Broad -band nonlinear optical response in BiTeS alloys based on alloy engineering.

Alloy engineering plays an important role in regulating the optoelectronic properties of materials. This work demonstrates that BiTeS alloys can extend nonlinear optical response to the near-infrared range. Te alloying at S sites can narrow the band gap, as proved by density functional theory (DFT) calculations, leading to a broadband saturable absorption response ranging from ultraviolet (350 nm) to near-infrared (1100 nm) wavelength with negative nonlinear optical absorption coefficient ranging from -0.12 cm GW to -1.28 cm GW. Moreover, the broadband carrier dynamic of BiTeS alloys was investigated femtosecond transient absorption (TA) at an excitation of 325 nm. A faster carrier dynamic at near-infrared wavelength was observed because of an increase in electron density at the conduction band minimum due to the additional Bi-Te interaction, which was corroborated by DFT calculations. These results suggest that alloy engineering provides an effective way for the development of broadband nonlinear optical devices.