Plasmon vortices around isolated nanoholes cratered on a gold film: polarization dependent coupling of SPPs and excitation beam

Understanding the role of surface plasmon polaritons (SPP) in extraordinary light transmission through subwavelength holes in metallic layers [1,2] is crucial for the development of subwavelength optics and nanophotonics [3]. It has been shown that nanoholes in Au thick films act as point like sources of SPPs in near-field. SPPs propagate in directions determined by the polarization of incident beam and interference fringes are produced [4]. The resulting light can be controlled by changing the polarization and angle of incident light beam [4]. In this context, we study the electromagnetic near-field distribution around isolated elliptical nanoholes in 88 nm thick Au film using near-field scanning optical microscopy by changing the polarization directions of both the exciting and revealed light. We performed transmission-mode SNOM measurements. The measurements show interesting optical effects related to de-polarization process of incident light through the elliptical nanohole. Specifically, when the incident and collected light are cross-polarized, the near-field intensity around the hole exhibits a vortex-like interference pattern as shown in Fig. 1(a). A comparison with near-field transmitted light intensities and interference patterns detected at wavelengths were SPPs play a negligible role, provide indication that the observed effects origins from SPPs.