Generation of multivortex ring beams by inhomogeneous effective diffusion

By means of numerical simulations, we demonstrate generation of multivortex ring beams (MVRBs), i.e., circular chains built of small vortices, by launching necklace-shaped inputs, carrying angular momentum, into a dissipative optical medium modeled by the cubic-quintic complex Ginzburg-Landau equation with effective diffusion, which is periodically modulated in the longitudinal direction. The MVRB chains keep the original angular momentum. The number of external vortices is equal to the number of “beads” in the input necklace pattern. The individual small vortices are produced by the centrifugal force, which is originally induced by the angular momentum applied to the input necklace. Upon the propagation, the inner part of the MVRBs keeps initial counterclockwise direction of the rotation, while the outer part reverses to the clockwise direction (the present dissipative system does not conserve the angular momentum). Individual small vortices in the chain carry topological charge (“spin”) + 1, whose sign agrees with the clockwise rotation. MVRB states do not exist in the usual model with constant diffusion. The results offer a method to produce new kinds of vortex beams, which may find applications in optics.