Refractive index measurements of films with biaxial symmetry. 1. Determination of complex refractive indices using polarized reflectance/transmittance ratio

A new method to extract complex refractive indices of films with biaxial symmetry from polarized transmission and reflection spectra is described. Theoretical analysis demonstrates that the reflectance/transmittance ratio (R/T ratio) of two films of different thicknesses but with the same optical anisotropy is a simple function of the refractive index (n) and extinction coefficient (k). For films with biaxial symmetry, components of n and k on symmetric axes can be extracted from either s- or p-polarized R/T ratios if the film thickness values are known. The R/T ratio method can generate n and k spectra within a particular wavelength range without assuming a specific relationship among n, k, and wavelength, which is an advantage over many currently available techniques. The R/T ratio method is used to extract the anisotropic complex refractive indices of a polyimide film with known uniaxial symmetry. The resultant n and k spectra compare well with simulations based on known n and k values. The accuracy of n and k spectra is affected mostly by data error in reflection and transmission spectra collection, thickness variation across sample films, and error in sample alignment.     

Theoretical calculations of refractive indices and optical effects in spectra of nitric acid and nitric acid monohydrate crystals

The theoretical infrared refractive indices of two systems related to atmospheric research, nitric acid (NA) and nitric acid monohydrate (NAM) crystals, have been computed using a methodology based on first-principles. The effects of lack of coherence in the infrared beam in RAIR and transmission spectra have also been treated using a model based on classical optics. The optical constants of NA crystals are presented for the first time; the results on NAM are compared to empirical values previously published with good general agreement. With the optical constants of NA, polarized reflection-absorption infrared spectra are predicted and compared to experimental spectra recorded also for the first time, for a set of varying film thickness. The global agreement is satisfactory. The effects of a number of experimental factors in transmission spectra of NAM are assessed, in an attempt to explain observed differences among experimental spectra. It is concluded that the spectral disparities are probably due to differences in the nature of the samples.     

Orientation effects on nitric acid dihydrate films

An investigation of orientation effects in films of nitric acid dihydrate (NAD) is presented, based on a systematic study of transmission and reflection-absorption infrared (RAIR) spectra of samples of varying thickness. The samples are prepared by vapor deposition on Ge (for transmission spectroscopy) and on Al substrates (for RAIR spectroscopy) at 175 K to produce crystalline alpha-NAD films. Transmission spectra were recorded at normal incidence, and RAIR spectra were recorded at a grazing angle of 75 degrees, with polarized radiation. The observed spectra are compared with predictions of a classical Fresnel model, to test the available optical indices of NAD, which are of great importance for the accurate interpretation of data from remote sensing measurements. Whereas the procedure yields satisfactory results for transmission and s-polarized RAIR spectra, it is found that the agreement is not acceptable for p-polarized RAIR spectra. An explanation is suggested in terms of a preferential alignment of the films, with the (10-1) crystallographic plane of the crystal situated parallel to the substrate. The infrared activity of a band at approximately 1170 cm(-1) is explained in terms of a preferential orientation of the crystal domains in the film.     

Preparation of thin silica films with controlled thickness and tunable refractive index

Silica films with controlled thickness and refractive index have been formed by the sequential adsorption of a cationic polyelectrolyte and silica sols. The conditions used to prepare the sol were varied, and allowed films with refractive indices as low as 1.16 to be obtained. The sequential adsorption technique allows the thickness of these films to be controlled in increments of 5-10 nm, depending on the desired refractive index. Scanning electron microscopy revealed that a low packing density of constituent silica particles was responsible for the low indices of these films. The as-adsorbed films are thermally robust; calcination at 500 degrees C resulted in only very small decreases in film thickness (by < or =1.8%) and refractive index (to as low as 1.14). After calcination, the silica films remained hydrophilic and sorbed water vapor from the atmosphere. As a result, the refractive indices of these films increased with increasing relative humidity (RH). The dependence of the refractive index on RH was eliminated by treating the calcined films with trimethylchlorosilane.     

Measurement of anisotropic refractive indices of free-standing polymer film

A new technique has been developed to measure the optical refractive indices in the parallel and perpendicular directions to the film surfaces and applied to measure the refractive indices of free-standing polymer films. The refractive indices were obtained by measuring the reflectivity as a function of the incident angle. The angle dependence of the reflectivity which results from the interference of the light beam reflected from the front and rear interfaces of the film was analysed by means of the Fresnel equation. This technique can be used to measure the refractive indices in three major axes, i.e. one out-of plane axis and two in-plane axes of the sample. This technique gives values for the refractive indices and the thickness simultaneously with an accuracy comparable to that measured by optical wave-guide technique. The validity of the technique has been tested with polymer films of known refractive indices such as poly(methyl methacrylate) and polyimide (PMDA-ODA).     

