Subwavelength nondiffraction beam generated by a plasmonic lens

We experimentally examined the near-field and far-field optical properties of a subwavelength annular aperture (SAA) made on silver film and undertook finite difference time domain simulations. In our near-field measurements, an interference pattern with a period very close to half of the surface plasmon (SP) wavelength (λSP∕2) was found on the surface of the silver film. Moreover, we observed that the transmitted light of the 442nm incident laser was focused at several micrometers behind the silver SAA structure at a tiny spot (354nm) and with a remarkable 31μm depth of focus. This implies that the silver SAA structure can generate a nondiffraction beam and can be used to fabricate high aspect ratio subwavelength structures.