Mid-infrared (mid-IR) nonlinear optical (NLO) crystals have potential in high-tech laser applications, such as air pollution monitoring and medical diagnostics. However, many commercially available mid-IR NLO crystals have a narrow band gap that makes them susceptible to laser damage.

Zheshuai Lin, Beijing Center for Crystal R&D, China, Xingguo Chen, Wuhan University, China, and colleagues have discovered a new type of NLO crystal with a large bandgap, good mid-IR transmission, and a strong NLO effect. Using a hydrothermal method, the team prepared 2D double-layered frameworks of ABi₂(IO₃)₂F₅ (A = K, Rb, and Cs) containing halide, oxide, and oxyhalide polar anions.

The anionic groups align the crystalline framework so that a strong second-harmonic generation (SHG) response (where photon energy amplifies) occurs when the NLO crystal is irradiated by a 1064 nm laser. The SHG responses of the crystals surpass that of potassium dihydrogen phosphate (KDP), which is widely used in nonlinear optics. Electronegative fluorine atoms and alkali metal ions with strong ionicity increase the bandgap of the NLO crystals to more than 3.75 eV, suggesting that the crystals will have a high laser damage threshold.

• ABi₂(IO₃)₂F₅ (A=K, Rb and Cs): A Combination of Halide and Oxide Anionic Units to Create a Large Second-Harmonic Generation Response with a Wide Bandgap,
  Hongming Liu, Qi Wu, Xingxing Jiang, Zheshuai Lin, Xianggao Meng, Xingguo Chen, Jingui Qin,
  Angew. Chem. Int. Ed. 2017.
  DOI: 10.1002/anie.201705672