  We theoretically investigate the optical second-order sideband generation
(OSSG) in an optical parity-time (PT) symmetric system, which consists of a
passive cavity trapping the atomic ensemble and an active cavity. It is found
that near the exceptional point (EP), the efficiency of the OSSG increases
sharply not only for the blue probe-pump detuning resonant case but also for
the red one. Using experimentally achievable parameters, we study the effect of
the atomic ensemble on the efficiency of the OSSG. The numerical results show
that the efficiency of the OSSG is 30% higher than that of the first-order
sideband, which is realized easily by simultaneously modulating the atom-cavity
coupling strength and detuning. Moreover, the efficiency of the OSSG can also
be tuned effectively by the pump power, and the efficiency is robust when the
pump power is strong enough. This study may have some guidance for modulating
the nonlinear optical properties and controlling light propagation, which may
stimulate further applications in optical communications.
