Nonlinear switching and solitons in PT-symmetric photonic systems

One of the challenges of the modern photonics is to develop all-optical devices enabling increased speed and energy efficiency for transmitting and processing information on an optical chip. It is believed that the recently suggested Parity-Time (PT) symmetric photonic systems with alternating regions of gain and loss can bring novel functionalities. In such systems, losses are as important as gain and, depending on the structural parameters, gain compensates losses. Generally, PT systems demonstrate nontrivial non-conservative wave interactions and phase transitions, which can be employed for signal filtering and switching, opening new prospects for active control of light. In this review, we discuss a broad range of problems involving nonlinear PT-symmetric photonic systems with an intensity-dependent refractive index. Nonlinearity in such PT symmetric systems provides a basis for many effects such as the formation of localized modes, nonlinearly-induced PT-symmetry breaking, and all-optical switching. Nonlinear PT-symmetric systems can serve as powerful building blocks for the development of novel photonic devices targeting an active light control. The paper provides an overview of the topic of parity-time (PT) symmetric optical structures incorporating spatially distributed regions of gain and loss, and it covers both theoretical and experimental studies of nonlinear optical phenomena in such structures. The concepts of nonlinearity enhancement and symmetry-breaking transitions are also discussed. Various effects and applications of PT-symmetric photonics are considered in detail, including all-optical switching, tuneable signal amplification, scattering and unidirectional propagation, as well as self-trapping and soliton effects.