Extended Data Fig. 2: Ensemble spectroscopy of Er³⁺:CaWO₄.

a Site-selective excitation spectrum of Er³⁺:CaWO₄. Green arrows indicate when the excitation laser is resonant with different excited state crystal-field levels, where the corresponding excitation path is labeled using the matching number in (b). Solid orange arrows denote the fluorescence energies due to decay from the ⁴I_13/2Y₁ level, whereas the dashed arrows denote decay from the ⁴I_13/2Y₂ level. No other Er³⁺ sites were observed. The prominent line with unity gradient corresponds to laser scatter and has been re-scaled in intensity for clarity. b Assigned transitions. The performed spectroscopy yielded energies of four ⁴I_15/2 and three ⁴I_13/2 levels.