Spectroscopic properties of as-grown and γ-irradiated undoped and Bi-doped α-BBO $\left(\mathrm{Ba}{\mathrm{B}}_2{\mathrm{O}}_4\right)$ single crystals were investigated. ${\mathrm{Bi}}^{2+}$ and color centers in $\mathrm{Bi}:\alpha$-BBO crystals were investigated to be nonluminescent in the near-infrared (NIR) region. Broadband NIR luminescence at $1139\mathrm{nm}$ with a FWHM of $108\mathrm{nm}$ and a decay time of $526\mu \mathrm{s}$ was realized in $\mathrm{Bi}:\alpha$-BBO crystal through γ irradiation. ${\mathrm{Bi}}^{+}$ was attributed to be responsible for the NIR emission, which can be bleached by thermal annealing. The involved physical processes in $\mathrm{Bi}:\alpha$-BBO crystal during the courses of irradiation and heat annealing were tentatively established.

We observed two ultrabroadband near-infrared (NIR) luminescence bands around 1.2 and $1.5\mathrm{\mu m}$ in as-grown bismuth-doped CsI halide crystals, without additional aftertreatment. Dependence of the NIR emission properties on the excitation wavelength and measurement temperature was studied. Two kinds of NIR active centers of subvalent bismuth and color centers were demonstrated to coexist in Bi:CsI crystal. The eye-safe $1.5\mathrm{\mu m}$ emission band with an FWHM of $140\mathrm{nm}$ and lifetime of $213\mathrm{\mu s}$ at room temperature makes Bi:CsI crystal promising in the applications of the ultrafast laser and ultrabroadband amplifier.

The passive Q-switching characteristics of a diode-pumped Tm,Ho:YAP laser at 2 μm based on a MoS ₂ saturable absorber (SA) is presented for the first time. Pulses as short as 435 ns under a repetition rate of about 55 kHz were generated at the incident pump power of 8.4 W, corresponding to the pulse peak power up to 11.3 W. This is, to the best of our knowledge, the highest pulse peak power, ever obtained from MoS ₂ based passively Q-switched 2-μm lasers. The experimental results sufficiently validated the feasibility of MoS ₂ as SA for solid-state 2-μm lasers.

The absorption, excitation, and ultrabroadband near-infrared luminescence spectra of Bismuth were investigated in H₂-annealed and γ-irradiated Bi:α-BaB₂O₄(α-BBO) single crystals, respectively. Energy-level diagrams of the near-infrared luminescent centers were fixed. The electronic transition energies of near-infrared active centers are basically consistent with the multiplets of free Bi⁺ ions. The minor difference of the energy-level diagrams of Bi⁺ ions in H₂-annealed and γ-irradiated Bi:α-BaB₂O₄ crystals can be ascribed to the difference of the local lattice environments. The involved physical and chemical processes were discussed. The effect of Ar-, air-annealing and electron-irradiation on Bi:α-BaB₂O₄ crystal were also investigated.