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Exceptional point (EP) degeneracies in coupled cavities with gain and lossprovide on-chip photonic devices with unconventional features and performance.However, such systems with realistic structures often miss the exact EPs evenin simulation, and the mechanism of this EP disruption has yet to be thoroughlyidentified. Here, we extend the coupled-mode theory of one-dimensionalnon-Hermitian resonator arrays to study the effects of the imaginary part ofthe inter-cavity coupling, which is a second-order term and attributed tomaterial amplification, absorption, and radiation. By taking an appropriategauge for the model, we clarify that the imaginary coupling components have asymmetric form in the effective Hamiltonian and hence representnon-Hermiticity. These additional factors can lift the gain- and loss-based EPdegeneracies. However, they are proportional to the sum of the imaginarypermittivities for involved cavity pairs. Thus, when the amplification andabsorption of adjacent cavities are balanced, their contribution to theimaginary coupling is canceled, and the EP singularity can be restored.Radiation-induced imaginary couplings measure the change in net radiation lossby the interference between cavity modes. Their impact on the EP can also becounteracted by small cavity resonance detuning even in loss-biased cases. Weshow and analyze eligible simulation examples based on photonic crystalnanocavities, and highlight design of an ideal EP degeneracy that is protectedby generalized PT symmetry and induced by radiation.

Imaginary couplings in non-Hermitian coupled-mode theory: Effects on exceptional points of optical resonators