Tilted fiber Bragg gratings (TFBGs) are refractometry-based sensor platforms that have been employed herein as devices for the real-time monitoring of chemical vapour deposition (CVD) in the near-infrared range (NIR). The coreguided light launched within the TFBG core is back-reflected off a gold mirror sputtered onto the fiber-end and is scattered out into the cladding where it can interact with a nucleating thin film. Evanescent fields of the growing gold nanostructures behave differently depending on the polarization state of the core-guided light interrogating the growing film, therefore the resulting spectral profile is typically decomposed into two separate peak families for the orthogonal S- and P-polarizations. Wavelength shifts and attenuation profiles generated from gold films in the thickness regime of 5-100 nm are typically degenerate for deposition directly onto the TFBG. However, a polarization-dependence can be imposed by adding a thin dielectric pre-coating onto the TFBG prior to using the device for CVD monitoring of the ultrathin gold films. It is found that addition of the pre-coating enhances the sensitivity of the P-polarized peak family to the deposition of ultrathin gold films and renders the films optically anisotropic. It is shown herein that addition of the metal oxide coating can increase the peak-to-peak wavelength separation between orthogonal polarization modes as well as allow for easy resonance tracking during deposition. This is also the first reporting of anisotropic gold films generated from this particular gold precursor and CVD process. Using an ensemble of x-ray techniques, the local fine structure of the gold films deposited directly on the TFBG is compared to gold films of similar thicknesses deposited on the Al2O3 pre-coated TFBG and witness slides.
Effectively size-dependent refractive index of ultrathin gold film deposited by chemical vapour deposition (CVD) is experimentally investigated at infrared wavelength. By coating the gold film on tilted fiber Bragg grating (TFBG), the wavelength and amplitude of the TFBG cladding modes are modulated by the interaction between their evanescent fields and the gold film. Then, the complex refractive index of the gold film in in-plane and out-of-plane directions can be calculated from the effective indices of the cladding modes with azimuthally and radically polarized electric fields at cladding boundary, respectively. The obtained real parts of the complex refractive indices are about 10 times higher than that of bulk gold, for the gold films with thickness from 6 to 65 nm, while the imaginary parts are 2 orders of magnitude lower than the bulk value in the both of directions. Based on the Atomic force microscope and scanning electron microscope images of the gold films with different thicknesses, the aggregation of gold nanoparticles (NPs) caused by high substrate temperature and low deposition rate is considered as the main contribution to the anomalous refractive indices.