Low-toxicity, air-stable cesium bismuth iodide Cs₃Bi₂X₉ (X = I, Br, and Cl) perovskites are gaining substantial attention owing to their excellent potential in photoelectric and photovoltaic applications. In this work, the lattice constants, band structures, density of states, and optical properties of the Cs₃Bi₂X₉ under high pressure perovskites are theoretically studied using the density functional theory. The calculated results show that the changes in the bandgap of the zero-dimensional Cs₃Bi₂I₉, one-dimensional Cs₃Bi₂Cl₉, and two-dimensional Cs₃Bi₂Br₉ perovskites are 3.05, 1.95, and 2.39 eV under a pressure change from 0 to 40 GPa, respectively. Furthermore, it was found that the optimal bandgaps of the Shockley–Queisser theory for the Cs₃Bi₂I₉, Cs₃Bi₂Br₉, and Cs₃Bi₂Cl₉ perovskites can be reached at 2–3, 21–26, and 25–29 GPa, respectively. The Cs₃Bi₂I₉ perovskite was found to transform from a semiconductor into a metal at a pressure of 17.3 GPa. The lattice constants, unit-cell volume, and bandgaps of the Cs₃Bi₂X₉ perovskites exhibit a strong dependence on dimension. Additionally, the Cs₃Bi₂X₉ perovskites have large absorption coefficients in the visible region, and their absorption coefficients undergo a redshift with increasing pressure. The theoretical calculation results obtained in this work strengthen the fundamental understanding of the structures and bandgaps of Cs₃Bi₂X₉ perovskites at high pressures, providing a theoretical support for the design of materials under high pressure.

Lead-free perovskites of Cs₃Sb₂X₉ (X = Cl, Br, or I) have attracted wide attention owing to their low toxicity. High pressure is an effective and reversible method to tune bandgap without changing the chemical composition. Here, the structural and photoelectric properties of Cs₃Sb₂X₉ under high pressure were theoretically studied by using the density functional theory. The results showed that the ideal bandgap for Cs₃Sb₂X₉ can be achieved by applying high pressure. Moreover, it was found that the change of the bandgap is caused by the shrinkage of the Sb-X long bond in the [Sb₂X₉]³⁻ polyhedra. Partial density of states indicated that Sb-5s and X-p orbitals contribute to the top of the valence band, while Sb-5p and X-p orbitals dominate the bottom of the conduction band. Moreover, the band structure and density of states showed significant metallicity at 38.75, 24.05 GPa for Cs₃Sb₂Br₉ and Cs₃Sb₂I₉, respectively. Moreover, the absorption spectra showed the absorption edge redshifted, and the absorption coefficient of the Cs₃Sb₂X₉ increased under high pressure. According to our calculated results, the narrow bandgap and enhanced absorption ability under high pressure provide a new idea for the design of the photovoltaic and photoelectric devices.

Ultrathin inorganic halogenated perovskites have attracted attention owing to their excellent photoelectric properties. In this work, we designed two types of Ruddlesden–Popper hybrid perovskites, Cs_n+1Sn_nBr_3n+1 and Cs_nSn_n+1Br_3n+2, and studied their band structures and band gaps as a function of the number of layers (n = 1–5). The calculation results show that Cs_n+1Sn_nBr_3n+1 has a direct bandgap while the bandgap of Cs_nSn_n+1Br_3n+2 can be altered from indirect to direct, induced by the 5p-Sn state. As the layers increased from 1 to 5, the bandgap energies of Cs_n+1Sn_nBr_3n+1 and Cs_nSn_n+1Br_3n+2 decreased from 1.209 to 0.797 eV and 1.310 to 1.013 eV, respectively. In addition, the optical absorption of Cs_n+1Sn_nBr_3n+1 and Cs_nSn_n+1Br_3n+2 was blue-shifted as the structure changed from bulk to nanolayer. Compared with that of Cs_n+1Sn_nBr_3n+1, the optical absorption of Cs_nSn_n+1Br_3n+2 was sensitive to the layers along the z direction, which exhibited anisotropy induced by the SnBr₂-terminated surface.

The photodarkening (PD) and transverse mode instability (TMI) effects are two main factors limiting the power increase and long-term stability of high-power fiber lasers. A prolonged burn-in test for an all-fiber laser oscillator below the TMI threshold was carried out. We observed the PD-induced TMI effects, which manifested as a sudden decrease in the output power due to higher-order mode leakage. After several minutes of exposure to a high-power density 450 nm laser diode (LD), the output power returned to its initial state, significantly enhancing the oscillator’s stability. The 450 nm LD probably mitigates the accumulation of thermal effects by inhibiting the photodarkening effect, thus preventing the occurrence of the TMI effects and improving the stability of the oscillator’s output power. Our work provides useful guidance for the development of high-stability fiber laser oscillators.

Fiber lasers are increasingly employed in radiative environments, such as outer space and nuclear facilities. However, their performance is significantly compromised under irradiation due to the accumulation of defects in fiber components. Despite various radiation-hardening strategies, including optimized fiber designs and post-treatment techniques, these approaches have been unable to fully mitigate performance degradation. In this study, we demonstrate near 100% power recovery in a 150 W fiber laser exposed to radiation, achieved through an in-situ photobleaching technique utilizing low-power blue light. This all-fiber photobleaching strategy not only restores laser performance but also provides a robust, compact, and scalable solution for developing high-performance radiation-resistant fiber lasers. The proposed approach holds significant potential for advancing laser applications in challenging radiative environments, such as space exploration, nuclear power plants, and medical radiotherapy.

Under gamma irradiation, the output power of the fiber laser gradually decreases due to the rise of radiation induced attenuation (RIA) also known as the radiation-induced darkening effect. This phenomenon may lead to a decreased threshold of transverse mode instability, reducing the stability and beam quality of the fiber lasers. Understanding the darkening effect on fiber lasers, particularly high-power oscillators, is necessary. This work comprehensively investigates the irradiation effect on the ytterbium-doped fiber (YDF) oscillator operating at around 200 W online and the annealing process afterward. During the irradiation process, we find that the RIA of the rare-earth doped fiber increased linearly with the total radiation dose, due to the generation of color centers. The output power of the 1060 nm oscillator decreased by nearly 12% after 6 krad(Si) irradiation, which recovered to 90.6% after annealing for 15 minutes. For the 1080 nm oscillator, its power dropped by almost 13% under 3 krad(Si) irradiation, recovering to 91.2% after 30-min annealing. We further constructed a rate equation model and calculated the radiation sensitivity of the fiber irradiation position and fiber pumping scheme. We find that the radiation-resistant performance of the backward pumping is better than that of the forward pumping. Our result is valuable to understand the radiation-induced darkening effect and for the development of radiation-resistant fiber lasers.