Nanooptics and photonics
Fluorescence of Nd3+ optical centers close to cubic symmetry in a calcium fluoride crystal co-doped with Na+
E. Vinogradova1, L. Dolgov1, V.A. Konyushkin2, E.O. Orlovskaya2, E.A. Vagapova1,2, A. Treshchalov1, V. Peet1, V. Hizhnyakov1, Yu.V. Orlovskii1,2
1 Institute of Physics, University of Tartu, W. Ostwaldi Street 1, Tartu 50411, Estonia
E-mail: elena.vinogradova@ut.ee
2 Prokhorov General Physics Institute of the Russian Academy of Sciences, 38 Vavilov street, Moscow 119991, Russia
E-mail: vasil@lst.gpi.ru

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. [1] 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 for the first time by the method of time-resolved site-selective fluorescence laser spectroscopy. New optical centers exhibit inhomogeneous splitting and can be attributed to nearly cubic sites with symmetry close to Oh.
Also numerous new 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.
We found that the optical sites with τR = 8.5 ms in the Nd3+ (0.05 at.%): Na+ (0.2 at.%): CaF2 crystals have C2ν symmetry, where Na+ ion replaces the Ca2+ ion in one of the positions adjacent to the Nd3+ ion. Since optical sites with C4ν symmetry have τR = 1.4 ms, it 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 shorter than at sites with lower symmetry. This result was explained by the different distance between the Nd3+ ion and the ion, which compensates for the excess of the extra charge using the Judd-Ofelt approach.

1.	G. Vincow, W. Low. Paramagnetic resonance spectra of f3 ions in a cubic site // Phys. Rev. 122(5) (1961) 1390-1392