Potential Laser\nCrystal of Zn<sub>4</sub>B<sub>6</sub>O<sub>13</sub>:Mn<sup>2+</sup> with Low Thermal Expansion and High\nThermal Conductivity: Single-Crystal Growth and Characterization

Laser crystals, serving as the laser gain medium, are\nkey materials\nin all-solid-state lasers. Due to considerable thermal expansion and\nlow thermal conductivity, the performance of conventional laser crystals\nstrongly depends on thermal management systems to improve the laser\nquality and power, which inevitably restricts the development of laser\ntechnology. In this work, a new potential laser crystal of Mn<sup>2+</sup>-doped Zn<sub>4</sub>B<sub>6</sub>O<sub>13</sub> (ZBO) with\na low thermal expansion and high thermal conductivity was grown and\ncharacterized. The strong luminescence emission at 539 nm in ZBO:Mn<sup>2+</sup> was observed, with a long lifetime of 16.20(7) ms, and the\ncorresponding absorption bands in the range of 410–460 nm fall\nwithin the spectra of GaN- or InGaN-based laser diodes. In variable-temperature\nfluorescence spectra, promoted by relaxation on the parity-forbidden <sup>4</sup>T<sub>1</sub>-to-<sup>6</sup>A<sub>1</sub> transition from\nthe increased phonon number, an abnormal negative thermal quenching\nwas manifested below 140 K, with the lifetime consistently greater\nthan 10 ms over the whole temperature range of 80–500 K. Meanwhile,\nZBO:Mn<sup>2+</sup> manifests very low thermal expansion (3.1 MK<sup>–1</sup>) and high thermal conductivity (27.44(6) W/(m·K))\nat room temperature (300 K). These excellent thermal properties, combined\nwith good optical properties, make ZBO:Mn<sup>2+</sup> an outstanding\ngain medium for green laser generation. Our study confirms that ZBO\nis a promising laser matrix crystal.