Fusion of fractional vortex pairs and their transition to integer vortex controlled by optical power

The evolution of fractional vortex pairs in free space and nonlocal nonlinear media is studied. In free space, the off-axis fractional vortex pairs of the-same-sign topological charge (TC) will be merged to one integer vortex at the beam center, which is drastically different from the dynamics of integer vortex pairs. In nonlocal nonlinear media, the conversion between the fractional vortex pair and the conventional integer vortex can be readily achieved by only tuning the input optical power. Therefore our approach provides a convenient way to control the number of vortices and thus the number of optical tweezers by adjusting the input optical power. These results may find potential applications in optical manipulation of particles.