Mirror Material Research Articles

Design and investigation of photo-induced super-hydrophilic materials for car mirrors

During the past decades, interests in various properties in titanium dioxide thin films have been growing rapidly. There have been several reports for TiO 2 thin films prepared on various media with photocatalytic and hydrophilic properties, in order to function as self-cleaning and/or anti-fogging materials. An obvious application is usually found in side-view car mirrors in the automobile industries. In this study, a number of photocatalytic TiO 2 films are prepared on soda-lime glasses for car mirrors by an electron-beam evaporation. The designs and development of the photocatalytic TiO 2 films, based on crystallinity, deposition rate, film thickness, film structure, and surface roughness are discussed. In comparison to the commercialized products, a systematic investigation procedure for the super-hydrophilic properties of the light-induced TiO 2 films for car mirrors has been developed, based on super-hydrophilicity, sustainability, self-cleaning property, and degradation of the samples. In addition, physical characterization by X-ray diffraction and surface roughness are also discussed. It has been found that most commercial products attain super-hydrophilicity only after exposed to ultraviolet and solar irradiation in less than 1 h. They can also maintain hydrophilicity after rigorous cleaning process. On the other hand, our prepared TiO 2 thin films demonstrate super-hydrophilic and photocatalytic properties after exposed to ultraviolet light for more than 2 h. According to the study, their anatase crystallinity, small grain size, and surface conditions all contributes to the excellent results. However, the prepared samples do not attain sufficient retention property to maintain their hydrophilicity. Conclusively, the designs of the TiO 2 films on car mirrors prove adequate to produce super-hydrophilic materials, which still degrade over normal usage. Nevertheless, our proposed investigation methods prove useful in quality evaluation in order to differentiate the results of super-hydrophilic materials of interests.

Gas-phase study of the reactivity of optical coating materials with hydrocarbons by use of a desktop-size extreme-ultraviolet laser

The reactivity of prospective capping-layer extreme-ultraviolet (EUV) mirror materials with hydrocarbons is studied in the gas phase by use of mass spectroscopy of metal-oxide clusters. We report the results of chemistry studies for Sim, Tim, Hfm, and ZrmOn metal-oxide clusters in which the reaction products were ionized with little or no fragmentation by 26.5 eV photons from a desktop-size 46.9 nm Ne-like Ar laser. Hf and Zr oxides are found to be much less reactive than Si or Ti oxides in the presence of EUV light. The results are relevant to the design of EUV mirror capping layers that are resistant to carbon contamination.

Assessment of Laser Transmission Mirror Materials for ITER Edge Thomson Scattering Diagnostics

An assessment of in-vessel metallic mirror materials for the transmission of the laser beam used in the ITER edge Thomson scattering diagnostics is reported. The transient temperature increase due to the laser pulse irradiation on the laser transmission mirror is calculated by a one-dimensional heat conduction equation. Candidate mirror materials are discussed based on a comparison between the numerical calculation and current data relevant to the laser-induced damage threshold (LIDT). Gold, silver, and copper are considered promising because of its high reflectivity. The LIDT is evaluated considering multi-pulse effects and used to determine the necessary size for the laser transmission mirror for the ITER edge Thomson scattering diagnostics.

A particle-in-cell plus Monte Carlo study of plasma-induced damage of normal incidence collector optics used in extreme ultraviolet lithography

We present a kinetic simulation of the plasma formed by photoionization in the intense flux of an extreme ultraviolet lithography (EUVL) light source. The model is based on the particle-in-cell plus Monte Carlo approach. The photoelectric effect and ionization by electron collisions are included. The time evolution of the low density argon plasma is simulated during and after the EUV pulse and the ion-induced sputtering of the coating material of a normal incidence collector mirror is computed. The relation between the time and position at which the ions are created and their final energy is studied, revealing how the evolution and the properties of the sheath influence the amount of sputtered material. The influence of the gas pressure and the source intensity is studied, evaluating the behavior of Ar+ and Ar2+ ions. A way to reduce the damage to the collector mirror is presented.

