Multimode parity-time symmetry and loss compensation in coupled waveguides with loss and gain

Loss compensation via inserting gain is of fundamental importance in different branches photonics, nanoplasmonics, and metamaterial science. This effect has found an impressive implementation the parity-time symmetric (PT-symmetric) structures possessing balanced distribution loss gain. In this work, we generalize phenomenon to asymmetric systems demonstrating coupled multi-mode loss-gain dielectric waveguides radii. We show that similar PT-symmetric single-mode identical radii, support exceptional points called here (LC) thresholds where frequency spectrum undergoes a transition from complex real values. Moreover, LC-symmetry can be obtained for dissimilar modes excited providing additional degree freedom control system response. particular, changing waveguides, observe TM TE as well hybrid HE EH modes.