One- and two-photon spectra of Nd3+ clusters in CaF2 and SrF2 crystals

The polarised two-photon (IR) and one-photon (visible) luminescence excitation spectra of Nd³⁺ nanoclusters in CaF₂ and SrF₂ crystals are measured at 10 K using a F₂^-:LiF colour centre laser tunable in spectral ranges 1090 – 1230 nm and 545 – 615 nm with an emission linewidth of ~0.02 – 0.03 cm^-1, an average output power of ~55 mW, and a pulse repetition rate of 10 Hz. The two-photon excitation spectra at the ⁴I_9/2 → ⁴G_5/2 transition reveal the structure, which is absent upon one-photon excitation, which can be explained by different selection rules for some Stark – Stark transitions upon one- and two-photon absorption.