Chiral materials to control exceptional points in parity-time-symmetric waveguides

Parity-time $(\mathcal{PT})$ symmetry and chirality are both actively investigated in photonics due to the original behaviors they provide. We combine $\mathcal{PT}$ symmetry and chirality in a single photonic structure by inserting a chiral material in the narrow gap between $\mathcal{PT}$-symmetric coupled waveguides. We analyze the various effects of chiral coupling between the modes, especially in the vicinity of an exceptional point. By tuning the waveguide gap we tailor the chiral coupling between non-Hermitian modes with different polarizations, which would otherwise not interact. As a result, a rich variety of qualitatively differing dispersions is achieved, from typical anticrossings to symmetry-broken and associated symmetry-recovery zones, as well as a hybrid trimodal anticrossing. Furthermore, the slot effect in the gap leads to a very strong chiral coupling, reaching bulk sensitivity values near an exceptional point, which could be useful for sensing purposes. We employ a modified two-dimensional finite-element approach to include chirality in the simulations. Additionally, we propose a compact coupled-mode theory that elucidates the physics at play and provides opportunities for the study of more complex devices.