Optical constants and integrated intensities of liquid hexafluorobenzene between 4000 and 250 cm at 25 °C

The optical constants (real and imaginary refractive indices) of hexafluorobenzene were determined at 25 °C via transmission measurements. Experimental absorbance spectra measured on a Nicolet Impact 410 FTIR were converted to imaginary refractive indices using methods described in the literature. The real refractive indices were obtained by Kramers–Kronig transformation of the imaginary refractive indices. From the complex refractive indices, the molar absorption coefficient (E_m) and complex molar polarizability spectra were calculated. The integrated intensities for the E_1u fundamentals were obtained from the areas under the bands in the spectrum. These integrated intensities are compared to those for benzene and benzene-d₆ in the literature.     

Oriented polyolefin laminates: A refractometry and infrared study

A series of polypropylene-copolymer laminates produced by biaxial orientation in the 120–155°C range has been characterized by refractometry and infrared (IR) spectroscopy. The copolymer was a random ethylene-propylene resin and the IR techniques included tilted-film transmission as well as internal reflection, both using polarized radiation. The IR techniques yielded spectra corresponding to the machine, transverse, and through directions. Similarly, the refractometry gave refractive indices of both components of each laminate along all three axes. Results from the three techniques were compared and contrasted and the validity of the calculated internal reflectance spectra discussed. Practical methods to estimate the source and extent of errors in the latter technique from false radiation and contact problems were explored.     

Preparation and Optical Characterization of Thick-Film Zirconia and Titania Ormosils

Crack-free films with thicknesses of up to 30 m were prepared by the sol-gel process using the dip-coating technique. Thick films were obtained from various starting solutions based on two, three or four components, with particular emphasis on ternary systems. The ternary systems were composed of two tetraalkoxy precursors (a silicon tetraalkoxide and a zirconium or a titanium tetralkoxide ) and a trialkoxysilane with a non-hydrolyzable group. By using these trialkoxysilanes, the tendency of the films to crack during the drying process is reduced because of the stress absorption by the network structure. The use of zirconia or titania allows for control of the refractive index of the films. Optical parameters of the films including refractive index, thickness, surface roughness and UV-Vis and IR transmission spectra were determined for each composition and the structural characteristics of the films were inferred from the IR spectra. UV cut-off and antireflective properties were also studied for some compositions.     

Hafnia sol-gel films synthesized from HfCl4: Changes of structure and properties with the firing temperature

Thin sol-gel hafnia films have been synthesised from HfCl₄, the synthesis has revealed to be a simple route to fabrication of hafnia films with high transparency in the UV-visible range. The films have been fired at different temperatures in air up to 1000°C and have been characterized by X-ray diffraction and Fourier transform infrared spectroscopy. Infrared absorption spectra of hafnia films have allowed to follow the formation of monoclinic crystalline phases together with XRD. Formation of monoclinic hafnia crystallites has been observed upon calcination at temperatures higher than 600°C, as shown by infrared spectroscopy and XRD. The optical transmission and the refractive index as a function of the temperature of firing have been characterized by UV-Visible spectroscopy and spectroscopic ellipsometry. The hafnia films, after firing at 600°C, had a refractive index of 1.92 with a thickness of around 70 nm.     

IR reflection spectra of monolayer films sandwiched between two high refractive index materials

Monolayer films adsorbed on substrates with high refractive indices such as metals or semiconductors yield strongly enhanced infrared reflection spectra when they are contacted with a transparent, high refractive index ambient medium and are probed with p-polarized light at high incidence angles. The sensitivity increase arises from the enhancement of the perpendicular electric field within a thin, low refractive index layer sandwiched between two high refractive index materials and gives rise to signal intensity gains up to 2 orders of magnitude in combination with an essentially exclusive detection of only perpendicular surface vibrations. Experimental spectra of ordered monolayer films of octadecanethiol on gold and of octadecylsiloxane on silicon in this sandwich configuration yield enhancement factors between 15 (on Si) and 30 (on gold) compared to conventional grazing incidence external reflection spectra and are governed by a common, simple surface selection rule, which allows immediate quantitative evaluation and comparison of the film structures on different substrates.     

Optical determination of the thickness of single planar thin lipid films using the symmetrical three-layer model

The total thickness of thin lipid films can be determined by the method of absolute or relative reflectance measurements. The film is described by the symmetrical three-layer model. By assuming equal refractive indices for the hydrocarbon layer and for the organic solution, the thickness and refractive index of the head group layer can be estimated.     

