Nanooptics and Photonics

Nd3+ sites with extremely long-living (~13.6 ms) states in Nd3+:Na+:CaF2 crystals
Vinogradova E., Dolgov L., Konyushkin V.A., Orlovskaya E.O., Vagapova E.A., Treshchalov A., Peet V., Hizhnyakov V., Orlovskii Yu. V.
Institute of Physics, University of Tartu, W. Ostwaldi Street 1, Tartu, 50411, Estonia

Inorganic fluoride crystals activated by Nd3+ ions were traditionally considered as fluorescent and laser materials. Recently it was proposed that energy states of Nd3+ ions paired inside fluoride hosts can model states of entangled qubits, which can be prospective for realization of multi-qubit quantum calculations [1]. It actualized our interest to energy states of different Nd3+ sites and their fluorescent properties.
Energy states of individual Nd3+ sites having cubic symmetry in CaF2 host were confidently detected earlier by electron paramagnetic resonance technique [2, 3]. However, there were no articles about registration of cubic Nd3+ sites in CaF2 in fluorescence regime till recent time. Most probably it is because the cubic symmerty of Nd3+ site implies extremely long life time for excited metastable 4F3/2 energy state of Nd3+ ion and therefore very low probability of radiative 4F3/2  4I9/2 transition. It means that fluorescence intensity of this transition is so weak for cubic site, that sensitivity of equipment was not enough, untill recent time, to detect it.
Here we report about confident detection of long-living state of Nd3+ sites with near cubic symmetry in Nd3+:Na+:CaF2 crystal and determination of their life time ~ 13.6 ms. Sample was placed in cryostat at 7 K temperature in order to minimize homogeneous broadening of spectral lines. Excitation was done by dye laser with precisely tuned wavelength. Fluorescence spectra were registered by CCD Camera intensified with microchannel plate having Peltier cooling (Andor Technology, Oxford Instruments, GB). Fluorescence kinetics was detected by photomultiplier tube 6240-02 (Hamamatsu, Japan). Several new Nd3+ sites with long-living lifetimes between 1.5 and 8 ms were revealed also in course of our experiments. These sites could be assigned with clusters of Nd3+ ions having comparatively high symmetry. Acknowledgement. This work was supported by Estonian Research Council project PRG347.

1.	Orlovskii Yu. V. et al. // J. Lumin.-2020.-219.-116920.
2.	Vincow G., Low W. // Phys. Rev.-1961.-122(5).-P. 1390-1392.
3.	Kask N. et al. // Phys. Solid State (ru).-1964.- 6(2).-P. 549-553.