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

Highlights: • We detected fluorescence of Nd³⁺ nearly cubic sites in the Nd³⁺:Na⁺:CaF₂ crystal. • The ⁴F_3/2(1) CF level lifetime of Nd³⁺ of the nearly cubic sites is long, τ_R = 13.6 ms. • An inhomogeneous splitting of the spectral lines of the nearly cubic sites is found. • τ_R of the C_2v symmetry site is about six times that of the C_4v higher symmetry site. • We explained the result by different Nd³⁺-Na⁺ distances using the Judd-Ofelt approach. 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 O_h symmetry are formed. In the present study, the optical centers of the Nd³⁺ ion with an anomalously long radiative lifetime τ_R = 13.6 ms of the ⁴F_3/2(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 ⁴I_9/2(1) → ⁴G_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 τ_R = 240 μs ‒ 8.5 ms of the ⁴F_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 τ_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 (τ_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.