Using n- and p-Type Bi₂Te₃ Topological Insulator Nanoparticles To Enable Controlled Femtosecond Mode-Locking of Fiber Lasers

Mechanically triturated n- and p-type Bi₂Te₃ nanoparticles, the nanoscale topological insulators (TIs), are employed as nonlinear saturable absorbers to passively mode-lock the erbium-doped fiber lasers (EDFLs) for sub-400 fs pulse generations. A novel method is proposed to enable the control on the self-amplitude modulation (SAM) of TI by adjusting its dopant type. The dopant type of TI only shifts the Fermi level without changing its energy bandgap, that the n- and p-type Bi₂Te₃ nanoparticles have shown the broadband saturable absorption at 800 and 1570 nm. In addition, both the complicated pulse shortening procedure and the competition between hybrid mode-locking mechanisms in the Bi₂Te₃ nanoparticle mode-locked EDFL system have been elucidated. The p-type Bi₂Te₃ with its lower effective Fermi level results in more capacity for excited carriers than the n-type Bi₂Te₃, which shortens the pulse width by enlarging the SAM depth. However, the strong self-phase modulation occurs with reduced linear loss and highly nonsaturated absorption, which dominates the pulse shortening mechanism in the passively mode-locked EDFL to deliver comparable pulse widths of 400 and 385 fs with n- and p-type Bi₂Te₃ nanoparticles, respectively. The first- and second-order Kelly sidebands under soliton mode-locking regime are also observed at offset frequencies of 1.31 and 1.94 THz, respectively.