Optical Coatings Research Articles

Stable Method for Optical Monitoring the Deposition of Multilayer Optical Coatings

For optical monitoring of layer thickness in the deposition of multilayer optical coatings, a stable method is proposed that completely eliminates the cumulative effect of errors in the thicknesses of deposited layers. The considered monitoring method relies on a nonlocal algorithm for analyzing data measured in the course of coating deposition monitoring. Computer simulation of coating deposition is used to demonstrate the effectiveness of the proposed type of monitoring as compared with other optical monitoring methods.

Correlation of Errors in Monochromatic Monitoring of Optical Coatings Deposition

Correlation of Errors in Monochromatic Monitoring of Optical Coatings Deposition

Thin‐Metal‐Film‐Based Transparent Conductors: Material Preparation, Optical Design, and Device Applications

AbstractTransparent conductors are essential elements in an array of optoelectronic devices. The most commonly used transparent conductor – indium tin oxide (ITO) suffers from issues including poor mechanical flexibility, rising cost, and the need for annealing to achieve high conductivity. Consequently, there has been intensive research effort in developing ITO‐free transparent conductors over the recent years. This article gives a comprehensive review on the development of an important ITO‐free transparent conductor, that is based on thin metal films. It starts with the background knowledge of material selection for thin‐metal‐film‐based transparent conductors and then surveys various techniques to fabricate high‐quality thin metal films. Then, it introduces the spectroscopic ellipsometry method for characterizing thin metal films with high accuracy, and discusses the optical design procedure for optimizing transmittance through thin‐metal‐film‐based conductors. The review also summarizes diverse applications of thin‐metal‐film‐based transparent conductors, ranging from solar cells and organic light emitting diodes, to optical spectrum filters, low‐emissivity windows, and transparent electromagnetic interference coatings.

Forward Stimulated Brillouin Scattering Analysis of Optical Fibers Coatings

The proper function of protective coating layers is essential for the handling and application of brittle optical fibers. The elastic parameters of polymer coatings can be studied through off-line analysis of test samples. However, the monitoring of these properties on a working fiber during service is challenging. In this work, we use forward stimulated Brillouin scattering processes in standard single mode fibers to measure the acoustic velocity in several types of coating layers. Pump light launches short acoustic pulses outward from the core of the fiber. Multiple reflections at the boundaries between cladding and coating, and between coating and air, form a series of delayed acoustic echoes across the core. These echoes are monitored, in turn, by photo-elastic phase modulation of probe light. Data are collected at temperatures between 25-120 °C. The thermal dependence of the acoustic velocities in several coatings and of the F-SBS resonance frequencies is investigated. Observations are corroborated by calculations. The proposed technique is well suited for research and development of coating materials, production line quality control, reliability studies and preventive maintenance of working fibers.

Special Issue: “Optical Thin Films and Structures: Design and Advanced Applications”

This Special Issue is devoted on design and application of thin films and structures with special emphasis on optical applications. It comprises ten papers, five featured and five regular papers, authored by respective scientists all over the world. Diverse materials are studied and their possible applications are demonstrated and discussed: transparent conductive coatings and structures from ZnO doped with Al and Ga and Ti-doped SnO2, polymer and nanosized zeolite thin films for optical sensing, TiO2 with linear and non-linear optical properties, organic diamagnetic materials, broadband optical coatings, CrWN glass molding coatings and silicon on insulator waveguides.

Tunable Emission in Lanthanide-SURMOFs Heteroepitaxial Thin Films

By using a layer-by-layer (LbL) approach, we have fabricated lanthanide-based, monolithic metal-organic framework (MOF) thin films for optical applications. The thickness of the Ln-SURMOFs can be easily controlled, thereby maintaining its transparent properties, which makes them attractive for the field of optical coatings. Moreover, employing a hetero-epitaxy process, we demonstrated that Ln-SURMOF can be deposited on the top of other Ln-SURMOFs with different lanthanide ions, maintaining the crystallinity and orientation. The hetero-epitaxial multilayer architectures of Ln-SURMOF reduced the efficient energy transfer between different lanthanides ions and optimized the fabrication of Ln-MOF thin film with a straightforward modulation of the emission color. Meanwhile, the distance between lanthanide ions in Ln-SURMOFs was extended by replacing the linkers. These novel hetero-epitaxial Ln-SURMOF architectures, possess a bright future in the field of optics, photonics and optoelectronics.

