Parity-time-symmetric optical coupler with birefringent arms

In this work, we propose a parity-time-(PT-) symmetric optical coupler whose arms are birefringent waveguides as a realistic physical model which leads to a so-called quadrimer, i.e., a four-complex-field setting. We seek stationary solutions of the resulting linear and nonlinear model, identifying its linear point of PT-symmetry breaking and examining the corresponding nonlinear solutions that persist up to this point, as well as so-called ghost states that bifurcate from them. We obtain the relevant symmetry-breaking bifurcations between symmetric (circularly polarized) and asymmetric (elliptically polarized) states and numerically follow the associated dynamics which give rise to growth or decay even within the PT-symmetric phase. Our symmetric stationary nonlinear solutions are found to terminate in saddle-center bifurcations which are analogous to the linear PT phase transition. We found that the PT symmetry significantly changes the stability and dynamical properties of the modes with different polarizations. DOI: 10.1103/PhysRevA.87.033812