Study on the Energy Band Regulation of Bi2-xSbxTe3 and Its Application as Mode Locking Material in Low Gain Ultrafast Fiber Laser

Topological insulators (TIs) are widely used as saturable absorbers in ultrafast pulsed fiber lasers, but the low modulation depth and high threshold power of self-starting mode-locking limit their application, especially in the field of communication. Five Bi2Te3-based materials are prepared by designing different doping ratios. The modulation depth is tunable from 4.41% to 51.31%. It is noteworthy that the minimum pump power of self-starting mode-locking is only 27.66 mW. In the circumstance of low gain, Kelly sideband is still weak even if the pump power reaches 165 mW. This is significant for the application in the field of laser communication. Density functional theory calculations show that the doping effect leads to the rearrangement of electrons near the Fermi level, which further leads to an increase in the density of states. The average contribution trend of Te p(z) of the five materials in each energy band near the Fermi level is consistent with the trend of modulation depth. This study suggests that the nonlinearity of Bi2Te3-based materials can be improved by doping Sb, and stable self-starting mode-locking can be achieved at low gain and low pump power, providing a new idea for improving the nonlinearity of materials.