Thulium is only required in small quantities. Here is a back-of-the-envelope calculation: molar mass of thulium: 169g/mol, density: 9.32g/cm3, i.e. 0.055 mol/cm^3 [0]. That's ~ 3.3*10^22 atoms per cubic centimeter.
From the article: A thulium ion dopant concentration of 4.0 × 10^20 cm−3 was measured using Rutherford backscattering
spectrometry. [1], i.e. a concentration of roughly 1/100 compared to pure material.
Taking the first search result at face value, a cubic centimeter of 99.9% pure Thulium costs $85 [2]. Since the disks seem to have a diameter of 40μm, only a minuscule fraction of a cubic centimeter of material is needed for each.
I would conclude that the material costs for these disks are not as large as initially assumed.
From the article: A thulium ion dopant concentration of 4.0 × 10^20 cm−3 was measured using Rutherford backscattering spectrometry. [1], i.e. a concentration of roughly 1/100 compared to pure material.
Taking the first search result at face value, a cubic centimeter of 99.9% pure Thulium costs $85 [2]. Since the disks seem to have a diameter of 40μm, only a minuscule fraction of a cubic centimeter of material is needed for each.
I would conclude that the material costs for these disks are not as large as initially assumed.
[0] https://en.wikipedia.org/wiki/Thulium
[1] https://onlinelibrary.wiley.com/doi/10.1002/lpor.202100348#p... (page 8, left column halfway down the page)
[2] https://luciteria.com/metal-cubes/thulium-metal-cubes