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Chiral emission can be achieved from a circularly polarized dipole emitter ina nanophotonic structure that possess special polarization properties such as apolarization singularity, namely with right or left circularly polarization(C-points). Recently, Chen et al. [Nature Physics 16, 571 (2020)] demonstratedthe surprising result of chiral radiation from a linearly-polarized (LP) dipoleemitter, and argued that this effect is caused by a decoupling with theunderlying eigenmodes of a non-Hermitian system, working at an exceptionalpoint (EP). Here we present a quasinormal mode (QNM) approach to model asimilar index-modulated ring resonator working near an EP and show the sameunusual chiral power flow properties from LP emitters, in direct agreement withthe experimental results. We explain these results quantitatively withoutinvoking the interpretation of a missing dimension (the Jordan vector) and adecoupling from the cavity eigenmodes, since the correct eigenmodes are theQNMs which explain the chiral emission using only two cavity modes. By couplinga LP emitter with the dominant two QNMs of the ring resonator, we show how thechiral emission depend on the position and orientation of the emitter, which isalso verified by the excellent agreement with respect to the power flow betweenthe QNM theory and full numerical dipole solutions. We also show how a normalmode solution will fail to capture the correct chirality since it does not takeinto account the essential QNM phase. Moreover, we demonstrate how one canachieve frequency-dependent chiral emission, and replace lossy materials withgain materials in the index modulation to reverse the chirality.

acs photonics

Quasinormal mode analysis of chiral power flow from linearly polarized  dipole emitters coupled to index-modulated microring resonators close to an  exceptional point