Overcoming the Rayleigh Criterion Limit with Optical Vortices
  We experimentally and numerically tested the separability of two independent equally-luminous monochromatic and white light sources at the diffraction limit, using Optical Vortices (OV), related to the Orbital Angular Momentum (OAM) of light. The diffraction pattern of one of the two sources crosses a phase modifying device (fork-hologram) on its center generating the Laguerre-Gaussian (L-G) transform of an Airy disk. The second source, crossing the fork-hologram in positions different from the optical center, acquires different OAM values and generates non-symmetric L-G patterns. We formulated a criterion, based on the asymmetric intensity distribution of the superposed L-G patterns so created, to resolve the two sources at angular distances much below the Rayleigh criterion. Analogous experiments carried out in white light allow angular resolutions which are still one order of magnitude below the Rayleigh criterion. The use OVs might offer new applications for stellar separation in future space experiments. 
F. Tamburini, G. Anzolin, G. Umbriaco, A. Bianchini and C. Barbieri