Ultrafast nonlinear absorption and nonlinear refraction in few-layer oxidized black phosphorus

We experimentally investigated the nonlinear optical response in few-layer oxidized black phosphorus (OBP) by the femtosecond Z-scan measurement technique, and found that OBP not only possesses strong ultrafast saturable absorption but also a nonlinear self-defocusing effect that is absent in black phosphorus (BP). The saturable absorption property originates mainly from the direct band structure, which is still maintained in OBP. The emergence of self-defocusing might originate from the combined consequences of the oxygen-induced defects in BP. Our experimental findings might constitute the first experimental evidence on how to dynamically tune its nonlinear property, offering an inroad in tailoring its optical properties through chemical modification (oxidation, introducing defects, etc.). The versatile ultrafast nonlinear optical properties (saturable absorption and self-defocusing) imply a significant potential of the layered OBP in the development of unprecedented optoelectronic devices, such as mode lockers, optical switches, laser beam shapers, and wavelength converters.