Polymer nanocomposites with “ultralow” refractive index

Nanocomposites of gelatine and gold were prepared by reduction of AuCl₄H in a gelatine solution, followed by a spin coating process and a brief washing. The thickness of the films thus obtained was typically 200–400 nm. The surface of the nanocomposites was investigated by scanning electron microscopy (SEM) and surface profilometry. The size of the gold particles is in the range of ca. 2–10 nm, as elucidated by transmission electron microscopy (TEM). Refractive indices were measured by ellipsometry at wavelengths of 632.8 and 1300 nm. The refractive index depends linearly on the volume fraction of gold, at least at 1300 nm. The lowest refractive indices measured are 1.008±0.060 at 1300 nm and 0.963±0.062 at 632.8 nm (limits are 95% confidence interval). These values are, to our best knowledge, the lowest reported for a polymer or polymer composite.     

Stress in polyimide coatings

The effect of film thickness on in-plane molecular orientation and stress in polyimide films prepared from pyromellitic dianhydride with 4,4′-oxydianline was investigated using a prism coupling technique to measure the refractive index. Film thickness was controlled by varying both solution concentration and spinning conditions. Birefringence, the difference between the in-plane and out-of-plane refractive indices, was used to characterize the in-plane molecular orientation. The observed birefringence is a combination of the birefringence resulting from molecular orientation and the birefringence induced by the residual stress present in the films. The birefringence decreases with increasing film thickness over the range of thicknesses studied (3–20 μm) indicating that the molecular orientation decreases with increasing film thickness. The in-plane coefficient of linear thermal expansion (CTE), controlled by the level of orientation in the film, increases from 18 to 32 × 10^?6/°C over the same thickness range. The birefringence of free-standing films was lower than that of adhered films due to the release of residual stress in the film once the film is removed from the substrate. The residual film stress arises primarily from the mismatch in CTEs between the polyimide film and the substrate to which the film is adhered. Thus, since the film anisotropy decreases with increasing thickness, the film stress increases with increasing thickness. Residual stress calculated by integrating the product of the film modulus and the CTE mismatch assuming temperature-dependent properties is comparable to experimentally measured film stress. Ignoring the temperature dependence of the film properties leads to an overestimation of stress. Moisture uptake was used to study the stress dependence of the optical properties. Moisture uptake increases both the in-plane and out-of-plane refractive indices by equal amounts in free-standing films due to an isotropic increase in the polarizability. In adhered films, an increase in moisture uptake leads to a decrease in the birefringence due to a swelling-induced decrease in the residual film stress. © 1994 John Wiley & Sons, Inc.     

Yb∶YAB晶体的生长及性质

研究了ＹｂｘＹ１－ｘＡｌ３（ＢＯ３）４（Ｙｂ∶ＹＡＢ）晶体的生长及光学性质,结果表明,以Ｋ２Ｍｏ３Ｏ１０作为助熔剂生长Ｙｂ∶ＹＡＢ晶体效果较好,根据测定的Ｙｂ∶ＹＡＢ折射率计算的一类位相匹配角为θｍ（Ⅰ）＝３４°１２′,与实验测定值θｍ（Ⅰ）＝３４°３８′吻合较好。该晶体室温吸收谱在９５６和９７５ｎｍ处存在两个吸收带,适于ＩｎＧａＡｓ泵浦;室温π偏振吸收比σ偏振吸收略强;１０３和１０４μｍ处各存在一荧光峰,１０３μｍ处荧光寿命为１３７９ｍｓ;估算的发射截面数量级为１０－２１ｃｍ２     

Preparation and Refractive Index of Nano BST Thin Films

"Radio frequency magnetron sputtering technique is used to deposit Ba0:65Sr0:35TiO3 (BST) thin films on fused quartz substrates. In order to prepare the high quality BST thin films, the crystallization and microstructure of the films were characterized by X-ray diffraction, field emission scanning electron microscopy and atom force microstructure. The more intense characteristic diffraction peaks and better crystallization can be observed in BST thin films deposited at 600 oC and subsequently annealed at 700 oC. The refractive index of the films is determined from the measured transmission spectra. The dependences of the refractive index on the deposition parameters of BST thin films are different. The refractive index of the films increases with the substrate temperature. At lower sputtering pressure, the refractive index increases from 1.797 to 2.179 with the pressure increase. However, when the pressure increases up to 3.9 Pa, the refractive index instead reduces to 1.860. The oxygen to argon ratio also plays an important effect on the refractive index of the films. It has been found that the refractive index increases with the ratio of oxygen to argon increasing. The refractive index of BST thin films is strongly dependent on the annealing temperature, which also increases as the annealing temperature ascends. In one word, the refractive index of BST thin films is finally affected by the films microstructure and texture."     