Atomic layer deposition of Al2O3 and HfO2 for high power laser application

Atomic layer deposition has promising application in optical coatings because of the self-terminating feature. An ideally self-terminating ALD process is based on the saturate chemi-sorption and complete purging of the desorbed precursor molecules. The kinetics of adsorption and desorption are analyzed, providing instruction for the development of ALD process. The exposure length is related to the partial pressure and molecule mass, while the purging length is dependent on the physi-sorption potential energy and substrate temperature. The GPC versus pulse length of ALD Al2O3 and HfO2 are measured. With a set of properly selected parameters, film layers with distinctive self-terminating feature are obtained. A coating consisting of several nanolaminates is analyzed with TEM to study the ALD film structure. An anti-reflection coating is deposited, controlling the film thickness by counting the cycles. The laser resistance of the ALD Al2O3 film, HfO2 film and anti-reflection coating is studied. The results certify the analysis and indicate a promising application in developing a self-terminating ALD process for a new material on a new plant.

Nano- and micro-scale impact testing of zirconia, alumina and zirconia-alumina duplex optical coatings on glass

Optimising the mechanical properties of optical coatings to improve their durability will be critical if they are to be used successfully in harsh environments where they may be subject to degradation by mechanical contact. In this study zirconia, zirconia-alumina duplex and alumina experimental coatings were deposited on soda lime and borosilicate glass and their resistance to repetitive impact under different experimental conditions evaluated in nano- and micro-scale impact tests. The influence of changing probe geometry (sharp and blunt contacts) and applied load on the deformation was studied. Spheroconical indenters were found to be more suitable to study the load sensitivity of the impact response than sharp cube corner indenters. Increased resistance to plastic deformation in the coating-substrate system (H3/E2) proved detrimental to the damage tolerance to the repetitive nano- and micro-impact tests. To compare the deformation behaviour in nano-impact and nano-scratch, tests were performed using the same spheroconical probe, revealing cracking and blistering of the glass substrate in both types of test. The change in probe depth after the first impact was found to be a very useful metric to effectively compare the evolution of surface damage on continued impact in nano- and micro-impact tests at different applied load and probe geometry.

Correlation of errors in inverse problems of optical coatings monitoring

Abstract On-line optical monitoring of multilayer coating production requires solving inverse identification problems of determining the thicknesses of coating layers. Regardless of the algorithm used to solve inverse problems, the errors in the thicknesses of the deposited layers are correlated by the monitoring procedure. Studying the correlation of thickness errors is important for the production of the most complex optical coatings. We develop a general geometric approach to study this correlation. It is based on a statistical analysis of large numbers of error vectors obtained during computational experiments on optical coating production. The application of the proposed approach is demonstrated using computational manufacturing experiments on the production of a 50-layer filter with four different monitoring strategies. A special coefficient is introduced to evaluate the strength of the error correlation effect. The results obtained confirm that the introduced parameter can be used as a measure of the strength of the correlation effect in practical applications.

