The electronic structures, DOSs, carrier transport properties, and optical properties of CsGeX₃ (X = I, Br or Cl) perovskites under triaxial strains of −4% to 4% were investigated based on density functional theory.

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.

Hybrid organic-inorganic perovskite shows a great potential in the field of photoelectrics. Embedding methyl ammonium lead bromide (MAPbBr₃) in a mesoporous silica (mSiO₂) layer is an effective method for maintaining optical performance of MAPbBr₃ at room temperature. In this work, we synthesized MAPbBr₃ quantum dots, embedding them in the mSiO₂ layer. The nonlinear optical responses of this composite thin film have been investigated by using the Z-scan technique at a wavelength of 800 nm. The results show plural nonlinear responses in different intensities, corresponding to one- and two-photon processing. Our results support that composites possess saturation intensity of ~27.29 GW/cm² and varying nonlinear coefficients. The composite thin films show high stability under ultrafast laser irradiating. By employing the composite as a saturable absorber, a passively Q-switching laser has been achieved on a Nd:YVO₄ all-solid-state laser platform to generate a laser at ~1 μm.

The generation of vector beams using metasurfaces is crucial for the manipulation of light fields and has significant application potential, ranging from classical physics to quantum science. This paper introduces a novel dielectric metasurface composed of quarter‐wave plate (QWP) meta‐atoms, known as a QWP metasurface, designed to generate focused vector beams (VBs) of Bell‐like states under right circularly polarized illumination. The propagation phase imparted on both the co‐ and cross‐polarized components of the output field constructs hyperbolic and helical phase profiles with topological charge l _p , whereas the Pancharatnam–Berry (PB) phase acts only on the cross‐polarized component to construct another helical phase profile with topological charge l _g . Thus, the co‐ and cross‐polarized components form two orthogonal vector vortex (VV) modes with topological charges l _p and l _p + l _g , respectively. When the parameter conditions are satisfied by matching the incident polarization chirality σ and topological charges l _p and l _g , the focused VBs of Bell‐like states are generated by simultaneously manipulating the two VV modes, in contrast to existing QWP metasurfaces. The polarization states of the generated VBs are manipulated using the initial orientation angle θ ₀ of the meta‐atom. Overall, this research provides an innovative strategy for metasurface design, enhancing the functionality of metasurface devices for a broader range of application scenarios.

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.