Determination of optical constants and thickness of passivating films on nickel from multiple-angle-of-incidence reflectivity measurements

A method is proposed which allows to determine without non-optical assumptions the complex refractive index and the thickness of thin surface films from measurements of the relative changes of reflectivity due to the formation or removal of the film at different angles of incidence with monochromatic light polarized perpendicular or parallel to the plane of incidence. The exact equations for the reflectivity change are used, since the linear approximation introduces systematic errors which are too large in view of the experimental accuracy. The method is applied to the passivating film formed on nickel in 1 M sulphuric acid after 1 h at U_HSS=0.85 V. A complex refractive index n₂=2.10 (±0.05)–0.46 (±0.02) i and a film thickness d=2.1 (±0.1) nm were found. In the spectrum of the optical constants in the range 1.4≤hv/e V≤4.8 a characteristic peak of the absorption coefficient was observed at 3.1 e V. Both the values at γ=500 nm and the spectra of the optical constants are explained by a partly hydrated nickel oxide film.     

Structural Tailoring of Monomolecular Films on Evaporated Silver Surface

Monomolecular films with well-defined structure were prepared by self-assembling of n-alkanoic acids on an evaporated silver surface. Here we report the structural studies of thin films of acids containing aromatic chromophores of varied size and polarity. Information regarding the packing, orientation and conformation of the structural units within the assembly were deduced from ellipsometric thickness measurement, polarized reflection absorption infrared spectra and wetting property measurement. Implications of results to the design of functional thin films are discussed.     

Quasi-isotropic analysis of anisotropic thin films on optical waveguides

Thin films assembled on a substrate are often anisotropic. Nevertheless, because of experimental limitations, sufficient parameters to characterize the anisotropy, even in the simplest (and perhaps most common) case of uniaxial thin films, which are birefringent, are not usually available. This paper examines the consequences of treating them as isotropic thin films, with particular reference to their characterization via perturbation of the propagation constants (effective refractive indices) of optical waveguides. It is shown that the refractive index and geometrical thickness of a thin film thus calculated are often unrealistic (especially when the thin film is positively birefringent), but the mass per unit area may be quite precise, depending on the sign and magnitude of the birefringence.     

Optical anisotropy of polymeric films measured by waveguide propagation mode determination

Extruded thin films of a liquid-crystalline charge-conjugated rigid-rod polymer poly(p-phenylenebenzobisthiazole), PBT, and a semicrystalline thermoplastic polyethylene-terephthalate (Mylar) were fabricated and examined for film thickness, refractive index, and linear attenuation coefficient. Optical waveguide modes were successfully induced on the polymeric films using a prism coupler at λ = 633 and 1300 nm. Highly consistent thickness values were obtained for the polymeric films. In addition, the anisotropic nature of the optical properties was determined using TE and TM propagation modes. A refractive index as high as 2.3 was observed on PBT film. The refractive index data suggested that the PBT and Mylar films were optically anisotropic with refractive indices n? (out-of-plane) invariably smaller than n∥ (in-plane). Large anisotropy was also discovered in the linear attenuation coefficient α, with α_∥/α_? ≈ 50 for the Mylar films. Complementary polarimetric and spectroscopic interference measurements were also applied to investigate the optical anisotropy of the extruded polymeric films. © 1992 John Wiley & Sons, Inc.     

Chain orientation and anisotropies in optical and dielectric properties in thin films of stiff polyimides

Thin films of poly(p-phenylene biphenyltetracarboximide) (BPDA-PDA), prepared by thermal imidization of the precursor poly(amic acid) on substrates, have been investigated by optical waveguide, ultraviolet-visible (UV-VIS), infrared (IR), and dielectric spectroscopies. The polyimide films exhibit an extraordinarily large anisotropy in the refractive indices with the in-plane index n_∥ = 1.806 and the out-of-plane index n_⊥ = 1.589 at 1064 nm wavelength. No discernible effect of the film thickness on this optical anisotropy is found between films of ca. 2.1 and ca. 7.8 μm thickness. This large birefringence is attributed to the preferential orientation of the biphenyltetracarboximide moieties with their planes parallel to the film surface, coupled with the strong preference of BPDA-PDA chains to align along the film plane. The frequency dispersion of the in-plane refractive index n_∥ is consistent with the results calculated by the Lorentz–Lorenz equation from the UV-visible spectrum exhibiting several absorption bands in the 170–500 nm region. The contribution from the IR absorption in the range 7000–400 cm,^?1 computed by the Spitzer-Kleinmann dispersion relations from the measured spectra, adds ca. 0.046 to the in-plane refractive index n_∥. Tilt-angle–dependent polarized IR results indicate nearly the same increase for the out-of-plane index n_⊥. Application of the Maxwell relation then leads to the out-of-plane dielectric constant ε^⊥ ? 2.7 at 1.2 × 10¹³ Hz, as compared with the measured value of ca. 3.0 at 10⁶ Hz. Assuming this small difference to remain the same for the in-plane dielectric constants ε_∥, we obtain a very large anisotropy in the dielectric properties of these polyimide films with the estimated in-plane dielectric constant ε_∥ ? 3.4 at 1.2 × 10¹³ Hz, and ε_∥ ? 3.7 at 10⁶ Hz. © 1992 John Wiley & Sons, Inc.