Topological-Insulator Passively Q-Switched Double-Clad Fiber Laser at 2 $\mu$ m Wavelength

In this paper, Topological insulator (TI) Bi₂Se₃ as a saturable absorber (SA) is exploited to Q-switch fiber lasers at 2 μm wavelength for the first time. Few-layer TI:Bi 2Se 3 nanosheets in CS-HAc solution are prepared by the liquid-phase exfoliation method, and the thin 2-D structure with the thickness of 3-5 layers is well characterized. The open-aperture Z-scan experiment shows that the few-layer TI:Bi 2Se 3 has the saturable optical intensity of 41 MW/cm ² at 800 nm and the modulation depth of 3.7%. The optical deposition technique is used to efficiently assemble the TI:Bi 2Se 3 nanosheets in the solution onto a fiber ferrule, therefore constructing a fiber-compatible TI-based SA (FC-TISA). By further inserting the FC-TISA into a diode-pumped Tm ³⁺-doped double-clad fiber laser (TM-DCFL), stable Q-switching operation at 1.98 μm is successfully achieved with the shortest pulse width of 4.18 μs and the tunable repetition rate from 8.4 to 26.8 kHz. In particular, the TM-DCFL can deliver large-energy Q-switched pulses with the pulse energy as high as 313 nJ (corresponding to average output power of 8.4 mW). Our results suggest that TI-based SA is suitable for pulsed laser operation in the eye-safe region of 2 μm, and potentially develops as an ultra-broadband photonics device.