Halogen-Tuned BiOX (X = Cl, Br, I) Layered Crystals with Superior Broadband Nonlinear Saturable Absorption for Pulsed Laser Modulation.

Nonlinear optical (NLO) materials play indispensable roles in various fields, such as information, energy, and biology. However, the ideal NLO materials necessitate a balance between the band gap and the nonlinear coefficient, which severely restricts the further development of nonlinear optics. BiOX (X═Cl, Br, and I) crystals, with their layered structure, combine the advantages of nontoxicity, a tunable band gap, and remarkable optoelectronic properties, rendering them highly promising NLO materials. Nevertheless, the research on the NLO properties of BiOX principally focuses on nanosheets, which require substrate attachment like traditional two-dimensional (2D) materials, making them inconvenient and arduous to fabricate NLO devices. In response to these issues, a BiOX crystal series of NLO devices with facile exfoliation, a broad optical transmission window, a preeminent broad spectral range (visible to infrared), nonlinear properties, and a large laser damage threshold are prepared, and for the first time, the NLO properties of BiOX NLO devices with varying band gaps and their performance as saturable absorbers in near-infrared (1 and 2 μm) laser bands are studied, with results indicating that the nonlinear performance is enhanced with the increase of the halogen atomic radius. This work expands the nonlinear application of BiOX crystals, providing an alternative perspective for research on NLO crystal materials.