Effective Optical Properties of Ultrathin Gold Coatings Investigated by Tilted Fiber Bragg Grating

Tilted fiber Bragg grating (TFBG)-based refractometers are inexpensive, compact, lightweight, and immune to electromagnetic interference and have high chemical resistance. In recent years, many kinds of metal films-coated TFBG refractometers were investigated for enhancing the surrounding refractive index (SRI) measurement sensitivity, resulting from the several orders of magnitude enhancement of the electromagnetic field intensity in the immediate vicinity of various ultrathin metal films or nanostructures. Thus, the measurements of the optical properties of the ultrathin metal coatings (especially gold coatings) on optical fibers are essential for optimizing the TFBG sensing performance. In this thesis, we firstly measured the properties of the high order cladding modes of standard optical fibers in real-time during the deposition of gold nanoparticle layers by chemical vapour deposition (CVD). Then, the geometry and size-dependent effective medium properties of ultrathin gold CVD films deposited on the bare cladding of single mode optical fibers are characterized by measuring the polarized transmission spectra of TFBGs at near infared wavelengths. This allows the preparation of gold nanoparticle layers that strongly discriminate between radially and azimuthally polarized cladding mode evanescent fields, with important consequences in the plasmonic properties of these layers. Finally, the optical properties of an ultrathin discontinuous gold evaporation film in different dielectric surroundings are investigated experimentally by measuring the polarization-dependent wavelength shifts and amplitudes of the cladding mode resonances of TFBG. 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. The results of this thesis show that the TFBG probe can either work as a bulk witness sensor for monitoring the thin film growth on silica substrate or be used as an “ellipsometry” tool for the optical properties measurements of the thin films coated on TFBG surface.