Nonlinear optical properties of Bi₂Se₂Te for harmonic mode-locked and broadband Q-switched pulses generation

The nonlinear optical properties of bismuth selenium telluride (Bi₂Se₂Te), a few-layer two-dimensional topological insulator material, have been investigated in this work. An erbium-doped fiber laser (EDFL) based on Bi₂Se₂Te (BST) nanosheets as saturable absorber (SA) was demonstrated. The BST SA with saturation intensity and modulation depth of 4.96 MW/cm² and 2.45%, respectively, was obtained by mixing the layered BST nanosheets solution prepared by liquid phase exfoliation with a polyvinyl alcohol solution. On the basis of a 3.81 MHz fundamental repetition rate (RPR), seven types of high-order harmonic mode-locked (HML) at 300.2, 392.7, 424.2, 454.3, 516.9, 547.5 and 611.7 MHz were obtained, corresponding to 79th, 103rd, 111th, 119th, 135th, 144th and 160th harmonics, with the maximum 3-dB spectral width (2.31, 2.55, 2.55, 2.85, 2.96, 2.73 and 2.85 nm) and the narrowest pulse widths (1.11, 1.00, 1.00 ps, 896, 862, 935 and 895 fs), respectively. After reducing the cavity length by removing 30 m of single-mode fiber, a Q-switched operation with a maximum single pulse energy of 29.62 nJ and a tuning range of up to 81.91 kHz was obtained. The experimental results fully demonstrated that BST SA provided an important reference in the design of generating high RPR ultra-short pulses and Q-switched fiber lasers.