Broad -band nonlinear optical response in Bi₂Te_0.6S_2.4 alloys based on alloy engineering

Alloy engineering plays an important role in regulating the optoelectronic properties of materials. This work demonstrates that Bi₂Te_0.6S_2.4 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^-1 to -1.28 cm GW^-1. Moreover, the broadband carrier dynamic of Bi₂Te_0.6S_2.4 alloys was investigated via 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.