Photocatalytic Activity of Bilayer TiO<sub>2</sub>/ZnO and ZnO TiO<sub>2</sub> Thin Films

Thin film is a thin material that is resulting from the condensation of species through the deposition of atoms on the substrate. Thin films are usually used in the production of electronic devices, optical coatings, solar cells, and for decorative items. The bilayer of TiO2/ZnO and ZnO TiO2 thin films have some advantages such as can enhance the surface state and surface atomic mobility, which are useful for improving the photocatalytic activity. The motivation to a used double layer of ZnO and TiO2 is to enhance the properties and photocatalytic activity using the different deposition temperature between the layers. The structural of ZnO/TiO2 thin films were studied using X-Ray diffraction (XRD). Field Emission Scanning Electron Microscope (FESEM) was used to determine the surface morphology of ZnO/TiO2 thin films. The photocatalytic activity of ZnO/TiO2 thin films was analysed using the photodegradation of methylene blue (MB) solution. The XRD analysis revealed that highest anatase crystalline phase for TiO2 growth with orientation (1 0 1), while the ZnO crystal phase, zincite occurred at the highest intensity with (1 0 1) orientation.. The bilayer TiO2/ZnO thin film had the highest reaction rate, K, which is 0.0972 h-1 for photocatalytic activity. The characteristics of bilayer TiO2/ZnO and ZnO/TiO2 thin-film is strongly influenced by the calcination temperature and the presence and combination between the two types of materials.

МЕТОД ВИЗНАЧЕННЯ ДІЕЛЕКТРИЧНОЇ ПРОНИКНОСТІ ДІЕЛЕКТРИКІВ У ММ ТА СУБММ ДІАПАЗОНАХ ДОВЖИН ХВИЛЬ НА ПІДСТАВІ ВИМІРЮВАННЯ ПАРАМЕТРІВ ПЛАЗМОН-ПОЛЯРИТОННОГО РЕЗОНАНСУ

МЕТОД ВИЗНАЧЕННЯ ДІЕЛЕКТРИЧНОЇ ПРОНИКНОСТІ ДІЕЛЕКТРИКІВ У ММ ТА СУБММ ДІАПАЗОНАХ ДОВЖИН ХВИЛЬ НА ПІДСТАВІ ВИМІРЮВАННЯ ПАРАМЕТРІВ ПЛАЗМОН-ПОЛЯРИТОННОГО РЕЗОНАНСУ

Thermal stability of optical fiber coatings: comparison of experimental thermogravimetric approaches

Thermal decomposition of poly(methyl methacrylate) (PMMA) and four polymer coatings on optical fibers (single acrylate, dual acrylate, polyimide and a hard silicone) was studied via thermogravimetry (TG). Of main interest was the upper use temperature of the fibers, which can be estimated for a given continuous use time, assuming a certain failure criterion. Arrhenius parameters of the decomposition reaction (the activation energy and the pre-exponential factor) were determined via several model-free approaches: isothermal, Modulated TG (MTG) and four non-isothermal isoconversional approaches. The isothermal approach was considered as a benchmark for the other approaches. The MTG results disagreed markedly with the isothermal ones, and the discrepancy appears unrelated to the choice of the decomposition kinetics model. The non-isothermal isoconversion methods provide closer agreement with the isothermal data. Still, in a few examples, substantial discrepancies between the isothermal and non-isothermal data were observed. It is concluded that the isothermal approach is the best one to use for model-free determination of the Arrhenius parameters from TG. The temperatures for observation of the isothermal kinetics should be selected such that the preheating time is short in comparison with the decomposition time of the material under investigation.

Thin SiNC/SiOC Coatings with a Gradient of Refractive Index Deposited from Organosilicon Precursor

In this work, optical coatings with a gradient of the refractive index are described. Its aim was to deposit, using the RF PECVD method, films of variable composition (ranging from silicon carbon-oxide to silicon carbon-nitride) for a smooth change of their optical properties enabling a production of the filter with a refractive index gradient. For that purpose, two organosilicon compounds, namely tetramethyldisilazane and hexamethyldisilazane, were selected as precursor compounds. The results reveal better optical properties of the materials obtained from the latter source. Depending on whether deposited in pure oxygen atmosphere or under conditions of pure nitrogen, the refractive index of the coatings amounted to 1.65 and to 2.22, respectively. By using a variable composition N2/O2 gas mixture, coatings of intermediate magnitudes of “n” were acquired. The optical properties were investigated using both UV-Vis absorption spectroscopy and variable angle spectroscopic ellipsometry. The chemical structure of the coatings was studied with the help of Fourier transform infrared and X-ray photoelectron spectroscopies. Finally, atomic force microscopy was applied to examine their surface topography. As the last step, a “cold mirror” type interference filter with a gradient of refractive index was designed and manufactured.

