Figure.  Vacuum Ultraviolet Second-Harmonic Generation Device Based on NH₄B₄O₆F (ABF) Crystal

Supported by the National Natural Science Foundation of China (Grant No. 22335007), the research team led by Prof. Pan Shilie from Xinjiang Technical Institute of Physics and Chemistry, Chinese Academy of Sciences, has developed a novel fluorooxoborate crystal, NH₄B₄O₆F (ABF). This material offers an effective solution to the practical challenges associated with vacuum ultraviolet (VUV) nonlinear optical materials. The findings, titled “Vacuum ultraviolet second-harmonic generation in NH₄B₄O₆F crystal,” were published online in the journal Nature on January 28, 2026 (https://www.nature.com/articles/s41586-025-10007-z).

Vacuum ultraviolet (VUV, 100-200 nm) lasers hold significant application value in key fields such as the development of high-end scientific instruments and high-precision industrial processing. Frequency conversion of ultraviolet lasers using nonlinear optical crystals is one of the most straightforward and efficient methods for generating VUV lasers. However, the performance requirements for VUV nonlinear optical crystals are extremely stringent, and the development of practical crystals with excellent comprehensive performance has long been a major challenge in the field.

The research team innovatively proposed a fluorination strategy and regulation mechanism to overcome the challenge of synergistically balancing a large band gap, strong second-harmonic generation (SHG) effect, and high birefringence. This led to the development of a series of high-performance crystals, with ABF as a prime example. Building on this theoretical breakthrough, the team addressed technical bottlenecks in crystal growth and device processing, successfully growing high-quality, centimeter-scale ABF single crystals and fabricating angle phase-matched VUV SHG devices.

The ABF crystal integrates a variety of key performance features, including excellent VUV transmittance, a large SHG coefficient, high birefringence suitable for VUV phase matching, and a high laser damage threshold. In addition, the crystal exhibits stable physicochemical properties, can grow into large-sized single crystals, and offers good processability. No previously reported crystal has been able to combine all of these qualities. Through direct frequency doubling, the ABF crystal can generate VUV laser light at wavelengths as low as 158.9 nm. In a frequency doubling configuration from 355 nm to 177 nm, it achieves a pulse energy of 4.8 mJ with a maximum conversion efficiency of 7.9%, both setting world records for this technical route. This achievement establishes a new class of practical VUV nonlinear optical crystals and provides critical material support for the development of compact, high-efficiency all-solid-state VUV lasers, which will significantly advance fields such as high-end scientific instruments and laser precision processing.