Cr-doped Ca₃NbGa₃Si₂O₁₄: A promising near-infrared tunable laser crystal

Highlights

• Cr:Ca₃NbGa₃Si₂O₁₄ crystal was grown successfully by Czochralski method.

• Spectral properties of Cr:Ca₃NbGa₃Si₂O₁₄ crystal were presented.

• Cr ions exist in different valence states simultaneously in the crystal.

• Cr:Ca₃NbGa₃Si₂O₁₄ is a promising candidate for solid state tunable laser.

Abstract

Broad tunable solid-state lasers in the near-infrared region are urgently demanded due to their critical applications in optical communications, eye-safe imaging, medicine and spectroscopy, and Cr⁴⁺-based lasers exhibit good application prospect in this area. Herein, we reported a new near-infrared tunable laser crystal of Cr-doped Ca₃NbGa₃Si₂O₁₄ (Cr:CNGS). By employing the Czochralski method, single crystal of Cr:CNGS with dimensions of Φ 20 × 30 mm³ was grown. Its crystal structure, spectral properties were investigated in detail., Cr:CNGS exhibits attractive properties, including broad emission band at 800–1400 nm, long fluorescence lifetime (18 μs at 298 K) and good adaptability to LD pumping source, all of these characteristics enable it to be a potential laser crystal for the near-infrared tunable lasers. Notably, we discovered that the multi-sites in CNGS crystal make Cr ions exist in different valence states simultaneously, including Cr³⁺, Cr⁴⁺ and Cr⁵⁺, which is also partly responsible for its broadband emission.

Introduction

Benefitting from their specific spectroscopic properties, tunable solid-state lasers are widely applied in the scientific research and real-life, such as on-site diagnostic investigations in medicine [1], transmission, and reflection in telecommunication [2]. In addition to these, they are also used as coherent light sources of some crucial nonlinear processes, including second-harmonic generation, optical parametric oscillation, sum- and difference-frequency generation [3]. Currently, with the rapid developments of optoelectronic technology, the studies for new tunable laser crystals with outstanding physicochemical properties, broad emission band, and long fluorescence lifetime as well as suitable for LD pumping are increasingly needed.

In the past decades, much effort was made to investigate the optical properties of transition metal ions (Cr³⁺, Cr⁴⁺, Mn^2+, and Ni²⁺) doped crystals for discovering excellent tunable laser materials [[3], [4], [5]]. Among these ions, as the energy levels of Cr⁴⁺ ion are more susceptible to crystal field, and its ³T₁→³A₂ transition usually shows broadband emission in the near-infrared region in a weak crystal field, these make it get more and more attention. Cr⁴⁺-based broad tunable lasers are extensively used in optical communications, biological detection, environmental monitoring and so on, in which the core is Cr⁴⁺-doped tunable laser crystals. To date, several Cr⁴⁺-doped tunable laser crystals, like YAG [6], Mg₂SiO₄ [7], and Ca₂GeO₄ [8] have been found and applied. Nevertheless, some drawbacks in these materials, including difficulties in obtaining high-quality crystals, short fluorescence lifetimes (2.7 μs for Cr: Mg₂SiO₄ [9], 3.5 μs for Cr: YAG [10], and 12 μs for Ca₂GeO₄ [8]) and low laser output efficiency [9,11], limit their scope of applications. Therefore, it is imperative to search for new Cr⁴⁺-doped laser crystals with enhanced performances.

Single crystals with the Ca₃Ga₂Ge₄O₁₄ structure have become the hot topic in the field of crystal research because of its multifunctional natures [12]. Ca₃NbGa₃Si₂O₁₄ (CNGS) crystal is one of them, due to its excellent mechanical and thermal properties [13], easy to obtain large and high-quality single-crystal by Czochralski (Cz) method. It has been reported as an ideal host for solid-state laser crystals. In recent years, Nd³⁺:CNGS, Yb³⁺:CNGS and Tm³⁺:CNGS laser crystals have been grown by Cz method [[14], [15], [16], [17], [18], [19]], and their laser experiments have been performed. A maximum output power of 1.43 W was obtained in Nd³⁺:CNGS crystal with the optical conversion efficiency of 29.3% and the slope efficiency of 31.0%; for Yb³⁺:CNGS crystal, the maximum CW laser output power was 1.53 W at 1060 nm, the optical conversion efficiency was 37.9%, the slope efficiency was 47.8%; and for Tm³⁺:CNGS crystal, the laser output power was 740 mW at 2.0 μm with the slope efficiency of 17.07% and light-to-light conversion efficiency of 14%. In the structure of CNGS crystal, Nb⁵⁺ ion is bonded to six O atoms to form a NbO₆ octahedron, while both of Ga³⁺ and Si⁴⁺ ions coordinate with four O atoms to form GaO₄ and SiO₄ tetrahedrons, respectively. Cr⁴⁺ ions prefer to replace cations which lie in tetrahedral sites. As the ionic radius of Cr⁴⁺ (0.41 Å) is close to that of Ga³⁺ (0.47 Å) and is bigger than that of Si⁴⁺ (0.26 Å ) in tetrahedral group, it is easy for Cr⁴⁺ ion to replace Ga³⁺ ion, although they have different ionic valence states. For example, in Cr⁴⁺:Y₃Ga₅O₁₂ [20], Cr⁴⁺:LiGaO₂ [21] crystals and Cr⁴⁺:SrGa₄O₇ [22], Cr⁴⁺ ion occupies the lattice position of Ga³⁺ ion. Of course, there may be part of Cr⁴⁺ ions substitute for Si⁴⁺ ions such as in Cr⁴⁺:Mg₂SiO₄ [7], Cr⁴⁺:Y₂SiO₅ [23], and Cr⁴⁺:Li₂MgSiO₄ [24]. Therefore, when Cr ion is doped into the CNGS crystal, a new Cr⁴⁺-doped laser crystal is expected.

In this paper, we report the synthesis of Cr:CNGS polycrystalline powders with different Cr ion doping concentrations, the growth of 1 at.% Cr:CNGS single crystal, the crystal structure, and the spectra of Cr:CNGS crystal before and after annealing in the H₂ atmosphere in detail. Furthermore, the possible valence states of Cr ions in the CNGS crystal are speculated.