“…Passively mode-locked fiber lasers self-started by exploiting a variety of saturable absorbers (SAs) have been investigated for decades [ 1 , 2 , 3 , 4 , 5 , 6 , 7 , 8 , 9 , 10 ]. These SAs include carbon-based materials, such as carbon nanotubes (CNTs) [ 11 , 12 , 13 , 14 , 15 , 16 , 17 ], graphene [ 18 , 19 , 20 , 21 , 22 , 23 ], graphite [ 24 , 25 , 26 , 27 , 28 ] and charcoal nanoparticles [ 29 , 30 , 31 ], and topological insulators, such as molybdenum disulfide (MoS 2 ) [ 32 , 33 , 34 , 35 ], antimony telluride (Sb 2 Te 3 ) [ 36 ], Bi 2 Te 3 [ 37 , 38 , 39 ], and bismuth selenide (Bi 2 Se 3 ) [ 40 , 41 ], etc. In addition to the well-known slow and fast SAs for fiber lasers, recently, germanium (Ge) was also considered as another promising candidate to serve as a fast SA due to its ultrafast recovery time (sub-picosecond degree) and nonlinear saturable absorption [ 42 , 43 , ...…”