5d–4f luminescence of Er3+ in YAG:Er3+

Interconfigurational 4f105d ↔ 4f11 transitions of the Er3+ ion in the YAG host were studied under both VUV photon (synchrotron radiation) and electron beam excitation. It was found that the lowest low-spin 5d level of the Er3+ ion has a rather large energy gap to the next lower 4f2D(2)5/2 crystal-field level, which results in a relatively low rate of nonradiative transitions from this 5d level leading to the appearance of weak spin-allowed 5d–4f luminescence at low temperature. The lowest high-spin 5d level, from which spin-forbidden 5d–4f radiative transitions could occur potentially, is situated only at ∼500 cm−1 above the 4D1/2 level. Such close location allows fast depopulation of the 5d level resulting in the absence of spin-forbidden 5d–4f luminescence and appearance of 4D1/2 4f–4f luminescence.