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

TL;DR

This paper extends the concept of parity-time symmetry in coupled waveguides to asymmetric systems, demonstrating loss compensation and exceptional points across various modes, offering new control mechanisms in photonic structures.

Contribution

It introduces asymmetric coupled waveguides supporting loss compensation and exceptional points, broadening PT-symmetry concepts to dissimilar mode systems with enhanced control.

Findings

Asymmetric waveguides exhibit loss compensation similar to PT-symmetric systems.

Loss compensation thresholds can be achieved for different mode types.

System response can be controlled via loss and gain adjustments.

Abstract

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