TiO2 - based decorative interference coatings produced at industrial conditions

The color of material is one of its important feature and can be produced using optical interference coatings consisting of dielectric film (upper layer) and substrate (mostly bulk metal plate or metallic layer). These coatings are alternative to layers prepared based on paints or lacquers. In the contrast to the lacquer, the interference coatings are significantly thinner and, apart from the color effect, they can fulfill the other functions (e.g. protective, anti-corrosion, self-cleaning). The colors of interference coatings are significantly sensitive to variations in the thickness of their individual components. Optical constants of the dielectric film and metallic substrate are also essential. Because of desirable properties of particular films, to effective production of the decorative interference systems the suitable technology (providing high quality of layers and uniformity of their properties) is required. Currently, magnetron sputtering is an environmentally friendly technology used in the industrial scale, allowing the production of coatings either on a large number of details or on large surfaces with maintaining of their repeatability. In addition to the concept of interference coatings and materials used, we would like to present the results obtained for layers deposited at industrial conditions on a large-scale glass with a fascinating narrow thickness tolerance.

Quantitative investigation of laser-induced damage fatigue in HfO2 and ZrO2 single layer coatings.

The decrease of laser-induced damage threshold (LIDT) of optical materials when irradiated with multiple laser pulses is an important phenomenon commonly known as the optical fatigue effect. In case of pulsed femtosecond irradiation fatigue is usually attributed to incubation of laser-induced lattice defects. In this study, standard S-on-1 LIDT test was complimented with in situ time-resolved digital holographic microscopy (TRDHM) to quantitatively investigate fatigue of catastrophic damage for HfO2 and ZrO2 single layer ion-beam-sputtered optical coatings. It was identified that ablation (critical damage) was preceded by exponential increase in optical path length visible as positive phase shift (subcritical damage). Atomic force microscopy was used to show that physical damage originates as localized 100 nm wide nanogrooves perpendicular to laser polarization. A novel link was established between LIDT fatigue and mechanical fatigue crack growth from cyclic loads which allowed construction of a unified numerical fatigue model that reproduced both S-on-1 and TRDHM experimental data.

Low-temperature synthesis of tetragonal phase of hafnium oxide using polymer-blended nanofiber precursor

Hafnium oxide (HfO2), which is known as hafnia, is considered one of the best materials for various future applications such as neuromorphic computing systems and optical coatings. Although tetragonal phase (t-phase) in the multiple crystallographic structures of HfO2 outperforms monoclinic phase (m-phase) most common phase that can be formed at low temperature in physicochemical properties such as dielectric constant, bandgap, high resistance to corrosion and hardness, it is extremely hard to stabilize t-phase below 1670 °C. Herein, we synthesized oxygen-deficient tetragonal hafnium oxide nanofibers (t-HfO2–x NFs) using the electrospinning technique, which included the polymer-blended precursors and a magnesium-thermic reaction at a low processing temperature (<600 °C). The small grain size (<10 nm) of the t-HfO2–x NFs and oxygen vacancies (Voxygen) synergistically stabilized t-phase in the nanofibrous structure. Oxygen reduction reaction (ORR) and oxygen evolution reaction (OER) performances of the t-HfO2–x NFs in alkaline and acid media were evaluated as a potential application, which were higher than those of the m-HfO2 NFs. Our straightforward approach to make t-phase in HfO2-based materials can be broadly applied for many future applications.