Rhodium coated mirrors deposited by magnetron sputtering for fusion applications

Metallic mirrors will be essential components of all optical spectroscopy and imaging systems for ITER plasma diagnostics. Any change in the mirror performance, in particular, its reflectivity, due to erosion of the surface by charge exchange neutrals or deposition of impurities will influence the quality and reliability of the detected signals. Due to its high reflectivity in the visible wavelength range and its low sputtering yield, rhodium appears as an attractive material for first mirrors in ITER. However, the very high price of the raw material calls for using it in the form of a film deposited onto metallic substrates. The development of a reliable technique for the preparation of high reflectivity rhodium films is therefore of the highest importance. Rhodium layers with thicknesses of up to 2 microm were produced on different substrates of interest (Mo, stainless steel, Cu) by magnetron sputtering. Produced films exhibit a low roughness and crystallite size of about 10 nm with a dense columnar structure. No impurities were detected on the surface after deposition. Scratch tests demonstrate that adhesion properties increase with substrate hardness. Detailed optical characterizations of Rh-coated mirrors as well as results of erosion tests performed both under laboratory conditions and in the TEXTOR tokamak are presented in this paper.

First mirrors for diagnostic systems of ITER

The majority of optical diagnostics presently foreseen for ITER will implement in-vessel metallic mirrors as plasma-viewing components. Mirrors are used for the observation of the plasma radiation in a very wide wavelength range: from about 1 nm up to a few mm. In the hostile ITER environment, mirrors are subject to erosion, deposition, particle implantation and other adverse effects which will change their optical properties, affecting the entire performance of the respective diagnostic systems. The Specialists Working Group (SWG) on first mirrors was established under the wings of the International Tokamak Physics Activity (ITPA) Topical Group (TG) on Diagnostics to coordinate and guide the investigations on diagnostic mirrors towards the development of optimal, robust and durable solutions for ITER diagnostic systems. The results of tests of various ITER-candidate mirror materials, performed in Tore-Supra, TEXTOR, DIII-D, TCV, T-10, TRIAM-1M and LHD under various plasma conditions, as well as an overview of laboratory investigations of mirror performance and mirror cleaning techniques are presented in the paper. The current tasks in the R&D of diagnostic mirrors will be addressed.

Substrate-dependent deposition efficiency on mirrors exposed in the TCV divertor

Given the planned extensive use of metallic mirrors in the optical diagnostics systems of ITER, the study of reflectivity changes induced by erosion and/or redeposition of impurities on the mirror surfaces is of primary importance for the reliability of the diagnostics signals. This contribution will demonstrate that the mirror material choice can exert a significant influence on the relative importance of erosion/redeposition affecting the mirror reflectivity. A dedicated manipulator has been designed to allow exposure of mirror samples in the divertor region of the Tokamak à Configuration Variable (TCV) tokamak. Mirrors from different materials have been exposed both during short experimental campaigns and boronisation procedures. Before and after exposures the mirrors were characterized with different surface analysis techniques (XPS, SIMS, SEM, EDX, ellipsometry). Under identical exposure conditions, the mirror material can strongly influence the deposit thickness found on the sample: the carbon layer thickness on a Si sample is much higher than on a Mo sample. These results have potentially important consequences for the first mirror material choice in ITER.

Reflections on ionic liquids

Ionic liquids are generally regarded as solvents, but these modular, tunable compounds have far greater technological potential. With a coat of silver, they become ideal materials for the liquid mirror of a space telescope.

Fabrication and testing of a complex-shaped mirror constructed with silicon mirror elements

We have fabricated a complex-shaped mirror array and evaluated its fabrication accuracy. The mirror is constructed with 16 mirror elements with an arc-shaped contour and a spherical reflective surface, and is part of an arc-shaped fly-eye mirror for use in an extreme ultraviolet lithography system. All the mirror elements are manufactured individually, using single-crystal silicon as the mirror material, and are then arranged side by side on a base plate by magnetic attraction. The spherical mirror elements are manufactured to have highly accurate and smooth surfaces, although they have a different shape from those of general optics, and are assembled to form the fly-eye mirror. To evaluate the positioning errors of the mirror elements of the assembled fly-eye mirror, an optical testing system using visible rays are developed. From the measured results using this system, the average tilt error of the mirror elements is 69 arcsec, assuming that the system has a collimation error, which we corrected through calculation. Thus, the fly-eye mirror is fabricated with a high accuracy, demonstrating that the fabrication process is useful in realizing complex-shaped mirrors.

Fabrication of Lightweight SiC Space Mirror

Silicon carbide ceramic has many attributes including high strength and stiffness, low thermal expansion coefficient, high thermal conductivity, low density and good resistant for radiation of space particles. These attributes make SiC ceramic becoming the third generation space mirror material. This paper describes the fabrication of lightweight SiC mirror blanks. By reducing the shrinkage of green body and controlling the drying conditions, lightweight SiC mirror blank with a diameter of 612mm has been prepared by using gelcasting process. The forming green body is infiltrated with liquid silicon at the temperature of 1600°C, then a CVD SiC layer is coated on the surface of SiC mirror blank. After grinding and polishing, the mirror shows good optical properties.

