Optical Interference Coatings Research Articles

A-Si/SiO2 nanolaminates for tuning the complex refractive index and band gap in optical interference coatings: publisher’s note

This publisher’s note reports corrections in Appl. Opt. 63, 1641 (2024)APOPAI0003-693510.1364/AO.515083.

Thermal radiation transferred to the guided modes of optical interference coatings.

An electromagnetic model is developed to predict the thermal radiation which is trapped in a multilayer structure and transferred to its guided modes. The theory is based on the electromagnetic power supplied by the thermal currents given by the fluctuation-dissipation theorem. The source of the radiation is the ambient temperature or that caused by the optical absorption of the component subjected to spatio-temporal illumination. A numerical example is given for a multi-dielectric mirror at thermodynamic equilibrium. It is shown that the thermal radiation transferred to the guided modes of the multilayer can be much larger or lower than the radiation emerging in free space outside the component.

A-Si/SiO2 nanolaminates for tuning the complex refractive index and band gap in optical interference coatings.

A-S i/S i O 2 nanolaminates are deposited by magnetron sputtering and show a decreasing absorption when the a-Si single-layer thickness is reduced from 2.4n m to 0.7n m. Moreover, an increase of the Tauc band gap by 0.18e V is measured. Experimental Tauc band gaps are compared to calculated effective band gaps, utilizing a numerical Schrödinger solver. Further, it is demonstrated that the refractive index can be controlled by adjusting the a-Si and S i O 2 single-layer thicknesses in the nanolaminates. The nanolaminates are optically characterized by spectroscopic ellipsometry, transmittance, and reflectance measurements. Additionally, TEM images reveal uniform, well-separated layers, and EDX measurements show the silicon and oxygen distribution in the nanolaminates.

Mid-infrared supermirrors with finesse exceeding 400 000

For trace gas sensing and precision spectroscopy, optical cavities incorporating low-loss mirrors are indispensable for path length and optical intensity enhancement. Optical interference coatings in the visible and near-infrared (NIR) spectral regions have achieved total optical losses below 2 parts per million (ppm), enabling a cavity finesse in excess of 1 million. However, such advancements have been lacking in the mid-infrared (MIR), despite substantial scientific interest. Here, we demonstrate a significant breakthrough in high-performance MIR mirrors, reporting substrate-transferred single-crystal interference coatings capable of cavity finesse values from 200 000 to 400 000 near 4.5 µm, with excess optical losses (scatter and absorption) below 5 ppm. In a first proof-of-concept demonstration, we achieve the lowest noise-equivalent absorption in a linear cavity ring-down spectrometer normalized by cavity length. This substantial improvement in performance will unlock a rich variety of MIR applications for atmospheric transport and environmental sciences, detection of fugitive emissions, process gas monitoring, breath-gas analysis, and verification of biogenic fuels and plastics.

Influence of fitting algorithms on thickness determination during monitoring of optical coatings

The monitoring of thickness evolution during the deposition of optical interference coatings is widely performed by transmittance measurements and continuous comparison with theoretical models. The fitted thickness of the actual layer is influenced by the quality of the signal. Effects from light path, substrate and spectrometers result in deviations because these are often not part of the models. We show an implementation of these unmodeled effects in the fitting algorithms. The outcome of different configurations while coating the same optical filter design is compared. Additionally, the time consumption of these strategies is investigated to verify their suitability for production.

Photo-induced thermal radiation of optical interference coatings submitted to a spatio-temporal illumination.

We present an electromagnetic model for photo-induced thermal radiation in multi-layer interference filters subjected to arbitrary pulsed illumination with limited beam size. Numerical calculation is used to analyze various structures affecting thermal radiation, such as multi-dielectric mirrors in the mid-infrared range. Other zero-admittance structures are shown to strongly confine and enhance the thermal radiation with an emissivity close to unity at pre-defined frequencies (wavelength and angles). Calculation tools are chosen that encourage the use of techniques for synthesizing thin-film multilayers able to control thermal radiation.

Model-free approach to finding index of refraction values of optical coatings from spectral measurements.

Mathematical models for fitting the refractive index versus the wavelength, such as the Cauchy, Sellmeier, and Drude equations, or physical models, such as the Lorentz model, are commonly used to fit the index properties of measured spectra of optical thin film witness samples for use in the design and production of optical interference coatings. The degree of agreement of the coating reflectance and transmittance with the design when the coatings are produced with these data will depend on the accuracy of the spectral measurements and index fittings. As thin-film coating technology has progressed, many cases are now encountered where no simple model is adequate to fit the actual index dispersion. This work shows an approach to finding the refractive index versus wavelength, which is independent of any mathematical or physical models.

Optical interference coatings for coloured building integrated photovoltaic modules: Predicting and optimising visual properties and efficiency

The presented work is focusing on optical interference coatings on the back side of the front glass for crystalline silicon-based photovoltaic modules. We present results on how such coatings can be designed to obtain desired visual properties combined with a limited loss in power conversion efficiency. This colouring technology may therefore be very attractive for building integrated photovoltaics. Coating design parameters such as number of layers, design wavelength, layer thicknesses, and layer refractive indices affect both the visual appearance parameters and the efficiency. Important colour parameters are lightness, chroma, and hue, and we demonstrate how these colour parameters are influenced by the design parameters of the optical interference coating.Using optical interference coatings to create colours results in colours that are dependent on the observation angle. We have introduced a new parameter for quantification of angular colour dependence. Our results show that the angular colour dependence can be reduced by increasing the refractive indices of the interference coating.For many building projects it may be highly desired to combine high power conversion efficiency with certain visual properties. The results from our work also indicate some of the possibilities and challenges in this regard.

