Mirror Material Research Articles

Analytic expressions for the reflection delay, penetration depth, and absorptance of quarter-wave dielectric mirrors

The authors analyze the operation of high-reflectivity quarter-wave (QW) dielectric mirrors at the band-stop center (Bragg) frequency, relevant for the design of small-cavity optoelectronic structures. The energy penetration depth concept is used to determine a first-order linear approximation for the reduction of the mirror peak reflectivity of the QE mirror as a function of the mirror material parameters and the number of layers. The expression can be applied in the limit of small loss. The mathematical analysis and expressions for the absorptance and the peak reflectivity of a dielectric mirror with weak material absorption are presented. The use of the results is illustrated for a typical vertical cavity surface-emitting laser structure.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

High-power operation of an argon excimer laser with a MgF/sub 2/ and SiC cavity

An intense coherent light pulse whose energy, height and width (FWHM) were 80 mJ, 16 MW, and 5 ns, respectively, was obtained as 126 nm from an argon excimer laser. A superpoished SiC mirror was used in combination with a MgF/sub 2/ output coupler for an electron-beam-pumped argon excimer laser. The SiC mirror had a reflectivity at 126 nm as high as 47% and was highly damage resistant. Laser mirror materials for high-power short-wavelength lasers are discussed. Surface damage on the MgF/sub 2/ output mirror could not be avoided; this prevented stable operation of the argon excimer laser. In order to obtain such high power from the argon excimer laser, it is necessary to construct the resonator without a MgF/sub 2/ coupler. An unstable resonator is considered as the best candidate for the stable operation of the argon excimer laser.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

Combustion synthesis of fine-particle ceria

Fine-particle ceria having narrow size distribution is of interest as polishing material for ophthalmological lenses, television tubes, plates and mirrors [1]. The preparation has been reported of such ceria by the hydrolysis of cerium isopropoxide [2] and cerium nitrate [3, 4] followed by thermal decomposition, as well as by the thermal decomposition of cerium oxalate hydrazinate [5] and cerium hydrazine carboxylate [6]. In this letter we report the preparation of fine-particle, large-surface-area ceria by the combustion process developed for the preparation of $\alpha$-alumina [7], aluminates [8], chromites [9] and ferrites [10]. The process involves rapid decomposition of a saturated aqueous solution containing ceric ammonium nitrate and urea $(CH_4N_20)$, carbohydrazide $(CH_6N_4O (CH))$, oxalyl dihydrazide $(C_2H_{16}N_4O_2 (ODH))$ or tetraformal trisazine $(C_4H_{16}N_6O_2 (TFTA))$ redox mixtures.

Chemically vapor deposited silicon carbide (SiC) for optical applications

In this paper we present important physical, thermal, mechanical, and optical properties of cubic (β) silicon carbide produced via a bulk chemical vapor deposition (CVD) process developed at CVD Incorporated. This CVD SiC has been identified as the leading mirror material for high energy synchrotron radiation because of its high thermal conductivity, low thermal expansion, high polishability, and high reflectance in the vacuum UV. However, it has been difficult to obtain high quality, monolithic, CVD SiC mirrors in large sizes, i.e., greater than 10–20 cm. Recently, CVD Incorporated has been successful in scaling an SiC CVD process to produce large monolithic pieces of SiC up to 60 cm (24 in.) in diameter and plates up to 76 cm (30 in.) long by 46 cm (18 in.) wide with thicknesses up to 13 mm (0.5 in.). The properties of this material that make it attractive for optical applications, such as synchrotron optics, will be discussed.

Stainless steel solar mirrors — A material feasibility study

The possibility to use stainless steel solar mirrors for increased energy collection in thermal energy systems was studied. Various grades of austenitic, ferritic and one martensitic steel were considered. The bulk reflectance was measured and the austenitic grades were selected for further analysis. The integrated solar reflectance, 0.67, was considered too low in comparison with alternative materials. The optical constants were used to calculate the solar reflectance at oblique incidence and with dielectric overlayers. Only with a double overlayer of the high-low index type an increase in R sol to 0.72 is noted. It is concluded that stainless steel will not be competitive as material for solar mirrors without such coatings. Careful consideration must also be given to long term stability properties.