A method to measure the mirror reflectivity of a prime focus telescope

We have developed a method to measure the mirror reflectivity of telescopes. While it is relatively easy to measure the local reflectivity of the mirror material, it is not so straightforward to measure the amount of light that it focuses in a spot of a given diameter. Our method is based on the use of a CCD camera that is fixed on the mirror dish structure and observes simultaneously part of the telescope’s focal plane and the sky region around its optical axis. A white diffuse reflecting disk of known reflectivity is fixed in the telescopes focal plane. During a typical reflectivity measurement the telescope is directed to a selected star. The CCD camera can see two images of the selected star, one directly and another one as a spot focused by the mirror on the white disk. The ratio of the reflected starlight integrated by the CCD from the white disk to the directly measured one provides a precise result of the product of (mirror area × mirror reflectivity).

Diagnostic mirrors for ITER: A material choice and the impact of erosion and deposition on their performance

Metal mirrors will be implemented in about half of the ITER diagnostics. Mirrors in ITER will have to withstand radiation loads, erosion by charge-exchange neutrals, deposition of impurities, particle implantation and neutron irradiation. It is believed that the optical properties of diagnostic mirrors will be primarily influenced by erosion and deposition. A solution is needed for optimal performance of mirrors in ITER throughout the entire lifetime of the machine. A multi-machine research on diagnostic mirrors is currently underway in fusion facilities at several institutions and laboratories worldwide. Among others, dedicated investigations of ITER-candidate mirror materials are ongoing in Tore-Supra, TEXTOR, DIII-D, TCV, T-10 and JET. Laboratory studies are underway at IPP Kharkov (Ukraine), Kurchatov Institute (Russia) and the University of Basel (Switzerland). An overview of current research on diagnostic mirrors along with an outlook on future investigations is the subject of this paper.

First mirror tests for ITER: Influence of material choice on the erosion/deposition mechanisms affecting optical reflectivity

Almost all optical diagnostics systems on ITER will be based on in-vessel metallic first mirrors. The possible deterioration of their surface reflectivity as a result of erosion by charge-exchange neutrals and re-deposition of material eroded from the plasma-facing components represents a serious concern for the reliability of spectroscopic and laser signals. A concerted effort within the tokamak community has been initiated to characterize these effects and seek mitigation methods. To date, the different damaging effects (erosion and deposition) have always been considered independently, neglecting any role that may be played by the substrate. In this contribution, we attempt to assess the influence of substrate material using different approaches: laboratory simulation experiments, mirror exposures in the divertor region of the TCV tokamak and numerical simulations using the Monte-Carlo code TRIDYN. Our results have potentially important implications for the choice of ITER first mirror materials.

Optical exposure characterization and comparisons for discharge produced plasma Sn extreme ultraviolet system

A critical issue for EUV lithography EUVL is the minimization of collector degradation from intense plasma erosion, debris deposition, and hydrocarbon/oxide contamination. Collector optics reflectivity and lifetime heavily depend on surface chemistry and interactions between fuels and various mirror materials, such as silicon, in addition to high- energy ion and neutral particle erosion effects. As a continuation of our prior investigations of discharge-produced plasma DPP and laser- produced plasma LPP Xe plasma interactions with collector optics sur- faces, the University of Illinois at Urbana-Champaign UIUC has ana- lyzed collector samples before and after exposure in a Sn-upgraded Xtreme Technologies EUV source. Sn DPP postexposure characteriza- tion includes multiple samples, Si/Mo multilayer film with normal inci- dence, 200-nm-thick Ru film with grazing incidence, as well as a Gibb- sian segregated GS Mo-Au alloy developed on silicon using a dc dual- magnetron cosputtering system at UIUC for enhanced surface roughness properties, erosion resistance, and self-healing characteris- tics to maintain reflectivity over a longer period of mirror lifetime. Surface analysis draws heavily on the expertise of the Center for Microanalysis of Materials at UIUC, and investigates mirror degradation mechanisms by measuring changes in surface roughness and film thickness as well as analysis of deposition of energetic Sn ions, Sn diffusion, and mixing of multilayer. Results from atomic force microscopy AFM and auger elec- tron spectroscopy AES measurements show exposure effects on sur- face roughness and contamination. The best estimates of thickness and the resultant erosion measurements are obtained from scanning electron microscopy SEM. Deposition, diffusion, and mixing effects are ana-

Design and finite element analysis of lightmass silicon carbide primary mirror

Primary mirror is one of the key components in the space remote sensing system. To minimize the mass of the mirror without compromising its stiffness and decrease the deformation of the mirror surface at the different temperatures are the mainly two objects in the development of the primary mirror. Silicon carbide (SiC), the most promising optical material, was used as the material of the primary mirror with triangle lightmass structure in a Cassegrain system. By using finite element method, the properties of the SiC mirror were compared with that of the traditional Be mirror and fused silica mirror. The results of static, dynamic and thermo-mechanical analysis indicate that the deformation of the mirror surface caused by temperature field is much bigger than that caused by gravity field. The SiC mirror has the best overall properties, and the SiC material is much suitable for the primary mirror.

