Research provides insights into aluminum’s optical properties
(Download Image)
A magnetron sputtering deposition system at LLNL. Capabilities such as this system allowed researchers to produce the stable, well-characterized aluminum films used in the study.
Aluminum is an important material for a variety of scientific and technological applications, including plasma physics, astrophysics, semiconductor photolithography, and instrumentation for short wavelengths (ultraviolet [UV], extreme ultraviolet [EUV], and x-ray). Despite this common use—and a wealth of experimental data about aluminum—there is still a lack of accurate information about aluminum’s optical constants (refractive index), or the properties that determine how aluminum interacts with light. Understanding these properties is important for designing aluminum-based optics and predicting their performance.
In an article published in the Journal of Applied Physics and selected as an “Editor’s Pick,” a team of researchers from LLNL, Université Paris-Saclay, and Lawrence Berkeley National Laboratory describe their work to provide more accurate optical constants for aluminum. Their experiments focused on the EUV and x-ray spectral ranges.
The team constructed freestanding aluminum films with varying thickness, protected by carbon layers to prevent oxidation. Using the Advanced Light Source, they measured the EUV/soft x ray transmittance (photoabsorption) of each film. They then combined these new measurements with existing data from other spectral regions to calculate how the aluminum dispersed light at different wavelengths.
The result of the team’s analysis was a new set of values for the refractive index of aluminum in a wide spectral range, extending from the near-UV to the x-ray region, with ultrahigh resolution near the L absorption edge (L1 and L2,3 regions). The team validated the new optical constants by comparing the experimental results with simulations based on aluminum optical constants from different sources. The agreement between the experimental data and the model using the new optical constants demonstrates increased accuracy compared to models using existing optical constants values from the literature.
The refractive index values from this work support the design, modeling, and calibration of EUV/x-ray instruments. Researchers can also use the values to validate and advance atomic and molecular physics models, such as ab initio calculations.
Funding for this work was provided in part by the Laboratory Directed Research and Development program (20-FS-026).
[C. Burcklen, F. Delmotte, J. Alameda, F. Salmassi, E. Gullikson, and R. Soufli, Optical constants of magnetron sputtered aluminum in the range 17–1300 eV with improved accuracy and ultrahigh resolution in the L absorption edge region, J. Appl. Phys. (2024), doi: 10.1063/5.0233781.]
–Physical and Life Sciences Communications Team
