Volume 3, Issue 1 - Optical Coatings / Guest Editors: Norbert Kaiser, Xu Liu and Angus Macleod

Buildings use as much as 40% of the world’s total primary energy. This huge energy consumption is mainly tied to poor designs and entrenched practices for buildings’ envelopes. One major road toward more energy efficient buildings is to employ design principles that are in harmony with the radiation in our natural surroundings. Glass coatings with spectral selectivity can lead to windows and glass facades with good thermal insulation and capability to transmit mainly visible light or, alternatively, across the full solar spectrum. Chromogenic glazings, especially if based on electrochromism, can regulate the inflow of visible light and solar energy between widely separated limits and achieve further energy efficiency. The new fenestration technologies are able to improve indoor comfort.

Transparent plastics have been used for optical applications with growing demand. This development is accompanied by a desire for extended surface functionalities. Most important optical surface function is antireflection (AR), which is performed mainly by applying plasma-assisted processes. Critical considerations for coating polymers include interaction with emission from plasma and thermal stress. State-of-the-art vacuum processes for coating on plastic, as well as new results of research and development in the fields of AR design and AR structures will be introduced and discussed.

The density of optical coatings is one of the most crucial material-related parameters in interference coating science and technology. It has an impact on the refractive index, the transparency range, and the mechanical stress of a coating material. This tutorial provides a background on the classical theory relating the coating density to the mentioned parameters. Simple models are presented that highlight the correlations between optical constants, stress, and shifting behavior of different oxide coatings. Comparison with the experiment is performed on the basis of numerous experimental data, which stem from hafnium oxide, zirconium oxide, tantalum pentoxide, and silicon dioxide.

Nowadays, dispersive mirrors are able to cover the wavelength range of 4.5 optical octaves and can be used from 220 nm up to 4500 nm. Various design approaches to dispersive mirrors in visible and near IR are briefly discussed. We consider in more detail two dispersive mirrors representing extreme cases. The first one is a mirror working in the range of 290−360 nm and providing group delay dispersion of -75 fs 2 . The second one is a mirror working in the range of 2500−4500 nm and providing +500 fs 2 of group delay dispersion.

Defects in optical coatings are a major factor degrading their performance. Based on the nature of defects, we classified them into two categories: visible defects and non-visible defects. Visible defects result from the replication of substrate imperfections or particulates within the coatings by subsequent layers and can increase scattering loss, produce critical errors in extreme ultraviolet lithography, weaken mechanical and environmental stability, and reduce laser damage resistance. Non-visible defects mainly involve a decrease in laser damage resistance but typically have no influence on other properties of optical coatings. In the case of widely used HfO 2 /SiO 2 dielectric coatings, metallic Hf nano-clusters, off-stoichiometric HfO 2 nano-clusters, or areas of high-density electronic defects have been postulated as possible sources for non-visible defects. The emphasis of this review is devoted to discussing localized defect-driven laser-induced damage (LID) in optical coatings used for nanosecond-scale pulsed laser applications, but consideration is also given to other properties of optical coatings such as scattering, environmental stability, etc. The low densities and diverse properties of defects make the systematic study of LID initiating from localized defects time-consuming and very challenging. Experimental and theoretical studies of localized defect-driven LID using artificial defects whose properties can be well controlled are highlighted.

The optical properties of opaque sputtered silver films are investigated and related to their surface morphology. As reference, an evaporated sample produced by thermal flash evaporation has been studied, too. Standard optical and electrical characterizations as well as scanning electron microscopy (SEM) and X-ray reflectometry have been carried out with unprotected silver films directly after deposition and reveal strong correlations between the corresponding characterization results. The aging behavior of the normal incidence reflectance has been studied over a time period of 36 weeks. Protected silver mirrors have been prepared by sputtering using silica and alumina as the protective coating. It is shown by model calculations that the main spectral features occurring in the reflectance spectra of as-deposited unprotected coatings, unprotected coatings after aging, and protected coatings can be reproduced assuming a rough silver surface covered by a corresponding ultrathin absorbing interface layer or overlayer.

Aluminum-doped zinc oxide (AZO) is one of the most promising transparent conductive oxide (TCO) materials that can substitute the high-quality but costly indium tin oxide (ITO). To ensure high-quality films as well as moderate production costs, inline DC magnetron sputtering was chosen to deposit thin AZO films. The influence of sputter gas pressure, substrate temperature, and film thickness on the electrical, optical, and structural properties was analyzed. The resistivity reaches a minimum of 1.3×10 -5 Ωm at around 1 Pa for a substrate temperature of 90°C. A maximum conductivity was obtained by increasing the substrate temperature to 160°C. An annealing step after deposition led to a further decrease in resistivity to a value of 5.3×10 -6 Ωm in a 200 nm thin film. At the same time, the optical performance could be improved. Additionally, simulations of the transmittance and reflectance spectra were carried out to compare carrier concentration and mobility determined by optical techniques with those from Hall measurements.

Light scattering of optical components caused by residual imperfections can be a critical factor for their practical application. In particular, the scattering properties of optical interference coatings are rather complex. Yet, simple theoretical models and comparisons with experimental results provide valuable insight into the main impact factors and mechanisms. The magnitude of scattering and the dominating factors strongly depend on the wavelength of application in connection with the types of coatings used in the corresponding ranges. The paper, therefore, gives an overview of the scattering properties of coatings in different spectral regions including the visible, deep ultraviolet, and extreme ultraviolet and discusses strong in-band variations of the scattering characteristics that have been neglected so far.