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Hafnium dioxide (HfO 2) thin films are generally used as high refractive index layers in optical coating devices. Aging effects on optical and mechanical properties of the films are very crucial for long-standing performance and stability of the devices, therefore knowledge of the aging effects is very important. In the present work, HfO 2 thin film has been prepared using electron beam evaporation. The optical properties and residual stress of the film have been investigated in six months interval over a period of one-year in ambient condition to probe its aging behaviour. The measured refractive index increases with aging and its change is Δ n = 0.011 at wavelength of 550 nm. The dispersion parameters such as ε L , N / m *, ε s , and E d increase with aging, while E 0 decreases. The measured film residual stress is tensile in nature. The residual stress decreases with aging and the change in stress in the film is around 78% in a year. A simple model has been utilized to establish and explain a correlation between the measured stress and the film porosity which varies with aging time. The results suggest that the change in the residual stress is significant in contrast to a very small change in the refractive index with aging. [ABSTRACT FROM AUTHOR]

In present work, structural, morphological and optical properties of glancing angle deposited (GLAD ~ 82°) ZrO 2 thin films by RF magnetron sputtering at different substrate rotation (ω) have been investigated and compared with normally deposited (ND ~ 0°) ZrO 2 films. Both GLAD and ND ZrO 2 films exhibit preferential structural growth of monoclinic phase oriented in different directions. GLAD films also depict a tetragonal peak which has been attributed to the presence of fine nano-crystallite size (~ 13 nm). Surface correlation length of such films decreases with the increase in ω. The decrease has been attributed to the change in columnar microstructure with ω. RMS roughness values for GLAD and ND ZrO2 films are 4.6–5.1 nm and 1 nm, respectively. Dominant atomic shadowing is responsible for high roughness of GLAD films. Refractive index derived from ellipsometric measurements manifests a decreasing trend with parameter, ω. The trend has also been explained in terms of varying columnar micro-structure. Refractive index for GLAD ZrO 2 films varies from 1.901 and 2.011 on varying ω from 0.50 to 4 rpm. ND ZrO 2 film exhibits refractive index value of 2.178 which is substantially greater than that of GLAD films. Lowering in refractive index of GLAD films has been attributed to dominant atomic shadowing at glancing angles. Residual stress switches from large compressive to small tensile as the deposition angle changes from 0° to 82°. For GLAD films, tensile stress increases with the increase in ω except for the film deposited at the highest substrate rotation. Lowering in stress for GLAD films compared to ND films and trend of stress with ω have been explained in terms of varying inter-molecular forces with inter-columnar distance. [ABSTRACT FROM AUTHOR]

Oblique angle deposited oxide thin films, in which refractive index profiles can be tailored across depth by tuning their microstructure using varying angle of deposition, have opened up new dimensions in fabrication of optical interference devices. Since surface morphology plays an important role for the qualification of these thin film devices for optical or other applications, it is important to investigate morphological properties of obliquely deposited thin films. In the present work, a set of HfO 2 thin films have been deposited at several oblique angles. Morphological parameters of such thin films viz., correlation length, intrinsic roughness, fractal spectral strength, etc., have been determined through suitable modelling of extended power spectral density measured by atomic force microscopy. It has been found that intrinsic roughness and fractal spectral strength show an interesting behaviour with deposition angle and the same has been discussed in the light of atomic shadowing and re-emission and diffusion of ad-atoms. Further refractive index and thickness of such thin films have been estimated from optical transmission spectra through suitable modelling. Refractive index of such thin film varies from 1.93 to 1.37 as the deposition angle varies from normal to glancing angle (80°). Further, refractive index and grain size depict an opposite trend with deposition angle. This variation has been explained in terms of varying film porosity and column slanting with angle of deposition. [ABSTRACT FROM AUTHOR]

One dimensional photonic crystal (1DPC) of TiO 2 /SiO 2 multilayer has been fabricated by sequential asymmetric bipolar pulsed dc magnetron sputtering of TiO 2 and radio frequency magnetron sputtering of SiO 2 to achieve wide omnidirectional photonic band in the visible region. The microstructure and optical response of the TiO 2 /SiO 2 photonic crystal have been characterized by atomic force microscopy, scanning electron microscopy and spectrophotometry respectively. The surface of the photonic crystal is very smooth having surface roughness of 2.6 nm. Reflection and transmission spectra have been measured in the wavelength range 300 to 1000 nm for both transverse electric and transverse magnetic waves. Wide high reflection photonic band gap (∆ λ = 245 nm) in the visible and near infrared regions (592–837 nm) at normal incidence has been achieved. The measured photonic band gap (PBG) is found well matching with the calculated photonic band gap of an infinite 1DPC. The experimentally observed omnidirectional photonic band 592–668 nm (∆ λ = 76 nm) in the visible region with band to mid-band ratio ∆ λ/λ = 12% for reflectivity R > 99% over the incident angle range of 0°–70° is found almost matching with the calculated omnidirectional PBG. The omnidirectional reflection band is found much wider as compared to the values reported in literature so far in the visible region for TiO 2 /SiO 2 periodic photonic crystal. [ABSTRACT FROM AUTHOR]

