“…From the viewpoint of materials, SAs used so far can be grouped into three categories, namely transition-metal-ion-doped materials, semiconductors and nanosheets materials. Transition-metal-ion-doped SAs mainly include Cr 4+ -doped host crystals like Cr:YAG [1], Cr:GSGG [2], Cr:YSO [3], Cr:SFS or Cr:CGS [4], but also Cr 2+ -, V 3+ -and Co 2+ -doped crystals like Cr:ZnSe, Cr:ZnS [5], Cr:CdMnTe [6], V:YAG [7], Co:MALO and Co:LMA [8]. SESAMs (semiconductor saturable absorber mirrors) [9] are the representative of semiconductor SAs.…”