Analysis of Optical Vortices in the Near Field of a Thin Metal Film

Optical vortices are shown to be generated in the near-field of a slab lens with a realistic thin metal film due to the amplification of the evanescent wave by the metal film in the TM polarization. The vortices are connected to two saddle points near the output interface of the lens. By means of varying the position of the object with respect to the lens and the wavelength, the strength of circulation of the power flow, the position and the rotation of the vortices can be well controlled. The influence of the gain to the optical vortices is also illustrated.