We prepared ${\mathrm{WS}}_2$ nanosheets by using the thermal decomposition method and demonstrated for the first time its nonlinear saturable absorption property at around 2 μm. With the as-prepared ${\mathrm{WS}}_2$ nanosheets as saturable absorber (SA), a passively Q-switched Tm:LuAG laser was realized successfully, and 660 ns laser pulses with an average output power of 1.08 W and pulse peak power of 26 W at a repetition rate of 63 kHz were obtained for an incident pump power of 7 W. Our experimental results definitely demonstrate that ${\mathrm{WS}}_2$ could be a kind of promising SA for solid-state 2 μm lasers.

The electronic structure and optical properties of ternary GaAs:Bi alloy are investigated by first principles calculations. It is found that the band gap of GaAs_1-xBi_x decreases monotonously with the increasing of Bi concentration, resulting in the fundamental absorption edge and main absorption peaks of GaAs_1-xBi_x shifting toward lower energy with the increase of the Bi content. The optical constants of GaAs_1-xBi_x, such as the optical absorption coefficient, refractive index, extinction coefficient and optical conductivity, are greater than those of pure GaAs when x > 3.1%, but less than those of pure GaAs when x < 3.1%, which is primarily decided by the intraband level repulsions between Bi-induced states and host states on the valence bands; the contribution of Bi-6s, Bi-6p orbitals and Ga-4p, Ga-4s orbitals on conduction bands is also crucial. Bi doping plays an important role in the modulation of the static dielectric constant and the static refractive index. These results suggest a promising application of GaAs_1-xBi_x alloy as a semiconductor saturable absorber.

In this paper, the fabrication process and characterization of ${\mathrm{Bi}}_2{\mathrm{Te}}_3$ topological insulators (TIs) synthesized by the spin-coating-coreduction approach (SCCA) is reported. With this approach, high-uniformity nano-crystalline TI saturable absorbers (TISAs) with large-area uniformity and controllable thickness are prepared. By employing these prepared TIs with different thicknesses as SAs in 2-μm solid-state $Q$-switched lasers, thickness-dependent output powers and pulse durations of the laser pulses are obtained, and the result also exhibits stability and reliability. The shortest pulse duration is as short as 233 ns, and the corresponding clock amplitude jitter is around 2.1%, which is the shortest pulse duration in TISA-based $Q$-switched 2-μm lasers to the best of our knowledge. Moreover, in comparison with the TISA synthesized by the ultrasound-assisted liquid phase exfoliation (UALPE) method, the experimental results show that lasers with SCCA synthesized TISAs have higher output powers, shorter pulse durations, and higher pulse peak powers. Our work suggests that the SCCA synthesized TISAs could be used as potential SAs in pulsed lasers.

By simultaneously employing the electro-optic (EO) modulator and Cr⁴⁺:YAG saturable absorber, a diode-pumped dual-loss-modulated Q-switched and mode-locked (QML) YVO₄/NdYVO₄/KTP green laser is presented. In comparison with the singly passively QML green laser with Cr⁴⁺:YAG, the dual-loss-modulated QML green laser with EO and Cr⁴⁺:YAG can generate more stable pulse train with deeper modulation depth, shorter pulse width, greater pulse energy and higher peak power. For the dual-loss-modulated QML green laser, at a pump power of 18 W and a repetition rate of 1 kHz, the pulse width and the pulse energy of the Q-switch envelope as well as the peak power of QML green laser are 42.1 ns, 360 μJ and 382 kW, respectively, corresponding to the pulse width compression 62%, the pulse energy improvement 10 times and the QML peak power increase 40 times when compared with that of the singly passively QML green laser.