The optical characterization was carried out on the single crystals as depicted in Figure of the previous section. Both the 4f 3 ion Nd 3+ and the 4f 10 ion Ho 3+ can show luminescence in the visible (Ho 3+ ) and near infrared (Nd 3+ ) spectral range dependent on the cutoff phonon energy of the surrounding host compound (see Figure 7 a-d). Typically, gallate-based garnets are characterized by maximum phonon energies in the order of 700 cm -1 , which limits multi-phonon relaxation based on the energy-gap law. Consequently, luminescent decay times of energetically isolated levels such as the 4 F3/2 level of Nd 3+ or the 5 S2, 5 F4 levels of Ho 3+ are expectedly high, in the order of ms. Indeed, Nd 3+ shows dominant NIR emission at around 870 nm due to the 4 F3/2 → 4 I9/2 transition (and additional transitions to the higher energetic ground levels 4 I11/2 and 4 I13/2 at around 1063 nm and 1330 nm, respectively; not shown here).