Design of DBR Mirrors for GaN Vertical Cavity Surface Emitting Laser

In the fabrication of nitride-based laser, the cavity plays a vital role in order to enhance the fundamental modes while suppressing the higher order modes. The critical problem faced between the designer and the grower is the idealistic approach in the design laboratory, which more often cannot be met in the fabrication lines. This is because the design does not take into account the limitation in the growth chamber in achieving the targeted mirror thickness and material composition. This paper discusses methods of designing DBR mirrors for vertical cavity surface emitting GaN based lasers. Our design considers variance in the thickness and material composition of the mirrors. The simulation results are given for the investigation of several properties of DBR mirror, as well as a comparison of same types of symmetric DBR mirror.

Experimental investigation of dynamic photo-thermal effect

In an optical interferometer, a part of the laser power is absorbed by the mirrors and gives rise to surface displacements through thermal expansion. The position measurement sensitivity is, therefore, limited by the shot noise of the absorbed radiation. This phenomenon is called the photo-thermal effect and its typical frequency dependence and size can span over several orders of magnitude, depending on the mirror material and temperature, hence it is crucial to have an accurate model extending over such a large range. We present an extensive experimental investigation of dynamic photo-thermal effects, covering more than seven decades in frequency and including finite mirror size effects (low frequency) and coating effects (high frequency) and showing the dependence on the beam waist. A specific extension of the basic model is being developed that can well explain the results.

Behaviour of mirrors fabricated from amorphous alloys under impact of deuterium plasma ions

In-vessel mirrors are necessary for optical and laser diagnostics of plasmas in next-step fusion devices; however obtaining mirror materials that will maintain their performance in the harsh fusion environment remains problematic. At present, mirrors are fabricated from polycrystals or monocrystals, and metal films on metallic substrate mirrors are also being studied. In this paper, we report on a new family of bulk amorphous alloys that are being investigated in the search for new materials for the fusion environment. Their properties are very different from properties of well-known materials and are insufficiently investigated for scientific and technological applications. We present the results of investigation of modification of the surface and optical properties of amorphous mirror fabricated from Zr(41.2%)Ti(13.8%)Cu(12.5%)Ni(10%)Be(22.5%) alloy after bombardment by ions of deuterium plasma with different fluence and energy.

Improving the light extraction efficiency of red-emitting conjugated polymer light emitting diodes

We demonstrate a significant improvement in the external electroluminescence efficiency of red-emitting polymer light emitting diodes (LEDs) by modifying the optical structure of the device. By using a cathode composed of a thin (5nm) film of calcium backed with an optically thick film of silver, we measured improvements in the external efficiency of polymer LEDs by a factor of 1.6 times compared to a device using a cathode composed of calcium backed with lower reflectivity aluminum. By incorporating the LED into a microcavity structure (to form a resonant cavity LED) it is possible to obtain additional (but rather smaller) improvements in external efficiency of the order of 1.15 times, compared to a standard LED utilizing the same cathode. By combining high reflectivity cathode∕mirror materials with a low finesse cavity structure, we show that the external efficiency of a LED can be improved by as much as 1.8 times compared to a standard (noncavity) LED. Our results are in good agreement with those of theoretical calculations and demonstrate the real improvements in device external efficiency that can be achieved by reducing optical losses within the LED structure.

407 大型SiCミラーのELID研削(OS-12 ELID加工)

Silicon carbide (SiC) is the most advantageous as the material of various telescope mirrors, because SiC has high stiffness, high thermal conductivity, low thermal expansion and low density. Therefore, we developed the ultra lightweight mirror by ELID grinding method which can obtain the surfaces of very hard materials efficiently. If a mirror becomes large, shape of the mirror becomes complicated, and thickness of the mirror surface and a lib part becomes thin for lightweight. Accordingly, it is thought that surface roughness of the mirror deteriorates by influence of vibration. And then, to improve the precision of the surface roughness, we did the ELID grinding by cross cut that the axis of rotation and a progress direction of a grinding wheel were vertical.