[1] [1] Rotter I. Non-Hermitian Hamilton operator and the physics of open quantum systems［J］. Journal of Physics A: Mathematical and Theoretical, 2009, 42(15): 1359-1366.

[2] [2] Dembowski C, Dietz B, Graf H D, et al. Encircling an exceptional point［J］. Physical Review E, 2004, 69(5): 056216.

[3] [3] Stehmann T, Heiss W D, Scholtz F G. Guidebook of electronic circuits［M］. New York: McGraw-Hill Book Company, 1974: 20-27.

[4] [4] Peng B, Ozdemir S K, Rotter S, et al. Loss-induced suppression and revival of lasing［J］. Science, 2014, 346(6207): 328-332.

[5] [5] Ding K, Ma G, Xiao M, et al. The emergence, coalescence and topological properties of multiple exceptional points and their experimental realization［J］. Physical Review X, 2015, 6(2): 021007.

[6] [6] Peng B, Ozdemir S K, Lei F, et al. Parity-time-symmetric whispering-gallery microcavities［J］. Nature Physics, 2014, 10(5): 394-398.

[7] [7] Sun Y, Tan W, Li H Q, et al. Experimental demonstration of a coherent perfect absorber with PT phase transition［J］. Physical Review Letters, 2014, 112(14): 143903.

[8] [8] Wu Lihua, Wang Zhengping, Zhang Xiangli. Theoretical research on epsilon’s nonlinearity of a metamaterial’s permittivity［J］. Acta Optica Sinica, 2009, 29(4): 1038-1042.

[9] [9] Ding Yuan, Zhu Junwei, Guo Yuhan, et al. Experimental study of resonance characteristics of split ring resonators in terahertz band［J］. Acta Optica Sinica, 2015, 35(2): 0216002.

[10] [10] Demange G, Graefe E M. Signatures of three coalescing eigenfunctions［J］. Journal of Physics A, 2011, 45(2): 25303-25316.

[11] [11] Wiersig J. Sensors operating at exceptional points: General theory［J］. Physical Review A, 2016, 93(3): 033809.

[12] [12] Wiersig J. Enhancing the sensitivity of frequency and energy splitting detection by using exceptional points: Application to microcavity sensors for single-particle detection［J］. Physical Review Letters, 2014, 112(20): 203901.

[13] [13] Pendry J B, Holden A J, Robbins D J, et al. Magnetism from conductors and enhanced nonlinear phenomena［J］. IEEE Transactions on Microwave Theory & Techniques, 1999, 47(11): 2075-2084.

[14] [14] Peng B, Ozdemir S K, Liertzer M, et al. Chiral modes and directional lasing at exceptional points［J］. Proceedings of the National Academy of Sciences of the United States of America, 2016, 113(25): 6845-6850.

[15] [15] Dembowski C, Graf H, Hamey H L, et al. Experimental observation of the topological structure of exceptional points［J］. Physical Review Letters, 2001, 86(5): 787-790.