Phase-locked spatial domains and Bloch domain walls in type-II optical parametric oscillators

We study the role of transverse spatial degrees of freedom in the dynamics of signal-idler phase locked states in type-II optical parametric oscillators. Phase locking stems from signal-idler polarization coupling which arises if the cavity birefringence and/or dichroism is not matched to the nonlinear crystal birefringence. Spontaneous Bloch domain wall formation is observed numerically and the dynamics and chiral properties of the fronts are investigated. Bloch walls connect homogeneous regions of self-phase-locked solutions by means of a polarization transformation. The parameter range for phase locking is found analytically. The polarization properties and the dynamics of walls in one and two transverse spatial dimensions are explained. The transition from Bloch to Ising walls is characterized, the control parameter being the linear coupling strength. The wall dynamics governs spatiotemporal dynamical states of the system, which include transient curvature driven domain growth, persistent dynamics dominated by spiraling defects for Bloch walls, and labyrinthine pattern formation for Ising walls.