Passively Q-switched near-infrared lasers with bismuthene quantum dots as the saturable absorber

Bismuthene quantum dots (Bi-QDs) were synthesized via the liquid phase exfoliation (LPE) method for the passive Q-switching operation in near-infrared (NIR) region. The nonlinear optical properties of the prepared Bi-QDs were investigated by the open-aperture Z-scan technology. The modulation depths were 18.1% and 5.1% at 1.06 and 1.34 µm, respectively. Based on the Bi-QDs saturable absorber, passively Q-switched Nd:GdVO 4 lasers operating at 4 F 3/2  →  2 I 11/2 and 4 F 3/2  →  2 I 13/2 transitions were demonstrated, showing the wideband optical modulation in NIR regime. For the 4 F 3/2  →  2 I 13/2 transition lasing at 1.34 µm, the shortest pulse duration of 155 ns was obtained with a repetition rate of 457 kHz. With respect to the 4 F 3/2  →  2 I 11/2 transition at 1.06 µm, the minimum pulse duration was 150 ns with a repetition rate of 424 kHz, leading to a single pulse energy of 261 nJ and a peak power of 1.68 W. In addition, the ground state absorption cross section and the excited state absorption cross section of Bi-QDs were also investigated for the first time. The impact of the excited state lifetime on the output parameters was numerically stimulated by the coupled rate equations. Our work confirmed that the trap state in Bi-QDs played an important role in the pulse generation mechanism.