Smart gas dosage by a peristaltic pump for reactive RF sputtering of composition spread combinatorial hafnium-oxy-nitride layers

Thin metal-oxy-nitride layers are widely used for wavelength selective optical coatings. We aimed to prepare composition spread metal-oxy-nitride combinatorial layers by reactive radiofrequency sputtering for a highly efficient characterization of their optical properties in a wide range of O/N ratios. Affinity of metals requires oxygen proportion, in the Ar-O-N plasma, be set orders of magnitude lower compared to nitrogen. The required low partial pressure range of oxygen was covered by a new concept self-regulating gas inlet assembly based on a peristaltic pump delivering gas from a finite volume vial. Accordingly, oxygen partial pressure was gradually decreasing from 8 × 10−5 mbar towards the base pressure of the chamber. It effected a continuous oxygen depletion and a transition of the deposited film from oxide to nitride. The suitability of our combinatorial method was proved by both energy dispersive spectrometry mapping and ellipsometry of the correlated refractive indices along a composition-spread HfON sample.

Microfluidic synthesis of robust carbon dots-functionalized photonic crystals

Robust hybrid microbeads with multiple optical signals have garnered widespread interests for various applications. However, the inefficient physical combination and the potential toxicity of raw materials (heavy metal-based quantum dots) have limited their sustainable development. Herein, we report a microfluidic strategy to achieve the continuous production of environmentally friendly carbon dots (CDs)-functionalized photonic crystal (PC) microbeads. The efficient mass and heat transfer on microscales and the covalent bonding crosslink between CDs and poly (methyl methacrylate-butyl acrylate-methacrylic acid) (P(MMA-BA-MAA)) colloid particles ensure effective assembly of CDs-P(MMA-BA-MAA) PC microbeads in the microchannel. Benefiting from this, the CDs-P(MMA-BA-MAA) hybrid microbeads behave stable fluorescence properties, together with highly saturated and angle-independent structural color, which can overcome the optical information confusion and distortion. Meanwhile, the obtained CDs-P(MMA-BA-MAA) hybrid microbeads show dual optical signal responsiveness through the variation of fluorescence and structural color under different ionic environment. Based on this, we realize the dual ionic sensing by constructing CDs-P(MMA-BA-MAA) hybrid microbeads arrays, which broadens the application of optical hybrid microbeads. In addition, we also prepare robust hybrid film and flexible lamp belt. The obtained robust CDs-functionalized PC hybrid microbeads with various attractive optical properties have a broad prospect in optical display, sensors, biological coding and antireflection coatings.

Embossed Mie resonator arrays composed of compacted TiO2 nanoparticles for broadband anti-reflection in solar cells

Mie resonator arrays formed by embossing titanium dioxide (TiO2) nanoparticles (NPs) from solution are investigated as optical coatings for anti-reflection applications. Compacted nanoparticle assemblies offer unique possibilities to tailor the effective refractive index (RI). Here, we demonstrate a simple table-top, low pressure, and low temperature method to fabricate structured optical coatings. TiO2 nanostructures in the form of nanodisks support Mie resonances in the visible wavelength spectrum and exhibit strong forward scattering into the high index substrates, making them suitable as broadband anti-reflection coatings for solar cells. TiO2 NP-based nanodisk arrays are designed, fabricated, and characterized regarding their anti-reflection properties on Si, GaAs, and InP substrates and solar cells. Detailed finite-difference time-domain simulations are performed to optimize the TiO2 NP-based Mie resonator arrays for the broadband anti-reflection as well as to explain the measured reflectance spectra. The solar-weighted reflectance is used as a figure of merit (FoM). TiO2 nanodisk arrays on Si show a FoM of ~ 7% in the 400–1,100 nm wavelength spectrum; similar values are obtained for GaAs and InP substrates. TiO2 nanodisk arrays embossed directly on prefabricated planar single-junction Si, GaAs, and InP solar cells result in an appreciable increase (~ 1.3 times) in the short-circuit current densities.

Проєктування просвітлюючих оптичних покриттів для широкого спектрального діапазону при падінні світла під кутом

Проєктування просвітлюючих оптичних покриттів для широкого спектрального діапазону при падінні світла під кутом