In most cases authors are permitted to post their version of the article (e.g. in Word or Tex form) to their personal website or institutional repository. Authors requiring further information regarding Elsevier's archiving and manuscript policies are encouraged to visit: a b s t r a c t a r t i c l e i n f o We present extended experimental material about optical and mechanical properties of oxide optical coating materials, deposited by electron beam evaporation, ion and plasma ion assisted evaporation, sputtering and ion plating. A clear correlation between these experimental data is established and understood as being caused by the different degree of the porosity of the films. This assumption has been verified by investigation of the layer structure and accompanying simulations of the effect of porosity on refractive index, layer stress and thermal shift. As a practical conclusion, we find that a certain pore fraction in the films is essential in order to get a valuable balance between optical and mechanical coating properties. It is well known, that the performance of optical coatings crucially depends on the production accuracy and reproducibility of geometrical coating parameters such as film thickness, surface smoothness, defect (pinhole) concentration and the like. On the other hand, excellent, environmentally stable and reproducible values of optical thin film material constants like refractive indices and small extinction coefficients are required as well. Ideally, the films should not have gradients of optical properties, and their refractive index and extinction coefficient should be independent of the film thickness. Therefore, formation of amorphous layers is favoured. Additional requirements on non-optical material properties like small mechanical stress are equally important [1–3]. From thin film theory it is well known, that for any spectral target defined at normal incidence and practically non-absorbing materials, the optimal design is achieved with a pair of materials that reveal a refractive index contrast as high as possible [4]. As a consequence of this general design recipe, there arises a need to optimize deposition techniques and conditions with the goal to maximize achievable refractive indices of traditional high-index materials. In modern ion assisted versions of optical coating techniques, the high densification of the coating leads to film refractive indices close to or higher than reported bulk values [2]. Hereby the ion plating techniques and reactively pulsed magnetron sputtering are reported to deliver coatings with the highest refractive indices known today [5]. As …

We present extended experimental material about optical and mechanical properties of oxide optical coating materials, deposited by electron beam evaporation, ion and plasma ion assisted evaporation, sputtering and ion plating. A clear correlation between these experimental data is established and understood as being caused by the different degree of the porosity of the films. This assumption has been verified by investigation of the layer structure and accompanying simulations of the effect of porosity on refractive index, layer stress and thermal shift. As a practical conclusion, we find that a certain pore fraction in the films is essential in order to get a valuable balance between optical and mechanical coating properties.

Inhomogeneous antireflection coatings were prepared by simultaneous evaporation of high and low index materials during production of the interface region. Rates of evaporation for both materials were monitored by microprocessor controlled quartz crystals. Four designs were prepared, three of them consist of both oxides (Hafnia, Tantala, Zirconia / Silica) and the other one contains Cerium oxide in combination with Magnesium fluoride. Absorption and damage threshold values are presented.

This paper gives a comprehensive study of designing, fabrication and determination of damage thresholds of single and double layer antireflection coatings. Single quarter wave layers were prepared form MgF2 and mixtures of MgF2 with other fluorides. An oxide-fluoride mixture was also tested. Double layer "V" coatings were studied taking SiO2 and MgF2 as low refractive index material in combination with HfO2, ZrO2, Ta2O5, Nd2O3, CeO2 and TiO2 as a high refractive index material next to the substrate. All coatings were prepared in a Leybold-Heraeus vacuum coating pant at a base pressure of <= 1E-5 mbar on Suprasil substrates by conventional electron beam evaporation method. Film thickness was controlled by usual optical method. Reflectance, scattering, absorption and damage threshold were measured. Damage morphology was done by Nomarsky and scanning electron microscopy (SEM). Influence of barrier layers on damage thresholds of AR-2 systems was also investigated. Besides this, an overcoat effect on AR-2 systems with MgF2 as an outer layer was also observed. Substrate polishing effects on damage threshold of bare as well as AR-coated systems are also reported.

The key technologies for modern production processes with enhanced spatial resolution, require high performance DUV- excimer laser optics with enhanced optical properties. Major challenges imposed onto the requested new generation of optical elements are concentrated on lowest absorption and scattering as well as stability against highest pulse number throughput. These targets are the driving force within the German Joint Research Project 'OPUS II', which is dedicated to the development of high quality optical components for the DUV spectral range. As a major contribution to these investigations, sets of reflecting stacks with four different numbers of layer pairs of LaF3/MgF2 were produced by 6 partners of the consortium and characterized in respect to their optical performance and structural properties. The characterization includes spectrophotometric measurements from the VUV up tot eh mid RI range. calorimetric absorption measurements at 193 nm, and a comparative study in total scatter behavior at 193 nm, which was performed by three laboratories within the project. Also, besides the intrinsic stress and the surface topography of the layers, the non-linear absorption behavior of selected samples have been determined. The results are presented and discussed with respect to possible applications.

