# Spectroscopic characteristics of Pr-doped Na0.4Y0.6F2.2 crystals, promising for lasing in near IR and visible range

A.M. Tkachuk, S.E. Ivanova, A.A. Mirzaeva, Marie-France Joubert, Y. Guyot

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A.M. Tkachuk, S.E. Ivanova, A.A. Mirzaeva, Marie-France Joubert, Y. Guyot. Spectroscopic characteristics of Pr-doped Na0.4Y0.6F2.2 crystals, promising for lasing in near IR and visible range. 2014 International Conference Laser Optics, Jun 2014, Saint Pétersburg, Russia. 10.1109/LO.2014.6886255. hal-02317425

HAL Id: hal-02317425

https://univ-lyon1.hal.science/hal-02317425

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# Spectroscopic Characteristics of Pr-Doped Na<sub>0.4</sub>Y<sub>0.6</sub>F<sub>2.2</sub> Crystals, promising for lasing in near IR and visible range.

A.M. Tkachuk, S.E. Ivanova
ITMO University, Russia
e-mail: am tkachuk@mail.ru

A.A. Mirzaeva
Vavilov State Optical Institute,
St. Petersburg, Russia
e-mail: amirzaeva@mail.ru

M.-F. Joubert, Y. Guyot
Institut Lumière Matière, Université
de Lyon, Lion, France

***Abstract***—Spectroscopic characteristics of intracenter transitions in disordered crystals of sodium-yttrium double fluoride Na0.4Y0.6F2.2: Pr3+ with cubic structure, grown by Stockbarger-Bridgeman method, are investigated theoretically and experimentally in order to evaluate their potential for application as active media of solid state lasers emitting in visible spectral range.

***Keywords***— cubic fluoride crystals, praseodymium, absorption spectra, oscillator strengths, intensity parameters, branching coefficients, lifetimes, luminescence decay kinetics

## I. INTRODUCTION

In the last decade, interest in the spectroscopic properties of crystals doped by ions of rare-earth elements has rekindled. These crystals are especially promising for practical applications as active media for solid-state lasers in the UV, visible, and mid-IR ranges. The upconversion effect is widely used in quantum counters, visualizers, and nanomarkers. In recent years, development of laser diodes emitting in visible range of the spectrum, stimulated interest to Pr<sup>3+</sup> doped crystals, which have a broad absorption band in the range of 440–480 nm.

## II. RESULTS AND CONCLUSION

We studied the series of cubic fluoride crystals Na<sub>0.4</sub>Y<sub>0.6</sub>F<sub>2.2</sub>:Pr<sup>3+</sup> (NYF:Pr<sup>3+</sup>) with the praseodymium concentration 0.04–9 at % grown by using the Bridgman–Stockbarger technique. All grown single crystals were high optical quality without veils, visible inclusions, bubbles and scattering centers.

The optical spectra of NaYF:Pr<sup>3+</sup> crystals at room and low (7 K) temperatures in the range of 200–2800 nm were investigated. The low-temperature absorption spectra of NYF:Pr<sup>3+</sup> crystals have been shown to consist of broad weakly structured bands.

Based on the analysis of low-temperature absorption spectra, the positions of excited multiplets of Pr<sup>3+</sup> ions were determined. The structure of the Stark splitting of praseodymium levels has been represented in terms of a model of “quasi-centers”, which are characterized by an inhomogeneous broadening of Stark components. The energy gaps ΔE<sub>ji</sub> between praseodymium multiplets were further used to estimate the rates of nonradiative transitions from excited radiative levels.

From experimental absorption spectra at T = 300 K, we have calculated absorption cross-section spectra and the oscillator strengths for transitions from the ground state <sup>3</sup>H<sub>4</sub> to excited multiplets <sup>3</sup>H<sub>5</sub>, <sup>3</sup>H<sub>6</sub>, <sup>3</sup>F<sub>j</sub> (j = 2, 3, 4), <sup>1</sup>G<sub>4</sub>, <sup>1</sup>D<sub>2</sub>, and (<sup>3</sup>P<sub>p</sub>, <sup>1</sup>I<sub>6</sub>) (j = 0, 1, 2). The maximal values of the peak absorption cross-sections belong to bands that are located in the visible range (λ<sub>abs</sub> = 441 nm; transitions <sup>3</sup>H<sub>4</sub> → <sup>3</sup>P<sub>0,1,2</sub>, <sup>1</sup>I<sub>6</sub>) and in the near-IR range of the spectrum (λ<sub>abs</sub> = 1526 nm; transition <sup>3</sup>H<sub>4</sub> → <sup>3</sup>F<sub>3,4</sub>). Corresponding experimental values of the peak absorption cross sections from the ground state <sup>3</sup>H<sub>4</sub> are σ<sub>abs</sub>(441 nm) = 1.01 × 10<sup>-20</sup> cm<sup>2</sup> and σ<sub>abs</sub>(1526 nm) = 1.41 × 10<sup>-20</sup> cm<sup>2</sup>.

Using the Judd-Ofelt method, we have determined intensity parameters Ω<sub>4</sub> for NYF:Pr<sup>3+</sup> crystal: Ω<sub>2</sub> = 0, Ω<sub>4</sub> = 4.4 × 10<sup>-20</sup>, and Ω<sub>6</sub> = 2.28 × 10<sup>-20</sup> cm<sup>2</sup>. With these values, we have calculated the probabilities of radiative transitions, the branching coefficients, and the lifetimes of the radiative levels <sup>1</sup>D<sub>2</sub> and <sup>3</sup>P<sub>0</sub>. The probabilities of multiphonon nonradiative transitions in NYF:Pr<sup>3+</sup> crystals have been estimated.

Using the method of kinetic spectroscopy with selective excitation, we have investigated the luminescence decay kinetics of Pr<sup>3+</sup> from the <sup>3</sup>P<sub>0</sub> and <sup>1</sup>D<sub>2</sub> levels upon their selective resonant excitation by nano-second laser pulses. We showed that the decay of the luminescence from the <sup>1</sup>D<sub>2</sub> and <sup>3</sup>P<sub>0</sub> levels in NYF:Pr<sup>3+</sup> crystals with a Pr<sup>3+</sup> concentration of 0.4 at % has a nonexponential character at the initial stage of the decay. For the radiative <sup>3</sup>P<sub>0,1</sub> level, we obtained good agreement between the calculated and experimental radiative and nonradiative relaxation rates. For the <sup>1</sup>D<sub>2</sub> level we observed a considerable discrepancy between the experimental and calculated values of the lifetime. This discrepancy can be explained by the fact that the <sup>1</sup>D<sub>2</sub> level in the specimen with the concentration of 0.4 at % is partially quenched as a result of the self-quenching processes.

Based on the obtained results, we have inferred that NYF:Pr<sup>3+</sup> crystals are processable; their high incorporation coefficient of Pr<sup>3+</sup> ions K<sub>Pr</sub> ~ 0.9 allows to obtain crystals with a high praseodymium concentration, up to 90 at.% (1.3×10<sup>22</sup>cm<sup>-3</sup>) without worsening of the optical quality. These crystals can be considered as new materials that are promising for the use as active media for optical converters and solid-state lasers in the visible and IR ranges (~2.3 µm) with a continuous tuning of the radiation frequency.