Weexperimentallyinvestigatedthenonlinearopticalresponseinfew-layeroxidizedblackphosphorus(OBP)bythefemtosecondZ-scanmeasurementtechnique,andfoundthatOBPnotonlypossessesstrongultrafastsaturableabsorptionbutalsoanonlinearself-defocusingeffectthatisabsentinblackphosphorus(BP).Thesaturableabsorptionpropertyoriginatesmainlyfromthedirectbandstructure,whichisstillmaintainedinOBP.Theemergenceofself-defocusingmightoriginatefromthecombinedconsequencesoftheoxygen-induceddefectsinBP.Ourexperimentalfindingsmightconstitutethefirstexperimentalevidenceonhowtodynamicallytuneitsnonlinearproperty,offeringaninroadintailoringitsopticalpropertiesthroughchemicalmodification(oxidation,introducingdefects,etc.).Theversatileultrafastnonlinearopticalproperties(saturableabsorptionandself-defocusing)implyasignificantpotentialofthelayeredOBPinthedevelopmentofunprecedentedoptoelectronicdevices,suchasmodelockers,opticalswitches,laserbeamshapers,andwavelengthconverters.