Magnetron sputter deposition of Ta2O5-SiO2 quantized nanolaminates.

Quantized nanolaminates are a type of optical metamaterials, which were discovered only recently. Their feasibility was demonstrated by atomic layer deposition and ion beam sputtering so far. In this paper, we will report on the successful magnetron sputter deposition of quantized nanolaminates based on Ta2O5-SiO2. We will describe the deposition process, show results and material characterization of films deposited in a very wide parameter range. Furthermore, we will show how quantized nanolaminates deposited by magnetron sputtering were used in optical interference coatings such as antireflection and mirror coatings.

Optical interference coatings: measurement problem 2022 [Invited

The 2022 Optical Interference Measurement Problem comprised the determination of the refractive index of a thin tantalum pentoxide film at a wavelength of 532nm and the characterization of the UV band edge as an optional task.

Optical Interference Coatings 2022: introduction to the feature issue.

The fifteenth topical meeting on Optical Interference Coatings (OIC) was held June 19-24, 2022 in Whistler, British Columbia, Canada. This feature issue of Applied Optics collects selected papers presented during this conference. The OIC topical meeting takes place every three years and is a pivotal event for the international community working in the field of optical interference coatings. The conference provides premier opportunities for attendees to share knowledge on their new advances in research and development and to build bridges for further collaborations. The meeting topics cover a broad range, from fundamental research, coating design theory, new materials, deposition and characterization technologies to a vast catalogue of applications including green technologies, aerospace, gravitational wave detection, communication, optical instruments, consumer electronics, high power and ultra-fast lasers, and many others.

Ultrafast switching with nonlinear optics in thin films.

We demonstrate a novel, to the best of our knowledge, concept for an all-optical switch based on the optical Kerr effect in optical interference coatings. The utilization of the internal intensity enhancement in thin film coatings as well as the integration of highly nonlinear materials enable a novel approach for self-induced optical switching. The paper gives insight into the design of the layer stack, suitable materials, and the characterization of the switching behavior of the manufactured components. A modulation depth of 30% could be achieved, which prepares the way for later applications in mode locking.

2022 Optical Interference Coatings Conference: Manufacturing Problem Contest [Invited

Participants in the 2022 Manufacturing Problem Contest were challenged to fabricate an optical filter with a specified stepped transmittance spanning three orders of magnitude from 400 to 1100nm. The problem required that contestants be versed in the design, deposition, and measurement of optical filters to achieve good results. Nine samples from five institutions were submitted with total thicknesses between 5.9 and 53.5µm with between 68 and 1743 layers. The filter spectra were measured by three independent laboratories. The results were presented in June 2022 at the Optical Interference Coatings Conference in Whistler, B.C., Canada.

Optical Interference Coating Design Contest 2022: a black box coating and a filter for an outdoor 3D cinema challenge [invited

The design problems for the Optical Interference Coating (OIC) 2022 Topical Meeting include black box coatings to reverse engineer and a pair of white-balanced, multi-bandpass filters for three-dimensional cinema projection in cold and hot outdoor environments. There were 14 designers from China, France, Germany, Japan, Russia, and the United States, submitting 32 total designs for problems A and B. The design problems and the submitted solutions are described and evaluated.

Photo-induced temperature in optical interference coatings.

The photo-induced temperature in multilayer systems is calculated using an original analytical model based on optical/thermal analogies. Various illumination regimes are considered, ranging from ps pulses to a continuous regime, while taking into account a variable repetition rate. The temporal and spatial (3D) resolutions are quantified, and the distributions of temperature and stationary optical field are compared. The temperature spectra are given as a function of the illumination wavelength. Thermal and electromagnetic damage thresholds are compared. Lastly, the thermal fringes resulting from structured optical illumination are studied.

High-NA optical edge detection via optimized multilayer films

There has been a significant effort to design nanophotonic structures that process images at the speed of light. A prototypical example is in edge detection, where photonic-crystal-, metasurface-, and plasmon-based designs have been proposed and in some cases experimentally demonstrated. In this work, we show that multilayer optical interference coatings can achieve visible-frequency edge detection in transmission with high numerical aperture, two-dimensional image formation, and straightforward fabrication techniques, unique among all nanophotonic approaches. We show that the conventional Laplacian-based transmission spectrum may not be ideal once the scattering physics of real designs is considered, and show that better performance can be attained with alternative spatial filter functions. Our designs, comprising alternating layers of Si and SiO2 with total thicknesses of only , demonstrate the possibility for optimized multilayer films to achieve state-of-the-art edge detection, and, more broadly, analog optical implementations of linear operators.

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.

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.

Optical interference coatings measurement problem 2019 [Invited

The 2019 Optical Interference Coatings measurement problem comprised the determination of the total backscattering, forward scattering, reflectance, and transmittance spectra of a multilayer system.

Quantizing nanolaminates as versatile materials for optical interference coatings.

In this paper, the theoretical foundation of quantizing nanolaminates is explained, and the dependence of the optical band gap on quantum-well thickness is demonstrated. The production is investigated by applying molecular dynamics growth simulation and by correlating the results with layers deposited by ion beam sputtering and atomic layer deposition. The properties of manufactured nanolaminates are then compared to the theoretical behavior, and good agreement is found.