Temperature Dependence of AlF3 Protection on Far-UV Al Mirrors

Direct measurement of the extinction coefficient by differential transmittance

A new procedure to measure the extinction coefficient k of film materials that are relatively transparent is presented. This procedure does not require the use of an optical-constant model or the knowledge of extra physical properties of the material, such as the specific heat capacity. It involves preparing a sample with two areas, at least one of them coated with the film, whereas the other may remain uncoated or may be coated with a different thickness of the same material. The differential transmittance between the two sample areas is shown to be proportional to k of the film material in the following measurement conditions: the incident light is p polarized and it impinges at the film material Brewster angle. The differential transmittance is obtained with a single measurement by making the light beam or the sample to oscillate with respect to one another and by using a lock-in amplifier; for normalization purposes, the transmittance in one of the sample areas is also measured. The proportionality factor between the normalized differential transmittance and k only involves the wavelength, the film thickness, and the Brewster angle. The knowledge of the film Brewster angle requires that the film refractive index (n) is measured beforehand; this can be performed with standard procedures, such as ellipsometry, since such techniques are efficient at measuring n of a transparent material, but are inefficient at measuring a small k. The procedure is exemplified with the calculation of k in the far ultraviolet of AlF3 films deposited by evaporation. The dependence of the uncertainty of k obtained with this procedure is analyzed in terms of the uncertainty of the film n, of wavelength, and of the degree of polarization of the incident beam. The selection of a substrate with similar n to the film material is also discussed. The uncertainties involved with the present procedure were analyzed for a specific example and an uncertainty of 2 × 10-5 in k calculation is considered feasible.

Synthesis under Normal Conditions and Morphology and Composition of AlF3 Nanowires

AlF3 has interesting electrophysical properties, due to which the material is promising for applications in supercapacitors, UV coatings with low refractive index, excimer laser mirrors, and photolithography. The formation of AlF3-based nano- and micro-wires can bring new functionalities to AlF3 material. AlF3 nanowires are used, for example, in functionally modified microprobes for a scanning probe microscope. In this work, we investigate the AlF3 samples obtained by the reaction of initial aluminum with an aqueous hydrofluoric acid solution of different concentrations. The peculiarity of our work is that the presented method for the synthesis of AlF3 and one-dimensional structures based on AlF3 is simple to perform and does not require any additional precursors or costs related to the additional source materials. All the samples were obtained under normal conditions. The morphology of the nanowire samples is studied using scanning electron microscopy. We performed an intermediate atomic force microscope analysis of dissolved Al samples to analyze the reactions occurring on the metal surface. The surface of the obtained samples was analyzed using a scanning electron microscope. During the analysis, it was found that under the given conditions, whiskers were synthesized. The scale of one-dimensional structures varies depending on the given parameters in the system. Quantitative energy-dispersive x-ray spectroscopy spectra are obtained and analyzed with respect to the feedstock and each other.

Far UV narrowband mirrors tuned at H Lyman α

Imaging at H Ly-α (121.6 nm), among other spectral lines in the short far UV (FUV), is of high interest for astrophysics, solar, and atmosphere physics, since this spectral line is ubiquitously present in space observations. However, the lack of efficient narrowband coatings has mostly prevented such observations. Present and future space observatories like GLIDE and the IR/O/UV NASA concept, among other applications, can benefit from the development of efficient narrowband coatings at Ly-α. The current state of the art of narrowband FUV coatings lacks performance and stability for coatings that peak at wavelengths shorter than ∼135 nm. We report highly reflective AlF₃/LaF₃ narrowband mirrors at Ly-α prepared by thermal evaporation, with, to our knowledge, the highest reflectance (over 80%) of a narrowband multilayer at such a short wavelength obtained so far. We also report a remarkable reflectance after several months of storage in different environments, including relative humidity levels above 50%. For astrophysics targets in which Ly-α may mask a close spectral line, such as in the search for biomarkers, we present the first coating in the short FUV for imaging at the OI doublet (130.4 and 135.6 nm), with the additional requirement of rejecting the intense Ly-α, which might mask the OI observations. Additionally, we present coatings with the symmetric design, aimed to observe at Ly-α, and reject the strong OI geocoronal emission, that could be of interest for atmosphere observations.

Pinholes in Al/MgF2

Pinholes in A l/M g F ₂ were first inferred by the transmittance enhancement in Larruquert group's research. However, no direct proof was reported to verify the existence of the pinholes in A l/M g F ₂. In the past 80 years, pinholes were first observed by dark-field microscopy and bright-field microscopy in the transmission mode. They were small and on the order of several hundred nanometers to several micrometers. Essentially, the pinhole was not a real hole, partially because of the lack of the Al element. Increasing the thickness of Al cannot reduce the size of the pinholes. The occurrence of the pinholes was dependent on the deposition rate of the Al film and the substrate heating temperature, and it was independent of the substrate materials. This research eliminates an otherwise easily ignored scattering source, and it will benefit the development of ultra-precise optics, including mirrors for gyro-lasers, the detection of gravitational waves, and coronagraph detection.

Far UV-enhanced Al mirrors with a Ti seed film

A Ti seed film is investigated towards improving the far UV reflectance of Al/MgF₂ mirrors. Samples were initially coated with a Ti film in half of the area and they were later coated in the full area with an Al film and protected with MgF₂. All materials were deposited by evaporation. Samples were prepared with the MgF₂ layer deposited either at room temperature (RT) or at 225°C. A 3-nm thick Ti seed film was seen to significantly increase the reflectance of Al/MgF₂ mirrors at the well-known reflectance dip centered at ∼160 nm; this was attributed to a reduction of short-range surface roughness at the Al/MgF₂ interface, which is responsible for radiation absorption through surface-plasmon (SP) coupling. SP absorption was more efficiently reduced with a Ti seed film on samples fully deposited at RT. A Ti seed film as thin as 1 nm provided the largest SP absorption reduction, and the SP dip was almost completely removed.

Mirrors for Space Telescopes: Degradation Issues

Mirrors are a subset of optical components essential for the success of current and future space missions. Most of the telescopes for space programs ranging from earth observation to astrophysics and covering the whole electromagnetic spectrum from x-rays to far-infrared are based on reflective optics. Mirrors operate in diverse and harsh environments that range from low-earth orbit to interplanetary orbits and deep space. The operational life of space observatories spans from minutes (sounding rockets) to decades (large observatories), and the performance of the mirrors within the mission lifetime is susceptible to degrading, resulting in a drop in the instrument throughput, which in turn affects the scientific return. Therefore, the knowledge of potential degradation mechanisms, how they affect mirror performance, and how to prevent them is of paramount importance to ensure the long-term success of space telescopes. In this review, we report an overview of current mirror technology for space missions with a focus on the importance of the degradation and radiation resistance of coating materials. Special attention is given to degradation effects on mirrors for far and extreme UV, as in these ranges the degradation is enhanced by the strong absorption of most contaminants.