• TiO 2 /Ag/TiO 2 /Ag multilayer is prepared using asymmetric bipolar pulsed dc sputtering. • Annealing induced changes in transmission spectrum of the multilayer is investigated. • Reverse engineering of the spectrum is made to analyse and explain the observations. • Three different multilayer models are executed for reverse engineering of the spectrum. • Model suggests that interface roughness and absorption due to Ag varies with annealing. Measured optical transmission spectrum of photonic structure comprising TiO 2 /Ag/TiO 2 /Ag multilayer is reverse engineered by employing transfer matrix method with three different multilayer models to analyse the factors responsible for the annealing induced changes in the transmission. The multilayer is made to have a single peak or a passband in the visible region, so that the spectral changes due to annealing can be observed more vividly. Prior to the multilayer deposition, single layer thin film of Ag and TiO 2 are prepared using asymmetric bipolar pulsed dc sputtering and their optical constants such as refractive index and extinction co-efficient are determined. The multilayer is prepared by four-layer sequential deposition of Ag and TiO 2 on BK7 glass substrate. The deposited TiO 2 /Ag/TiO 2 /Ag multilayer is annealed in the temperature range 100–300 °C. The optical transmission of the annealed multilayers is measured using spectrophotometer. The measured peak transmission and spectral bandwidth of the multilayer filter decreases with annealing, which is ascribed to inter-diffusion at the interfaces, and increasing absorption at the interfaces and in the silver layers of the multilayer. The ascription is resolved from the layers properties and interface parameters obtained by reverse engineering the measured transmission spectra. The rough interfaces in the multilayer are modeled as thin homogeneous layers following Bruggeman effective medium approximation, and the interface parameters are found to be varying with annealing. [ABSTRACT FROM AUTHOR]

Abstract: Thin films of composite materials are being progressively explored for achieving tunability in the optical constants for application in multilayer optical devices. In the present study, a set of ZrO2–MgO binary mixed composite thin films have been prepared by reactive electron beam evaporation of solid solution of ZrO2 and MgO at different oxygen partial pressures. Since elastic properties of the thin films are very important for their environmental stability under high power laser application, elastic moduli (indentation moduli) of the films have been measured by atomic force acoustic microscopy measurements. The optical properties especially refractive index of such films has been determined from the optical transmission measurement using an inverse synthesis method, while the density of such films has been measured by grazing incidence X-ray reflectivity. The variation of the elastic moduli of the thin films as a function of oxygen partial pressure used during deposition has been studied and the above variation has been corroborated with the variation of density and refractive index of the thin films. [Copyright &y& Elsevier]

Abstract: Composite thin films of HfO2:SiO2 with wide range of relative composition from 100:0 (pure HfO2) to 10:90 have been deposited on fused silica substrates by co-evaporation technique and the optical properties of the films have been studied by measuring the transmission spectra of the samples by spectrophotometer. Different important optical parameters viz., band gap, refractive index and absorption coefficients of the samples have been obtained by fitting the measured optical spectra with theoretically generated spectra and the variation of the optical constants as a function of SiO2 content in the films have been obtained. Two different dispersion models viz., the single effective oscillator model and the Tauc–Lorentz model have been used to generate the theoretical spectra in the above fitting procedure. X-ray reflectivity (XRR) measurement technique has been used to find the densities of the films in order to explain the observed variation in optical properties of the films with increase in SiO2 content. [Copyright &y& Elsevier]

Abstract: A comparative morphology, grain structure and optical properties studies of reactive electron beam co-evaporated mixed thin films of hafnia–silica (HfO2:SiO2) and zirconia–silica (ZrO2:SiO2) systems have been carried out using atomic force microscopy and phase modulated ellipsometry. The addition of silica, especially with small fractions, has demonstrated altogether different types of evolutions in the microstructure and morphology in these binary thin film systems possibly forming new solid solution phases. Such morphological evolutions are probed through RMS roughness, power spectral density, height–height correlation and autocorrelation analyses of the topographic data acquired through atomic force microscopy. The present investigations indicated that with composition-control morphological quality improvement is more favourable in composite hafnia–silica over the zirconia–silica films. So for ultraviolet optical coating applications which demand low light scattering thin film microstructure, hafnia–silica binary composite system has a definite edge over the zirconia–silica counterpart. [Copyright &y& Elsevier]

