title: Anisotropic effective permittivity of an ultrathin gold coating on optical fiber in air, water and saline solutions.
abstract: The optical properties of an ultrathin discontinuous gold film in different dielectric surroundings are investigated experimentally by measuring the polarization-dependent wavelength shifts and amplitudes of the cladding mode resonances of a tilted fiber Bragg grating. The gold film was prepared by electron-beam evaporation and had an average thickness of 5.5 nm ( ± 1 nm). Scanning electron imaging was used to determine that the film is actually formed of individual particles with average lateral dimensions of 28 nm ( ± 8 nm). The complex refractive indices of the equivalent uniform film in air at a wavelength of 1570 nm were calculated from the measurements to be 4.84-i0.74 and 3.97-i0.85 for TM and TE polarizations respectively (compared to the value for bulk gold: 0.54-i10.9). Additionally, changes in the birefringence and dichroism of the films were measured as a function of the surrounding medium, in air, water and a saturated NaCl (salt) solution. These results show that the film has stronger dielectric behavior for TM light than for TE, a trend that increases with increasing surrounding index. Finally, the experimental results are compared to predictions from two widely used effective medium approximations, the generalized Maxwell-Garnett and Bruggeman theories for gold particles in a surrounding matrix. It is found that both of these methods fail to predict the observed behavior for the film considered.
authors.forename: Wenjun
authors.initials: W
authors.lastname: Zhou
authors.forename: David J
authors.initials: DJ
authors.lastname: Mandia
authors.forename: Seán T
authors.initials: ST
authors.lastname: Barry
authors.forename: Jacques
authors.initials: J
authors.lastname: Albert
mesh_terms: D065966:Absorption, Physicochemical; D000327:Adsorption; D000388:Air / Q000032:analysis*; D025802:Electric Capacitance; D004735:Energy Transfer; D004867:Equipment Design; D019544:Equipment Failure Analysis; D006046:Gold / Q000737:chemistry*; D008422:Materials Testing; D055100:Optical Fibers; D012542:Scattering, Radiation; D012965:Sodium Chloride / Q000737:chemistry*; D014867:Water / Q000737:chemistry*
chemical_list: D014867:Water; D012965:Sodium Chloride; D006046:Gold