New borate crystal boosts UV optical applications

The ability of inorganic crystals known as nonlinear optical (NLO) materials to alter the properties of a beam of laser light—for example, doubling its frequency—makes them indispensable for applications in fiber optics, photolithography, and laser micromachining. Few NLO materials can generate coherent light deep in the ultraviolet range (< 200 nm). KBe₂BO₃F₂ (KBBF) is an exception. But the toxicity of beryllium and the low intensity of KBBF’s NLO properties limit its application. A team of researchers led by Shilie Pan of Xinjiang Technical Institute of Physics & Chemistry and Kenneth R. Poeppelmeier of Northwestern University may have come up with a solution—NH₄B₄O₆F (ABF), a beryllium-free deep-ultraviolet NLO material (J. Am. Chem. Soc. 2017, DOI: 10.1021/jacs.7b05943). The researchers report that ABF’s nonlinear coefficients are roughly 2.5 times as large as KBBF’s values. They also note that their synthesis method, based on the high-temperature reaction of B₂O₃ with NH₄F, leads to high-quality crystals that tend to be thicker than typical KBBF crystals, which benefits applications in laser optics. One source of the improved crystal growth is hydrogen bonding between lattice layers, which results from replacing potassium ions in KBBF with ammonium ions in ABF.