Stability of optical solitons in parity-time-symmetric optical lattices with competing cubic and quintic nonlinearities.

The existence and stability of optical solitons in the semi-infinite gap of parity-time (PT)-symmetric optical lattices with competing cubic and quintic nonlinearities are investigated numerically. The fundamental and dipole solitons can exist only with focusing quintic nonlinearity; however, they are always linearly unstable. With the competing effect between cubic and quintic nonlinearities, the strength of the quintic nonlinearity should be larger than a threshold for the solitons' existence when the strength of the focusing cubic nonlinearity is fixed. The stability of both fundamental and dipole solitons is studied in detail. When the strength of the focusing quintic nonlinearity is fixed, solitons can exist at the whole interval of the strength of the cubic nonlinearity, but only a small part of the fundamental solitons are stable. We also study numerically nonlinear evolution of stable and unstable PT solitons under perturbation.