Elsevier

Journal of Crystal Growth

Volume 513, 1 May 2019, Pages 10-14
Journal of Crystal Growth

Impact of Tm3+/Ho3+ co-doping on spectroscopic and laser properties of Ca3(VO4)2 single crystal

https://doi.org/10.1016/j.jcrysgro.2019.02.054Get rights and content

Highlights

  • Single crystals of Ca3(VO4)2 co-doped with Tm3+ + Ho3+ ions were obtained by Czochralski method for the first time.

  • Optical quality, ferroelectric domain structure, dislocation density of the pure and doped crystals were investigated.

  • Spectroscopic and laser experiments were carried out.

Abstract

The series of Tm3+/Ho3+ co-doped vanadates based on Ca3(VO4)2 was obtained by Czochralski method. The trivalent doping ions were introduced into the melt in the form of oxides without any additional compensating impurities. The spectroscopic investigations showed anisotropic character of optical properties of the material. For optimal output mirror reflectivity of 94% the oscillation spectrum for holmium ions was measured to be centered at about 2070 nm with oscillation spectrum width of about 20-nm width at half maximum. Slope efficiency 5% for Ho3+ ions under 792 nm thulium pumping was obtained. Broadband tuning of Ho3+ ions oscillation wavelength within 2020–2115 nm spectral range was obtained in selective cavity with SiO2 filter.

Introduction

At the current stage of information technologies and wide application of coherent radiation sources in scientific, engineering, and other fields, researchers are focused not only on extending the functionalities and increasing the efficiency of already existing optical systems but also on creating new systems for controlling laser radiation and information flows. Particularly, great interest is attracted to solid-state nonlinear optical materials, the properties of which can be varied within a wide range due to changes in the main composition and degree of imperfection and due to isomorphous substitution of host ions by ions of rare-earth or transition metals. In the paper we consider the possibilities of creation of effective laser-nonlinear media based on Ca3(VO4)2 co-doped with Tm3+ and Ho3+ ions. It is known that using set of various Tm-doped materials it is possible to cover spectral region from 1.8 up to 2.05 μm [1], [2], [3], [4]. To generate longer wavelengths (around 2.1 μm) usually Ho-doped laser materials are utilized [5], [6], [7], [8], [9]. From other side, Ca3(VO4)2 possesses high nonlinear-optical properties (it can be used as for second harmonic generation as for Raman shifting of laser radiation) and exhibit favorable phase matching conditions in the range between 2 µm and 3 µm. Also, the crystals are characterized by the high damage threshold. Based on doped Ca3(VO4)2 crystalline material new multifunctional media with controllable physicochemical parameters and spectroscopic and laser-nonlinear characteristics can be created.

Section snippets

Experimental details

The Ca3(VO4)2 charge was prepared by solid-state synthesis from 5 N CaCO3 and 5 N V2O5 at 1100 °C during 6 h. Crystals, nominally pure and co-doped with Tm3+ and Ho3+ (in oxide form), were grown from the melt by Czochralski method using a «Nika-3» growth setup from inductively-heated Pt crucibles in air. A conical Pt screen was placed above the crucible to provide needed thermal gradients (60–90 °C/cm in the growth zone and 5–10 °C in the annealing zone). Growth direction was 90° to optical

Results and discussion

The series of rare-earth-doped vanadates based on Ca3(VO4)2 was obtained. Melting point of Ca3(VO4)2 is 1430 °C, it belongs to the R3c space group. This material is known as a high-temperature ferroelectric with Curie temperature Tc = 1113 °C [10]. The general formula rare-earth-doped calcium vanadate can be written as Ca3-3xM2x(VO4)2, where M is (Tm + Ho) element. In this whitlockite-type structure (Fig. 1a) divalent calcium was replaced by trivalent rare earth ions what needs some valence

Conclusion

Calcium orthovanadate crystals co-doped with Tm3+ and Ho3+ ions were successfully grown by Czochralski method. It was shown that the material has good optical homogeneity and can be successfully used as active medium for diode-pumped laser working in the range of 2000 nm. Rather efficient Tm-Ho energy transfer was observed in Ca3(VO4)2:Tm3+ (0.26 at.%)/Ho3+(0.09 at.%) crystal which allowed to obtain 2070 nm lasing of Ho3+ ions under 792 nm thulium pumping. Broad tuning range within about 100 nm

Acknowledgements

This research was supported by the VolkswagenStiftung (Grant № Az. 90.261 from 29.02.2016), the Russian Academy of Sciences (Program I.7) and Czech Science Foundation (project No. 18-11954S).

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