Growth and characterization of Ti:MgAl2O4 laser crystal by Czochralski method

Abstract

The melting point of Ti:MgAl<sub>2</sub>O<sub>4</sub> crystal is as high as 2130 °C, it is a challenge to obtain a large-sized and high-quality laser crystal. By optimizing the crystal growth process, Ti:MgAl<sub>2</sub>O<sub>4</sub> crystal with a size of 30 mm× 70 mm is successfully grown by the Czochralski method under the condition of weak reducing atmosphere. The X-ray diffraction pattern is studied, and the x-ray rocking curve indicates that the grown crystal has a high crystalline quality in terms of the lower full width at half maximum(FWHM) intensity, which provides a material basis for the next laser output experiment. In a range of 100–1000 cm<sup>–1</sup>, there are four Raman vibration peaks located at 312, 410, 675 cm<sup>–1</sup> and 771 cm<sup>–1</sup> respectively. The grown crystal has an absorption cutoff range of 250–318 nm and two wide absorption bands of 395–495 nm and 550–1100 nm. Excited by 271 nm, the grown crystal shows a strong broadband emission ina range of 340–650 nm with a peak centered at 480 nm. After annealing in hydrogen atmosphere, shape of the transmittance spectrum and emission spectrum are both unchanged, but the fluorescent emission intensity is significantly reduced. After annealing in air atmosphere, the original two absorption bands disappear while none of the characteristics of fluorescence emission in a 340–650 nm range changes significantly. In addition, a new fluorescence emission peak near 725 nm is observed. Combining with the ESR spectrum, what we canconfirm is that the Ti:MgAl<sub>2</sub>O<sub>4</sub> as-grown crystal contains Ti<sup>3+</sup> and Ti<sup>4+</sup> ions, and no ESR signal of Ti<sup>3+</sup> is observed after annealing in air atmosphere. Moreover, excitationspectrum is also recorded. The fluorescence lifetime is 14 μs at room temperature, which is 4–5 times that of Ti:Al<sub>2</sub>O<sub>3</sub> crystal and Ti:BeAl<sub>2</sub>O<sub>4</sub> crystal. Furthermore, the emission cross section of the grown Ti:MgAl<sub>2</sub>O<sub>4</sub> crystal is calculated from the Füchtbauer-Ladenburg (F-L) formula and its value is 2 × 10<sup>–20</sup> cm<sup>2</sup>, large emission cross section which is beneficial for realizing laser oscillation. All the above results show that the Ti:MgAl<sub>2</sub>O<sub>4</sub> crystal is a potential crystal material for realizing broadband tunable blue laser output.