Discrete thin-film multilayer design for X-ray and neutron supermirrors

Multilayer structures, analogous to broadband optical filters, may be used to reflect X-rays or neutrons at angles larger than the total external reflection angle intrinsic to the bulk mirror material. These so-called supermirrors have generally been designed using continuum theories, which predict that the thickness of the jth layer in the multilayer should vary smoothly as j −1/4. A new approach is proposed, based on considering the contribution of each bilayer to the extinction in a given stack of bilayers, and the discrete equations governing the choice of layer thicknesses are derived and solved. In the limit of zero layer thickness, the continuum result is recovered. The design produces essentially perfect reflectivity over the entire supermirror range. An optimal technique for trading reflectivity to gain angular range also emerges naturally from the physics of the design.

Grazing incidence imaging from 10 to 40 keV

The prospects for imaging x rays at energies from 10 to 40 keV with grazing incidence optics are explored. The scientific rationale and existing laboratory measurements are reviewed. Measurements of reflectivity using possible mirror materials are described. Iridium-coated float glass gives an improved performance over gold by the factor predicted by theory but both had a lower absolute level. This may be due to a lower density of the thin metal layer caused by the deposition method. The reflectivity of a sample of iridium-coated float glass was measured at small grazing angles (25-5 min of arc) at energies of 8, 17, and 26 keV. High reflectivity (>50%) was seen out to angles of 33, 16, and 11 min of arc, respectively. These are close to the theoretical values. A design for a high energy imaging telescope of the Explorer class is described.

Glow-Discharge Cleaning in Oxygen of Carbon-Contaminated Optical Elements

Carbon-contaminated mirrors and gratings have been cleaned by a d.c. glow discharge in oxygen. The reflectance of mirrors and the efficiency of gratings were measured in situ before and after cleaning in the photon-energy range 4 ∼ 40 eV or 100 ∼ 1000 eV using reflectrometers which incorporated a discharge electrode. Angle-resolved scattering of mirrors and diffraction patterns of gratings were also measured. The mirror reflectance was restored to that of original mirror materials and the scattering level was reduced rather than increased by the discharge cleaning. The grating efficiency of the inside first order was drastically improved by cleaning, even by nearly an order of magnitude in some energy regions. Furthermore, the effect of a blaze was shown to be recovered by examining changes in the efficiencies of other orders as well.

Transparent heat mirror characteristics of cadmium indate

Electrical and optical measurements on rf sputtered CdIn 2O 4 films show that this compound is a wide-bandgap semiconductor. Coatings have been prepared which have high visible transmission and high infrared reflectivity. The optical properties of these samples come close to those of the best known semiconductor heat mirror materials.

Metrology of reflection optics for synchrotron radiation

Recent years have seen an almost explosive growth in the number of beamlines on new and existing synchrotron radiation facilities throughout the world. The need for optical components to utilize the unique characteristics of synchrotron radiation has increased accordingly. Unfortunately, the technology to manufacture and measure the large, smooth, exotic optical surfaces required to focus and steer the synchrotron radiation beam has not progressed as rapidly as the operational demands on these components. Most companies do not wish to become involved with a project that requires producing a single, very expensive, aspheric optic with surface roughness and figure tolerances that are beyond their capabilities to measure. This paper will review some of the experiences of the National Synchrotron Light Source in procuring grazing incidence optical components over the past several years. We will review the specification process - how it is related to the function of the optic, and how it relates to the metrology available during the manufacturing process and after delivery to the user's laboratory. We will also discuss practical aspects of our experience with new technologies, such as single point diamond turning of metal mirrors and the use of SiC as a mirror material. Recent advances in metrology instrumentation have the potential to move the measurement of surface figure and finish from the research laboratory into the optical shop, which should stimulate growth and interest in the manufacturing of optics to meet the needs of the synchrotron radiation user community.

