We report passively $Q$-switched $\sim 2$ and $\sim 3\mathrm{\mu m}$ mid-infrared (MIR) solid-state lasers with a self-assembly solvothermal-synthesized ${\mathrm{Bi}}_2{\mathrm{Te}}_3/\text{graphene}$ heterostructure saturable absorber (SA) for the first time. Based on the oxidation resistance and high thermal conductivity of graphene, and large modulation depth of ${\mathrm{Bi}}_2{\mathrm{Te}}_3$ nanosheets, two high-performance $Q$-switching lasers were realized. One is a Tm:YAP laser with a maximum average output power of 2.34 W and a pulse width of 238 ns at $\sim 2\mathrm{\mu m}$. The corresponding maximum pulse peak power was 91 W, which was much improved in comparison with the pure graphene-based Tm laser. The other one is an Er:YSGG laser producing a pulse width of 243 ns, which is the shortest among the 2D SAs-based $\sim 3\mathrm{\mu m}$ solid-state lasers, as far as we know. Our results indicate that such a composite ${\mathrm{Bi}}_2{\mathrm{Te}}_3/\text{graphene}$ material is a promising SA for generating high-performance mid-infrared pulse lasers.

Er³⁺/Yb³⁺/Pr³⁺: SrGdGa₃O₇ crystal was firstly grown by Czochralski method. Detailed spectroscopic analyses of Er³⁺/Yb³⁺/Pr³⁺: SrGdGa₃O₇ were carried out. Besides better absorption characteristic, the spectra of Er³⁺/Yb³⁺/Pr³⁺: SrGdGa₃O₇ show weaker up-conversion and near-infrared emissions as well as superior mid-infrared emission in comparison to Er³⁺: SrGdGa₃O₇ and Er³⁺/Yb³⁺: SrGdGa₃O₇ crystals. Furthermore, the self-termination effect for Er³⁺ 2.7 μm laser is suppressed successfully because the fluorescence lifetime of the ⁴I_13/2 lower level of Er³⁺ decreases markedly while that of the upper ⁴I_11/2 level changes slightly in Er³⁺/Yb³⁺/Pr³⁺: SrGdGa₃O₇ crystal. The sensitization effect of Yb³⁺ and deactivation effect of Pr³⁺ ions as well as the energy transfer mechanism in Er³⁺/Yb³⁺/Pr³⁺: SrGdGa₃O₇ crystal were also studied in this work. The introduction of Yb³⁺ and Pr³⁺ is favorable for achieving an enhanced 2.7 μm emission in Er³⁺/Yb³⁺/Pr³⁺: SrGdGa₃O₇ crystal which can act as a promising candidate for mid-infrared lasers.

We reported on a diode-end-pumped single-longitudinal-mode microchip laser using a 600-μm-thick Er:GGG crystal at $\sim 2.7\mathrm{\mu m}$, generating a maximum output power of 50.8 mW and the maximum pulsed energy of 0.306 mJ, with repetition rates of pumping light of 300, 200, and 100 Hz, respectively. The maximum slope efficiency of the laser was 20.1%. The laser was operated in a single-longitudinal mode centered at about 2704 nm with a FWHM of 0.42 nm. The laser had a fundamental beam profile and the beam quality parameter $M^2$ was measured as 1.46. These results indicate that the Er:GGG microchip laser is a potential compact mid-infrared laser source.

Dirac-like topological insulators have attracted strong interest in optoelectronic application because of their unusual and startling properties. Here we report for the first time that the pure topological insulator Bi₂Te₃ exhibited a naturally ultrasensitive nonlinear absorption response to photoexcitation. The Bi₂Te₃ sheets with lateral size up to a few micrometers showed extremely low saturation absorption intensities of only 1.1 W/cm² at 1.0 and 1.3 μm, respectively. Benefiting from this sensitive response, a Q-switching pulsed laser was achieved in a 1.0 μm Nd:YVO₄ laser where the threshold absorbed pump power was only 31 mW. This is the lowest threshold in Q-switched solid-state bulk lasers to the best of our knowledge. A pulse duration of 97 ns was observed with an average power of 26.1 mW. A Q-switched laser at 1.3 μm was also realized with a pulse duration as short as 93 ns. Moreover, the mode locking operation was demonstrated. These results strongly exhibit that Bi₂Te₃ is a promising optical device for constructing broadband, miniature and integrated high-energy pulsed laser systems with low power consumption. Our work clearly points out a significantly potential avenue for the development of two-dimensional-material-based broadband ultrasensitive photodetector and other optoelectronic devices.

In this paper, we reported a multiwavelength passively $Q$-switched ${\mathrm{Yb}}^{3+}\text{:}{\mathrm{GdAl}}_3{({\mathrm{BO}}_3)}_4$ solid-state laser with topological insulator ${\mathrm{Bi}}_2{\mathrm{Te}}_3$ as a saturable absorber (SA) for the first time, to the best of our knowledge. ${\mathrm{Bi}}_2{\mathrm{Te}}_3$ nanosheets were prepared by the facile solvothermal method. The influence of three ${\mathrm{Bi}}_2{\mathrm{Te}}_3$ densities on the laser operation was compared. The maximum average output power was up to 57 mW with a pulse energy of 511.7 nJ. The shortest pulsewidth was measured to be 370 ns with 110 kHz pulse repetition rate and 40 mW average power. The laser operated at three wavelengths simultaneously at 1043.7, 1045.3, and 1046.2 nm, of which the frequency differences were within the terahertz wave band. Our work suggests that solvothermal synthesized ${\mathrm{Bi}}_2{\mathrm{Te}}_3$ is a promising SA for simultaneously multiwavelength laser operation.