Code to design a broadband multilayer sapphire HWP according to Komatsu et al. “Demonstration of the broadband half-wave plate using the nine-layer sapphire for the cosmic microwave background polarization experiment”. In: Journal of Astronomical Telescopes, Instruments, and Systems 5.4 (2019), pp. 1–14. url: https://doi.org/10.1117/1.JATIS.5.4.044008. The algorithm is described in T. Ghigna PhD thesis: https://ora.ox.ac.uk/objects/uuid:a8124ed3-683f-49ac-9ad7-7a366292490d.