The mirror material test programme at the Daresbury SRS

The present generation of high flux electron storage rings, purpose-built as synchrotron radiation sources, presents new problems in optical design, particularly where reflections other than in grazing incidence are required. A mirror test programme, in collaboration with the Photon Factory, Japan, has been under way on the Synchrotron Radiation Source at Daresbury Laboratory for two years. Many samples, ranging from glasses, through ceramics to metals such as molybdenum, have been exposed to radiation densities up to 70 W/cm 2, and long term changes in their optical properties measured. Considerable effort has been devoted towards developing a cooled mount satisfactory for use in an ultra high vacuum environment. Recommendations are made for the most suitable materials for future use based on the results of this test programme, bearing in mind the even greater intensities that will be available from undulator and multipole wigglers in the future. Finally, a programme for more detailed measurements on the effect of high density heating and radiation on the optical performance of mirrors is described.

Relative cost-effectiveness of CPC reflector designs suitable for evacuated absorber tube solar collectors

Specular reflectors of the fixed CPC type are compared in terms of yearly energy collection and relative cost-effectiveness. The reflector designs used are designed for use with a circular-cylindrical evacuated tubular absorber, and a gap is allowed between reflector and absorber to accommodate the tubular glass envelope and evacuated space. Stainless steel, aluminum, and thin, back-silvered glass mirrors were modelled. The results show that the choice of acceptance angle of a reflector for use with a moderately priced evacuated tube at water heating temperatures is not critical; almost any reflector acceptance angle will do so long as the aperture is carefully chosen, and both North-South or East-West orientations have approximately similar performance. Under such conditions, other factors such as mirror self-cleaning and manufacturing ease may be decisive in the choice of design. At temperatures above 100°C or for high tube costs, an East-West reflector design of concentration >1.4 is strongly indicated. At the time of writing, polished stainless steel is as cost-effective a choice as any other for a mirror material, and is probably more durable and amenable to mass production.

Thermal expansion uniformity of materials for large telescope mirrors

Uniformity of thermal expansion has been measured for fused quartz (Heraeus-Amersil TO8E) and borosilicate glass (Schott Duran and Ohara E6). The variation of expansion coefficient for three melts of TO8E was 5 x 10(-9)/K over a temperature range of 300 to 100 K and was found to vary linearly with position in the melt. This spatial gradient averaged 3.5 x 10(-11)/K cm. The room-temperature thermal expansivity variation of Duran (Tempax) glass was approximately 27 x 10(-9)/K, while that of E6 glass was approximately 52 x 10(-9)/K.

Nature of resonator mirror damage in high-power industrial electron-beam-controlled CO2lasers

An investigation was made of the nature of damage to resonator mirrors and of the change in the microstructure of the mirror material as a result of prolonged operation of a general-purpose industrial electron-beam-controlled CO2 laser with on output power ~ 10 kW. It was found that the damage to the mirror surfaces occurred at considerably lower radiant flux densities than in the case when radiation is focused on a mirror surface in vacuum. Characteristic forms of surface damage were classified according to the following basic scheme: damage to the surface by incandescent particles formed in the laser active medium from the materials in the discharge chamber and resonator; damage by pitting; appearance of deep craters resulting from breakdown at the mirrors; damage to and crystallization of an oxide film, accompanied by the formation of a dendritic structure. An analysis was made of deformation effects in the material of the mirrors.

