Enhancing optical Kerr nonlinearity by gain-loss balanced feedback loop

We present a method to enhance the optical Kerr nonlinearity by introducing a gain-loss balanced feedback control structure. Our gain-loss balanced system consists of two indirectly coupled whispering-gallery-mode resonators in which one is a loss resonator and the other is a gain resonator. We found that the Kerr nonlinearity in the loss resonator can be greatly enhanced in the vicinity of the so-called exceptional point. Our results have various possible applications in on-chip optical devices, such as the construction of on-chip isolators and circulators.