstrate theoretically the electric tunability due to coalescence of exceptional points in PT-symmetric waveguides bounded by imperfect conductive layers. Owing to the competition effect of multimode interaction, multiple exceptional points and PT phase transitions could be attained in such a simple system and their occurrences are strongly dependent on the boundary conductive layers. When the conductive layers become very thin, it is found that the oblique transmittance and reflectance of the same system can be tuned between zero and one by a small change in carrier density. The results may provide an effective method for fast tuning and modulation of optical signals through electrical gating. PACS numbers:A wide class of non-Hermitian Hamiltonians with parity-time (PT) symmetric complex potentials have been extensively studied since Bender et al. [1,2] showed such systems can exhibit purely real spectra. By tuning the degree of non-Hermiticity, PT-symmetric systems may experience an abrupt phase transition between PTsymmetric phase with a real spectrum and PT-broken phase with a complex spectrum. The transition point is called the exceptional point (EP), at which two or more eigenvalues coincide. The existence of EPs have been investigated in various types of PT-symmetric physical systems [3][4][5][6][7][8][9][10][11][12][13][14][15][16][17][18][19], giving rise to a wide range of counterintuitive wave phenomena, such as loss-induced transparency [3], nonreciprocal Bloch oscillation [4], unidirectional invisibility [5][6][7], coexisting coherent perfect absorption and lasing [8,9], enhanced spontaneous emission [10], and enhanced nano-particle sensing [11].Recently, there has been strong interest in more complex phenomena closely related to the occurrence of EPs in multistate systems and high-order EPs [10,[20][21][22][23][24][25][26][27]. Multiple optical waveguide systems [20,21] and photonic crystals [10,22] have been proposed for the realization of high-order EPs. For example, Ding et al. [22] demonstrated theoretically two different kinds of EP evolution process in one-dimensional PT-symmetric photonic crystals. One type is the occurrence of a ring of EPs, leading to the restoration behavior of PT symmetry phase. Another type is the coalescence of EPs to form high-order singularity. Zhen et al. [23] observed experimentally that a ring of EPs could be supported near a Dirac-like cone in a two-dimensional photonic crystal slab. Also, the emergence and interaction of multiple EPs have been studied in a four-state acoustic system [24], exhibiting richer physical behaviors than those seen in two-state systems.Taking the advantage of the abrupt change near an EP, one may tune the properties of a system drastically by a small change in certain parameters. However, previous studies on evolution process of EPs and various types of phase transitions, are mostly on tuning by geometrical parameters. In this Letter, we demonstrate that the PTsymmetric plasmonic waveguide bounded by imperfect conducting materials...

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