Abstract: Tailoring of the refractive index of optical thin films has been a very fascinating as well as challenging topic for developing new generation optical coatings. In the present work a novel Gd2O3/SiO2 composite system has been experimented and probed for its superior optical properties through phase modulated spectroscopic ellipsometry, spectrophotometry and atomic force microscopy. The optical parameters of the composite films have been evaluated using Tauc–Lorentz (TL) formulations. In order to derive the growth dependent refractive index profiles, each sample film has been modeled as an appropriate multilayer structure where each sub-layer was treated with the above TL parameterizations. All codeposited films demonstrated superiority with respect to the band gap and morphological measurements. At lower silica mixing compositions such as in 10–20% level, the composite films depicted superior spectral refractive index profile, band gap as well as the morphology. This aspect highlighted the fact that microstructural densifications in composite films can override the chemical compositions while deciding the refractive index and optical properties in such thin films. [Copyright &y& Elsevier]

Highlights: [•] SEIRA of uracil on AuNp film/glass slide using ATR geometry. [•] The deposition time of AuNp on glass slides varied to see its impact on SEIRA. [•] The small shift in SEIRA spectra indicates weak interaction of molecules with AuNp. [Copyright &y& Elsevier]

Abstract: Diffuse reflectance or optical scattering in thin films and multilayers can pose serious limiting factors to their desired or ultimate performances. Besides, such studies provide valuable information related to the buried microstructures and interfaces. Synchrotron radiation is the most appropriate source to record wavelength dependent polarized light scattering in thin films and multilayers. In the present experiment several gadolinia, silica thin films and multilayers were studied for their light scattering using the white light synchrotron beam. Various thin film layer geometries were selected to probe the results due to different types and combinations of interfaces. Due to phase coherent delay in certain optical non-wedged component used in the experimental setup very interesting spectral interference were noticed as the modulations in the diffuse reflectance signal. Appropriate modeling approach utilizing Gaussian function de-convolution technique is used to compute the pulse delay between the back reflected and forward propagating scattering signals that lead to such spectral interference. Alternatively inverse fast Fourier transform (IFFT) and analytical techniques were adopted to determine the group delays. The diffuse reflectance spectra were finally compared with their specular counter part and a shift both in the wavelength and phase were noticed. This may be explained on the basis of thin film roughness factors, different polarizations and incident geometries used in the measurements. However, all most all the spectral scattering signals exhibited the features resembling their specular reflection characteristic. [Copyright &y& Elsevier]

Abstract: Composites are pragmatic choices for tailoring the material to have a desired property. Besides, such thin films have scopes to display superior optical, microstructural and morphological properties which are otherwise not possible to obtain from the pure component films. Vapor-phase-mixed binary composite Gd2O3/SiO2 thin film is one such interesting system where band gap as well as refractive index superiority is observed simultaneously under certain compositional mixings. Such and similar observations in composites cannot be explained by Moss empirical rule. Our systematic study on the microstructure of this composite system based on ellipsometry and scanning probe microscopy has satisfactorily provided the information that can explain such optical properties supremacy. Morphological measurements and its derived parameters like autocorrelation and height–height correlation functions have provided several clues that represent the superior grain structures of the composites. Besides, refractive index modeling through effective single oscillator model has strongly supported such analysis results favoring the superior microstructure in composite films. [Copyright &y& Elsevier]

Abstract: Composite optical thin-film materials have received a significant amount of interest in order to relieve the material constraints on refractive indices as well as reducing the number of layers required in optical coating design. Amongst others binary zirconia–silica composite thin films have attracted considerable attentions due to their several favorable opto-mechanical properties. In the present studies such a composite system under certain compositional mixings displayed both refractive index and band gap supremacy over pure zirconia films violating the most popular Moss rule. This unexpected evolution has several practical applications one of which can be directly employed in extending the range of tunability of the refractive index. Besides, the probing of such a novel evolution through the analysis of ellipsometric refractive index modeling and morphological correlation functions has revealed several novel as well as superior microstructural properties in the composite thin film systems. All these characterization and analysis techniques distinctly indicate a strong interrelation between the microstructural ordering and superior optical properties of the present zirconia–silica codeposited composites. [Copyright &y& Elsevier]

Abstract: Characteristic topographic parameters like fractals and superstructures contribute substantially to the thin film morphology, which directly or indirectly influences the physical, optical and mechanical properties. Fractal geometry and scaling concepts can concisely as well as more effectively describe the complex rough surface morphology. In the present study, such microstructural analysis has been carried out for gadolinia thin films, a most promising candidate to develop multilayer optical coatings in the deep ultraviolet spectral region. Extended power spectral density (PSD) spectra for these films acquired through atomic force measurements have been modeled and analyzed using several thin film morphological functions appropriately modified to suit the present requirements. Analysis results of the PSD functions have distinctly indicated the presence of Brownian fractals and uniformly distributed superstructures in the most experimental films deposited through reactive electron beam deposition technique. Process variables such as the rate of deposition and the oxygen partial pressure have affected the microstructural and related morphological evolutions very differently. The rate parameter strongly influenced the intrinsic microroughness, whereas the oxygen pressure influenced the grain sizes of the gadolinia films. One can advantageously use such information in tailoring a desired surface morphology in thin films by choosing appropriate combinations of process variables. [Copyright &y& Elsevier]