Restoring robust binary switching operation and exceptional point using long-period grating-assisted parity-time symmetric couplers

The impact of imbalance in waveguide propagation constants among parity-time symmetric coupled waveguides and/or of a complex-valued coupling coefficient is assessed. The narrow tolerance found implies that attempts to tightly control waveguide parameters appear to be elusive because of fabrication technology limitations, calling for more feasible mitigation avenues. It is shown that a grating-assisted parity-time symmetric coupler design restores both technologically robust binary switching operations and exceptional points. In addition, the proposed design is compatible with birefringence compensation techniques, providing polarization-independent operation as well as coupling and/or gain-loss profile modulation techniques that extend the control of switching operation in the parity-time symmetric phase. Using the wavelength as an additional tuning parameter near exceptional points opens promising avenues for manipulating the trajectory on Riemann sheets in topological photonics applications. (paper)