Saturable nonlinearity-induced high-order sideband generation in a parity–time symmetric double-cavity system

A coupled active-passive double-cavity system with balanced effective gain and loss provides a powerful platform for sideband generation. In different-input limit, we explore the gain-saturation nonlinearity of the active cavity carefully in sideband generation. Through perturbation method, we give an explicit analytical expression of second-order sideband generation (SSG) not only with over 80% conversion efficiency, but also with the pump power as low as ~ µW. A remarkable mode-splitting is also observed during the SSG process in the unbroken parity-time (PT) symmetry phase. Changing the system parameters largely, our numerical method shows that high-order sideband generation (HSG) in the unbroken PT phase is controllable, including increasing its amplitudes and changing its order numbers, especially switching HSG between perturbative- and nonperturbative-regime. In the broken PT phase, the field localization effect leads to the stronger output spectra intensity than in the unbroken PT phase.