title: Propagation characteristics of silver and tungsten subwavelength annular aperture generated sub-micron non-diffraction beams.
abstract: We examined the optical properties such as propagation modes, focal length, side lobes, etc. of metallic subwavelength annular apertures (SAA) and used finite-difference time-domain (FDTD) simulation to compare our experimental findings. Using two different metals, silver and tungsten, we examined the different optical transmission properties of the two metallic SAA structures. The far-field propagation of the silver SAA structure was found to be a type of quasi-Bessel beam when compared with a quasi-Bessel beam generated by a perfect axicon. The propagation characteristics of these two beams were found to match qualitatively. The far-field transmitted light generated by the silver SAA structure was found to possess a 390 nm sub-micron focal spot with a 24 microm depth of focus, which was much smaller than the focal spot generated by a perfect axicon. We also found that a silver SAA structure can generate a sub-micron quasi- Bessel beam that has a much lower far-field side-lobe when compared to that of non-diffraction beams generated by a tungsten SAA structure.
authors.affiliation: Institute of Applied Mechanics, National Taiwan University, Taipei, Taiwan.
authors.forename: Tsung-Dar
authors.initials: TD
authors.lastname: Cheng
authors.forename: Ding-Zheng
authors.initials: DZ
authors.lastname: Lin
authors.forename: Jyi-Tyan
authors.initials: JT
authors.lastname: Yeh
authors.forename: Jonq-Min
authors.initials: JM
authors.lastname: Liu
authors.forename: Chau-Shioung
authors.initials: CS
authors.lastname: Yeh
authors.forename: Chih-Kung
authors.initials: CK
authors.lastname: Lee