Bi₃TeBO₉: A Promising Mid-Infrared Nonlinear Optical Crystal with a Large Laser Damage Threshold

Mid-infrared (mid-IR) nonlinear optical (NLO) crystals are key materials in the field of laser technology. Nevertheless, the applications of these significant optical materials are always limited by their low laser damage threshold (LDT). Here, an oxide mid-IR NLO crystal, Bi₃TeBO₉, with a large LDT was discovered. The centimeter-sized Bi₃TeBO₉ single crystal was successfully grown by the top-seeded solution growth (TSSG) method. The Bi₃TeBO₉ features a high LDT of 450 MW/cm² (∼15 × AgGaS₂) and the widest transparent range (0.35-7.10 μm) among known borate crystals. The second-harmonic generation (SHG) effect is about 1.6 times that of KDP. First-principles calculations and structural analysis show that the optical characteristics of Bi₃TeBO₉ are mainly affected by distorted [BiO₆] groups. Furthermore, its thermal characteristics including specific heat, thermal diffusivity, thermal conductivity, and thermal expansion were also measured.