NatureVacuum ultraviolet second-harmonic generation in NH4B4O6F crystalFangfang ZhangZilong ChenChen CuiZhihua YangMiriding MutailipuFuming LiXueling HouXifa LongShilie Pan2026-01-28Full textDOI https://doi.org/10.1038/s41586-025-10007-z