The paper analyses the dispersion characteristics of an infinitely long PT chain made of whispering gallery resonators with gain/loss modulation. The results show that the appearance of the threshold breaking point depends not only on both of coupling and gain/loss modulation but also on the Bloch phase. The Bloch phase is seen as an additional parameter that can reduce or even completely eliminate PT-breaking threshold.
Phang, S., Vukovic, A., Creagh, S., Benson, T. M., Gradoni, G., & Sewell, P. D. (2015). Parity-time chain of whispering-gallery mode resonators