Anisotropic effective permittivity of an ultrathin gold coating on optical fiber in air, water and saline solutions

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

Tris(dimethylamido)aluminum(III): An overlooked atomic layer deposition precursor

Aluminum oxide and aluminum nitride-containing films were grown by atomic layer deposition (ALD) and plasma-enhanced atomic layer deposition (PE-ALD) by employing an under-utilized tris(dimethylamido)aluminum(III) precursor. This compound has not been reported as a precursor for ALD of alumina previously, and has only been reported as an AlN precursor for a thermal process using ammonia as a coreagent. Thermogravimetric analysis demonstrates its excellent volatility and thermal stability, both of which are ideal characteristics for an ALD precursor. Aluminum oxide films were deposited thermally using water as a coreagent. By x-ray photoelectron spectroscopy, the films appeared nearly pristine with only adventitious carbon on the surface accumulated postdeposition that was easily removed with 2 min of Ar+ sputtering. The rest of the films contained a very low 1.4% impurity of carbon. Aluminum nitride films were attempted using the same aluminum precursor with nitrogen plasma as a coreagent; they contained large amounts of oxygen due to ambient exposure, possible oxidation during characterization, or the presence of incidental oxygen during the deposition of AlN, which allowed the formation of an aluminum oxynitride. Though the composition was not stoichiometrically AlN, the films also contained ∼1% carbon impurities, which is an improvement over many other AlN films reported, particularly those using TMA as a precursor. This precursor shows great promise for the deposition of low-impurity or impurity-free aluminum nitride by PE-ALD

Absolute near-infrared refractometry with a calibrated tilted fiber Bragg grating

The absolute refractive indices (RIs) of water and other liquids are determined with an uncertainty of±0.001 at near-infrared wavelengths by using the tilted fiber Bragg grating (TFBG) cladding mode resonances of a standard single-mode fiber to measure the critical angle for total internal reflection at the interface between the fiber and its surroundings. The necessary condition to obtain absolute RIs (instead of measuring RI changes) is a thorough characterization of the dispersion of the core mode effective index of the TFBG across the full range of its cladding mode resonance spectrum. This technique is shown to be competitive with the best available measurements of the RIs of water and NaCl solutions at wavelengths in the vicinity of 1550 nm

CVD on optical fibers: Tilted fiber Bragg gratings as real-time sensing platforms

The tilted fiber Bragg grating (TFBG), as a versatile and robust tool for many sensing applications with a particular focus on vapor deposition processes, is reviewed. Recent work employing the TFBG as an optical probe for monitoring the metal-organic (MO)CVD of noble-metal, single-source precursors is discussed extensively. This work also establishes the broad scope that utilizes TFBGs and other optical fiber configurations to interrogate thin film growth and the associated optical properties. While it cannot possibly cover the full scope of applications with respect to the TFBG device, this review highlights the recent advances of TFBG-based sensing for CVD, and progress towards the applicability of TFBGs as evanescent field-based sensors

Monitoring of the insulator-to-metal transition of ultrathin gold coatings on optical fibers

The anisotropy of the optical properties of ultrathin gold evaporated films (with massequivalent thicknesses from 3 to 8 nm) during the insulator-to-metal transition process is measured from the polarization-dependent response of coated tilted FBGs

Using a Vapor-Phase Surfactant to Control Gold Metal Plate Growth

Hexamethyldisilazide-1,3-diisopropylimidazolidine-2-ylidenegold(I) (1) is used to deposit gold microplates with (111) faces. In the absence of any secondary vapor-phase surfactant, these plates show secondary nucleation and growth of gold metal nanoparticles on the (111) faces. When tetrahydrothiophene (THT) is used as a secondary, vapor-phase surfactant, plate size increases, and secondary nucleation is controllable by temperature. Deposition of gold microplates at 370 °C using a 45 mTorr overpressure of THT shows the best experimental results, growing 20 μm2 plateaus with no apparent secondary nucleation. Computational modelling demonstrates that THT is a stronger surfactant than the carbene ligand (from the gold precursor) due to steric hindrance from the alkyl groups present in the carbene ligand

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

Anomalous refractive index of ultrathin gold nanoparticle film coated on tilted fiber Bragg grating

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

The effect of ALD-grown Al2O3 on the refractive index sensitivity of CVD gold-coated optical fiber sensors

The combined effect of nanoscale dielectric and metallic layers prepared by atomic layer deposition (ALD) and chemical vapor deposition (CVD) on the refractometric properties of tilted optical fiber Bragg gratings (TFBG) is studied. A high index intermediate layer made up of either 50 nm or 100 nm layers of Al2O3 (refractive index near 1.62) was deposited by ALD and followed by thin gold layers (30-65 nm) deposited from a known single-source gold (I) iminopyrrolidinate CVD precursor. The fabricated devices were immersed in different surrounding refractive indices (SRI) and the spectral transmission response of the TFBGs was measured. Preliminary results ind