Nonlinear reversal of PT-symmetric phase transition in a system of coupled micro-ring cavities

Abstract

Parity time (PT) symmetric systems are known to exhibit two distinct phases: those associated with an unbroken and broken symmetry. In the domain of optics, PT-symmetry can be established by incorporating a balanced distribution of gain and loss in a system. Under linear conditions, in a coupled dimer, composed of two cavities or waveguides, if the gain-loss contrast increases beyond a critical value with respect to the coupling constant, a transition is expected from the unbroken symmetry to the broken symmetry regime. However, in the presence of nonlinearity, this transition behavior can be drastically modified. We here study a system of two coupled semiconductor-based resonators that are lasing around an exceptional point. The quantum wells in such structures not only provide gain but also lead to strong levels of saturable loss in the absence of any optical pumping. Interestingly, in sharp contrast with linear PT-symmetric configurations, such nonlinear processes are capable of reversing the order in which the symmetry breaking occurs. If the ratio of the net loss to coupling is less than unity in one of the cavities, as the pumping level in the other resonator is increased, the nonlinear eigenmodes move from an unbroken symmetric state to a broken one. Moreover, in this nonlinear domain, the structural form of the resulting solutions are isomorphic to the corresponding linear eigenvectors expected above and below the phase transition point. Experimental results are in good agreement with these predictions.