Fluorescence of Nd3+optical centers close to cubic symmetry in a calcium fluoride crystal co-doped with Na

Earlier, it was shown by the EPR method that at a very low concentration of the Nd3+ ion in the Nd3+:CaF2 crystal, highly symmetric cubic centers with the Oh symmetry are formed. In the present study, the optical centers of the Nd3+ ion with an anomalously long radiative lifetime tau(R) = 13.6 ms of the F-4(3/2) (1) crystal-field (CF) level in the Nd3+ (0.05 at.%): Nat (0.2 at.%): CaF2 single crystal at T = 7 K were detected by the method of time- resolved site-selective fluorescence laser spectroscopy. Its fluorescence excitation at the I-4(9/2) (1) -> (4)G(5/2) (1) transition of Nd3+ is in a longer wavelength spectral range compared to optical tetragonal (C-4V)L-, orthorhombic (C-s) pair M-, and quartet N- centers. New optical centers exhibit inhomogeneous splitting and can be attributed to nearly cubic sites with symmetry close to O-h. Also numerous Nd3+ optical sites in the Nd3+ (0.03 at.%): CaF2 and Nd3+ (0.05 at.%): Na+ (0.2 at.%): CaF2 crystals at T = 7 K with tau(R) = 240 mu s - 8.5 ms of the F-4(3/2)(1) CF level of Nd3+ were detected in a shorter wavelength spectral range compared to the nearly cubic sites. Among them, the sites with the longest tau(R) = 8.5 ms except for the nearly cubic sites, which were identified as C-2V symmetry sites. Their radiative lifetime is about six times that of the C-4V site (tau(R) = 1.4 ms). This means that, for optical sites without a center of inversion, the radiative lifetime of the excited state at sites with higher symmetry can be almost an order of magnitude less than at sites with lower symmetry. We explained this result by the different distance between the Nd3+ ion and the ion, which compensates for the excess of its charge. Thus, we have demonstrated the use of the Judd-Ofelt approach to explain the difference in the rates of emission of the excited state in different optical sites of the same rare-earth ion in the same crystal matrix.

Automated summary

In this study, highly symmetric cubic centers with Oh symmetry were formed at very low concentrations of the Nd3+ ion in the Nd3+:CaF2 crystal. The optical centers of the Nd3+ ion with anomalously long radiative lifetime were detected in the single crystal, showing inhomogeneous splitting and attributed to nearly cubic sites with symmetry close to O-h. Additionally, optical sites with different radiative lifetimes were identified and explained using the Judd-Ofelt approach to understand the emission rates in different optical sites of the same rare-earth ion.