Parity-time anti-symmetric parametric amplifier.

Light propagation in waveguiding structures with spatially distributed sections of loss and gain can be analogous to quantum wavepacket dynamics governed by a parity-time (PT) symmetric Hamiltonian [1]. Below a certain gain/loss level, such systems support PT-symmetric optical modes, which then exhibit the same average loss or gain. However when gain or loss is increased, the PT-symmetry of modes breaks, and a mode with the strongest gain dominates [2]. The phase transition associated with such PT-symmetry breaking opens new possibilities for light manipulation, such as single-mode PT-symmetric lasers [3, 4]. Here, we reveal the potential of PT-symmetric systems for optical parametric amplification, which enables flexible wavelength conversion and tunable signal gain.