Selected Publications

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BYU Authors: David D. Allred and Matthew R. Linford, published in Society of Vacuum Coaters, 2019 Technical Conference Proceedings, Optical Coatings (August 22, 2019).

Aluminum enjoys broad band reflectivity and is widely used as an astronomical reflector. However, it oxides rapidly, and this oxide absorbs very short wavelength light, which limits the performance of aluminum mirrors. Accordingly, thin transparent layers, such as films of MgF2, are used to protect aluminum. In this study, we present an X-ray photoelectron pectroscopy (XPS) study of the chemical changes in MgF2 - protected aluminum that take place as it oxidizes (is exposed to the air). XPS reveals the rate of Al oxidation for different MgF2 thicknesses as determined from measurements obtained from 5 min to 8 months of air exposure. The degree of Al oxidation depends on the MgF2 over layer thickness.

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BYU Authors: David D. Allred, J. Gabriel Richardson, and R. Steven Turley, published in Optical Interference Coatings 2019, (Santa Ana Pueblo, NM, June 2019).

While no solid barrier layer is transparent below ~103nm, simulations show that ~9.5nm LiF on 8.5nm MgF2 on Al could reflect some hydrogen Lyman lines better than a single fluoride layer does. Experiments are promising.

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BYU Authors: Joseph B. Muhlestein, Benjamin D. Smith, Margaret Miles, Stephanie M. Thomas, Anthony Willey, David D. Allred, and R. Steven Turley, published in Opt. Express
We report optical constants of e-beam evaporated yttrium oxide Y2O3 thin films as determined from angle-dependent reflectance measurements at wavelengths from 5 to 50 nm. Samples were measured using synchrotron radiation at the Advanced Light Source. The experimental reflectance data were fit to obtain values for the index of refraction and thin film roughness. We compare our computed constants with those of previous researchers and those computed using the independent atom approximation from the CXRO website. We found that the index of refraction near 36 nm is much lower than previous data from Tomiki as reported by Palik. The real part of the optical constants is about 10% to 15% below CXRO values for wavelengths between 17 nm and 30 nm. Films were also characterized chemically, structurally, and optically by ellipsometry and atomic force microscopy.
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BYU Authors: Nathan D. Powers, Dallin S. Durfee, and David D. Allred, published in 2018 Conference on Laboratory Instruction Beyond the First Year of College, Part of the BFY Conference series, (Baltimore, MD, July, 2018).
There is a natural tendency for students to act first (e.g. - build and conduct experiments) and think later (e.g. - outline goals, identify challenges, predict outcomes, etc.). This is often apparent in labs that include student design components. We have developed a lab course structure that teaches students how to develop their ideas and make plans before beginning an experiment by providing multiple opportunities for peer and instructor feedback. As a result, we have seen significant improvements in the success rate and quality of student-designed experiments and presentations. We provide a detailed explanation of the course structure and rubrics and evidence of the impacts of this course structure.
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BYU Authors: Michael Greenburg, David D. Allred, and R. Steven Turley, published in J. Utah Acad. Sci.

We report on the results of computationally designing and optimizing

multilayer mirrors for broadband reflectivity in a region spanning from

the vacuum ultraviolet to the infrared. Such a mirror would open up

new possibilities for future space observatories. Because of the

immense number of possible layer combinations and thicknesses for a

multilayer mirror, we automated the mirror selection process with a

genetic algorithm. Starting with a random object population within the

simulation, a genetic algorithm iteratively selects and mutates the best

portion of a population of objects that fit given design criteria to create

a new population; this can be repeated as many times as desired. Our

genetic algorithm yielded a high broadband reflectance mirror, which

was then optimized via gradient search within the program. We found

that placing a few layers under an aluminum coating can significantly

increase extreme ultraviolet reflectivity, which would give access to

important spectral lines such as that of the dominant He-II transition.

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BYU Authors: David D. Allred, R. Steven Turley, Stephanie M. Thomas, Spencer G. Willett, Michael J. Greenburg, and Spencer B. Perry, published in Proc. SPIE
Protective layers on aluminum mirror surfaces which can be removed via the use of atomic hydrogen or hydrogen plasmas at the point of use in space may allow an expansion of broad-band mirrors into the EUV. LUVOIR (large, UV-optical-IR telescope) is a potential NASA flagship space-based observatory of the 2020’s or 30’s. It would utilize the largest mirrors ever flown1 . Their reflective coating will almost certainly be aluminum, since such telescopes would profit from truly broad-band mirrors. To achieve reflectance over the broadest band, the top surface of such aluminum mirrors, however, needs to be bare, without the oxide layers that naturally form in air. This will open the 11 to 15 eV band. Since thin aluminum films are largely transparent between 15 and 70 eV an EUV mirror under the aluminum could make EUV bands such as 30.4 nm available for space-based astrophysics without sacrificing mirror IR, visible and UV reflectance. The local space environment for the observatory is sufficiently oxygen-free that the surface should remain bare for decades. We discuss protecting as-deposited aluminum mirrors with robust, oxygenimpenetrable, barrier layers applied in vacuo to the aluminum immediately after deposition and before air contact. The goal is that the barrier could also be cleanly, and relatively easily, removed once the mirror is in space. We propose hydrogen atoms as the means for removing the overcoat, since they can be expected to meet the criteria that the means is gentle enough to not roughen the mirror surface, and does not redeposit material on the mirror or other spacecraft components. We have investigated both organic and inorganic (such as, a-Si) hydrogen-removable films that can be applied to the aluminum immediately after its deposition have been investigated. We also examined the REVAP technique, using Cd and Zn. Agglomeration limited their effectiveness as barrier layers. That and dealing with the reevaporated atoms may limit their utility as barrier materials.