High repetition rate solid-state laser systems are used in many scientific and industrial applications. Especially-Q- switched Nd:YAG lasers are gaining increasing importance as pump sources for frequency conversion processes. These lasers are a potential alternative for several classical gas laser systems. Presently, the development of Nd:YAG systems with higher output power and improved beam parameters suffers from limitations imposed by the power handling capability of the optical components. At the Laser Zentrum Hannover, a facility has been installed for the measurement of the multiple-pulse damage thresholds of optical surfaces according to ISO/DIS 11254-2. For an efficient determination of the threshold values, an on-line algorithm calculating the recommended energy for each site has been elaborated and implemented. The most important coating materials for the NIR spectral range were investigated, in respect to their S- on-1 damage threshold. The results indicate the materials dependent reduction of the damage threshold with an increasing number of pulses per site.

For high power laser applications, extinction coefficients and refractive indices of ion beam sputtered resp. ionplated Hf02-, Si02- and A1203- films were determined by spectrophotometric analysis in the wavelength range between 200 and 500 nm. Film qualities were related to the corresponding process parameters. Qualitative information about water content has been obtained by infrared spectroscopy. Impurities and film stoichiometry were determined by RBS analysis. Laser induced damage thresholds were measured with a (KrF*)excimer laser at 248 nm. The tested samples were single layers as well as high reflecting QWOT stacks. All results were compared with the corresponding values of e-beam coatings.

Single layers of hafnia, alumina, and silica on quartz substrates were deposited by reactive ion plating and ion beam sputtering. Refractive indices and extinction coefficients were determined by spectrophotometric analysis and related to the process parameters. Water adsorption was examined with the aid of optical transmission in the range from 2 to 4 µm. Laser-induced damage testing was performed by a KrF excimer laser at 248 nm. The tested samples were quarterwave stacks of Al2O3/SiO2 and HfO2/SiO2 on silica substrates. All the results are compared and related to the properties of conventional evaported coatings.

A study on thin film optical materials is presented. Single layers of oxides, fluorides, and mixtures of various dielectrics are fabricated on suprasil substrates. Optimized parameters are used for deposition of each material. Optical characteristics of dielectric materials are determined. A Nd-YAG laser with a maximum output energy of 250 mJ and pulse duration of 14 ns (FWHM) is used for damage experiments. X-ray diffraction analysis of various dielectric materials for bulk and film specimen is also presented. Morphology of damage sites is done by Nomarsky and scanning electron microscopy. Oxid-, Fluorid- und Mischschichten verschiedener Dielektrika werden auf Suprasil-Substraten abgeschieden, und es werden die optischen Eigenschaften dieser dielektrischen Materialien untersucht. Zur Abscheidung werden für jedes System optimierte Depositionsparameter verwendet. Ein Nd-YAG-Laser mit einer maximalen Energie von 250 mJ und einer Impulsdauer von 14 ns (Halbwertsbreite) wird für Damage-Experimente verwendet. Eine Röntgenbeugungsanalyse an Volumenmaterial oder an dünnen Schichten der hergestellten Dielektrika wird ebenfalls durchgeführt. Die Morphologie der Damage-Gebiete wird mit Nomarsky- und Raster-Elektronen-Mikroskopie untersucht.

Interfaces between adjacent layers contribute essentially to laser damage in dielectric film systems. Due to different thermal and mechanical properties of the adjacent materials stress is built up in the interface region by heating during laser exposition. Generally interface absorption exceeds volume absorption and an additional amount of heat is generated at the boundaries. In contrast to this layer adhesion is small at the interfaces. Therefore, in many cases, laser induced damage of dielectric systems starts at the boundaries of the layers.

A thermographic laser calorimeter for the measurement of absorptance in optical coatings is described. The heat transport equation is solved for a laser-heated disk with a thin-film coating. The solutions are used to calculate the output characteristics of the thermographic scanning system. The absorption and the thermal conductivity of oxide coatings (SiO2, TiO2, HfO2, Al2O3, and Ta2O5) were measured. Results lead to the conclusion that there is a considerable difference between the thermal properties of the coating and the corresponding bulk material.