Phase-locking of a Nonlinear Optical Cavity via Rocking: Transmuting   Vortices into Phase Patterns
  We report experimental observation of the conversion of a phase-invariant nonlinear system into a phase-locked one via the mechanism of rocking [G. J. de Valcarcel and K. Staliunas, Phys. Rev. E 67, 026604 (2003)]. This conversion results in that vortices of the phase-invariant system are being replaced by phase patterns such as domain walls. The experiment is carried out on a photorefractive oscillator in two-wave mixing configuration.A model for the experimental device is given that reproduces the observed behavior. 
Adolfo Esteban Martin, Manuel Martinez Quesada, Victor B. Taranenko,   Eugenio Roldan, and German J. de Valcarcel