Twin-Vortices in Clustered Speckles Modulated by a Spiral Multi-Pinhole Plate

We report an experimental study of a new vortex state in clustered speckles generated by modulating the scattering waves with a spiral multi-pinhole plate. This vortex state can be termed a twin-vortex state with two spatially neighboring singularities nested in a common vortex core. In our experiment, the spiral multi-pinhole plate with $N$ pinholes arranged along a spiral curve introduces a phase variation of an integer multiple of $2\pi$ into the waves. With different phase modulations, the waves scattered from random surfaces can be modulated to produce various forms of twin-vortices in clustered speckles. Based on the experimental data, we conduct a statistical analysis of both the density of twin-vortices and the distance between the neighboring singularities. The results indicate that the twin-vortex has the properties of low density and closely neighboring singularities. The particular vortices with two singularities in speckle fields are experimentally studied for the first time.