Abstract
Achieving exact balance between spatially separated gain and loss is generally regarded as a necessary condition for parity-time-(PT-) symmetric optical systems. We introduce generalized PT(GPT-) symmetric optical structures, which have an asymmetric and unbalanced gain-loss profile, while exhibiting similar scattering properties and phase transitions as their traditional PT-symmetric counterparts. In particular, we show that the concept of GPT symmetry may help to reduce the threshold gain in achieving newly discovered PT-enabled applications, such as the coherent perfect absorber laser and exceptional points. The concept proposed herein will facilitate the practice of PT-symmetric optical and photonic devices by offering greater design flexibility to tailor gain-loss profiles and their thresholds.