Fluorescence line narrowing (FLN) and site-selective fluorescence decay of Nd 3+ centers in CaF 2

Abstract

Site-selective and time-resolved fluorescence and excitation spectroscopy in the absorption bands of the M and N centers in CaF 2 : Nd 3+ (0.6%) were performed at 1.6 and 4.2 K using a pulsed tunable laser. The site-selective excitation and fluorescence spectra including the decay kinetics were studied for the crystal-field transitions 4 I 9/2(1)→ 4 G 5/2(1) in absorption and 4 F 3/2(1)→ 4 I 9/2(1) in emission for different Nd 3+ centers. It is shown that the M absorption band mainly formed by (Nd 3+–F i) 2 pair M centers contains also spectral lines of other pair modifications. Similarly, the N band includes not only lines from (Nd 3+–F i) 4 quartet N centers but also from higher aggregated Q and R centers as well as from lower aggregated S centers. These centers have different excitation and fluorescence spectra and different 4 F 3/2 fluorescence lifetimes. The weak absorption between the M and N bands is due to numerous so-called D(2a) centers. An attempt to identify some of the new centers on the basis of their fluorescence lifetimes and nonradiative quenching rates was made. It was proved that the 4 I 9/2(1) and the 4 G 5/2(1) Stark levels in these centers split up into several sublevels. For the pair M and the quartet N centers the total splitting could be verified to be about 2.5 and 3.5 cm −1, respectively. Values of 5 and 6 cm −1 for the Q and Q′ centers and a value of 5 cm −1 for the R center were found.