Fig. 1. (a) and (b) Schematics of the PT-symmetric systems in Case I and Case II. (c) and (d) four phase responses of Case I and Case II. ϕpp values in Case IB and Case IIB are labeled out as examples. (e) and (f) Amplitude responses of Case ID and Case IID. Detailed information about A, B, C, D is in the main text.

Fig. 2. ϕpp maps for different coupling-coefficient combinations that fulfill the EP conditions for (a) Case I and (b) Case II. The four coupling regimes of the coupled microrings, named A, B, C, and D, are shown in different colors.

Fig. 3. (a) and (b) show dψ/dω and ψ spectra in Case I. (c) and (d) show dψ/dω and ψ spectra in Case II.

Fig. 4. When the gain of Ring 1 in Case I and Ring 2 in Case II increases by 1% compared with gth at EPs. (a) and (b) Calculated Q factors for Case I and Case II; (c) and (d) four phase responses of Case I and Case II; insets, magnification of Case ID and Case IID around the resonant frequency; (e) and (f) amplitude responses of Case ID and Case IID. (c), (d), (e), and (f) have the same coefficients combinations as mentioned in Figs. 1(c) and 1(d).

Fig. 5. When the gain of Ring 1 in Case I and Ring 2 in Case II increases by 1% compared with gth at EPs. (a) and (b) Group delay versus frequency detuning responses for Case ID and Case IID. The corresponding transmission profiles are marked as dotted lines.