Effective Permittivity of Ultrathin Chemical Vapor Deposited Gold Films on Optical Fibers at Infrared Wavelengths

The geometry- and size-dependent effective medium properties of ultrathin gold films deposited on the bare cladding of single mode optical fibers by chemical vapor deposition are characterized by measuring the polarized transmission spectra of in-fiber gratings at wavelengths near 1550 nm. The real part of the complex refractive indices of films with average thicknesses ranging from 6 to 65 nm are about 10 times higher than that of bulk gold at these wavelengths, while the imaginary part values are 2 orders of magnitude lower. The films are essentially isotropic, apart from a small increasing dichroism between the in-plane and out-of-plane component of the imaginary part of the refractive index at thicknesses larger than 25 nm. Unlike gold films prepared by other means, the optical properties of the coatings do not converge rapidly toward bulk values at thicknesses larger than 10 nm but remain characteristic of gold films prepared by very slow physical deposition processes. The modified Clausius-Mossotti theory for anisotropic structures was used to confirm that the observed properties arise from a persistent granularity of the film at larger thicknesses, with metal filling fractions increasing from 30% to 68% and particle aspect ratios from 0.8 to 1.0 (spherical). These conclusions are supported by nanoparticle shape measurements obtained by atomic force microscopy and scanning electron microscope images.