We consider parity-time (PT) symmetric arrays formed by N optical waveguides with gain and N waveguides with loss. When the gain-loss coefficient exceeds a critical value γc, the PT-symmetry becomes spontaneously broken. We calculate γc(N) and prove that γc→0 as N→∞. In the symmetric phase, the periodic array is shown to support 2N solitons with different frequencies and polarisations.

http://arxiv.org/abs/1311.4123
Optics (physics.optics); Mathematical Physics (math-ph); Pattern Formation and Solitons (nlin.PS)