Application of silicon carbide to synchrotron radiation mirrors

Damage to conventional mirror materials exposed to the harsh synchrotron radiation (SR) environment has prompted the SR user community to search for more suitable materials. Next-generation insertion devices, with their attendant flux increases, will make the problem of mirror design even more difficult. A parallel effort in searching for better materials has been underway within the lasr community for several years. The technology for dealing with high thermal loads is highly developed among laser manufacturers. Performance requirements for laser heat exchangers are remarkably similar to SR mirror requirements. We report on the application of laser heat exchanger technology to the solution of typical SR mirror design problems. The superior performance of silicon carbide for laser applications is illustrated by various material trades studies and its superior performance for SR applications is illustrated by means of model calculations.

Optical Wavefront Analysis of Thermally Cycled 500 mm Metallic Mirrors

Warping is a major concern in the use of aluminium as a material for large mirrors. The warping characteristics are influenced by:*the type of material, i.e. pure aluminium or some alloy*the type of fabrication, i.e. cast, rolled or forged.So far there is little data available on the warping characteristics of aluminium mirrors. Therefore ESO has tested the warping behaviour of 18 aluminium mirrors under thermal cycling conditions with a glass ceramic mirror, which has not been cycled, as a reference mirror. The set of 500 mm mirrors selected for this experiment is described in [1].

Aspheric generator: an all air bearing machine

Methods for the production of aspheric surfaces for optical and infrared applications have been widely described 1. In this paper P. Langenbeck ∗ describes the design of a machine for production of aspheric surfaces by single point diamond machining. The substrates which have been machined include the known reflective mirror materials such as copper, aluminium and nickel, germanium and certain plastics. Parts sizes have ranged from a few mm up to 600 mm in diameter. The machine described here was first reported at the 1982 meeting of the German and Swiss Optical Societies in Luzern 2. In operation, a single diamond tool sweeps just once along a computed curve within the surface of revolution, producing immediately the final surface in terms of geometry and microstructure. Post-polishing is not necessary

Multilayer mirrors for soft X-ray and VUV radiation

Materials for multilayer mirrors have been selected and the limiting values for the reflection coefficient and resolution in the vacuum UV and soft X-ray wavelength ranges (λ=200−20 A ̊ ) have been calculated on the basis of the optical constant data. An attempt has been made to fabricate multilayer mirrors by a modified method of laser deposition. Conditions under which the dose of material evaporated during one laser pulse is stabilized are defined. The possibility of obtaining layers with given thickness without optical control during the film deposition is demonstrated. Mirrors for normal and grazing incidence optics have been fabricated and studied using synchrotron radiation. For wavelengths λ=60−65 A ̊ the reflection coefficients obtained were ≈ 7% for mirrors are normal incidence optics at α=2.5° and 20% for mirrors in grazing incidence optics at λ=53 A ̊ . Multilayer mirrors in the wavelength range 60÷110 Å have been fabricated. The size of focal spot has been shown to be completely determined by the divergence of synchrotron radiation.

Investigation of X-ray exposure using plane scanning mirrors

There are different methods of enlarging the small exposure field resulting from the slight divergence of the synchrotron beam. One possible way is the application of scanning X-ray mirrors in the grazing incidence region with reflectivities around 90% at wavelengths around 10 Å. The theoretical values and experimental behavior of some mirror materials (Zerodur, gold) have been compared. The distribution of the absorbed power in the X-ray resist as a function of the glancing angle, in the case of a plane mirror, has been calculated. The influence of the low-pass properties of the X-ray mirror on the quality of copied structures has been investigated.

Soft and Ultrasoft X-Ray Fluorescent Spectrometer Using Total Reflection Monochromator

Since specular reflection of characteristic X-rays can become nearly total external reflection at grazing incidence, total reflection analysis offers an alternative to the wavelength dispersive method based on Bragg reflection. Using curves of calculated and measured intensity versus glancing angle, a high-efficiency mirror material was selected and the optimized optical geometry and filtering conditions were determined. Short wavelength X-rays could be eliminated and overlapping interference of equal and longer wavelength X-rays was excluded using the equation for determining the element concentration. Analytical examples for Al, C and B are described.