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

Abstract

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 τR = 13.6 ms of the 4F3/2(1) crystal-field (CF) level in the Nd3+ (0.05 at.%): Na+ (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 4I9/2(1) → 4G5/2(1) transition of Nd3+ is in a longer wavelength spectral range compared to optical tetragonal (C4v) L-, orthorhombic (Cs) pair M-, and quartet N- centers. New optical centers exhibit inhomogeneous splitting and can be attributed to nearly cubic sites with symmetry close to Oh. 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 τR = 240 μs ‒ 8.5 ms of the 4F3/2(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 τR = 8.5 ms except for the nearly cubic sites, which were identified as C2v symmetry sites. Their radiative lifetime is about six times that of the C4v site (τR = 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.