Bi 2 O 2 Te Nanosheets Saturable Absorber‐Based Passive Mode‐Locked Fiber Laser: From Soliton Molecules to Harmonic Soliton

Abstract As a recent addition to the emerging 2D bismuth oxychalcogenides (Bi 2 O 2 X , where X = S, Se, and Te), atomically‐thin bismuth oxytellurium (Bi 2 O 2 Te) exhibits unique thermoelectric and photoelectric properties. In this study, high‐quality Bi 2 O 2 Te nanosheets (NSs)‐based saturable absorber (SA) with a modulation depth of 8.2% is developed using the liquid phase exfoliation (LPE) method and is successfully applied to a passively mode‐locked Er‐doped fiber laser (EDFL) for the first time (with reference to the available literature). Due to the excellent nonlinear saturable absorption property of Bi 2 O 2 Te NSs, various switchable and stable mode‐locking states can be realized in the same EDFL, including fundamental frequency conventional soliton (CS), soliton molecules, and harmonic mode‐locking (HML) states. Among them, the CS pulse at 7.48 MHz (fundamental frequency) with a high signal‐to‐noise ratio (SNR) of 67.25 dB is obtained. The 2nd, 3rd, and 4th‐order soliton molecules are observed with a soliton pulse separation of ≈6.2 ps. Moreover, for HML, a maximum repetition rate of 1.78 GHz (i.e., corresponding to the 238th‐order harmonic) is obtained. The results reveal that Bi 2 O 2 Te NSs demonstrate excellent nonlinear optical modulation properties and can serve as a fundamental basis for promotion of academic research and innovation of engineering applications involving ultrafast photonics.