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Using first principles calculations based on the density functional theory and the structural, electronic, and optical properties of the substitution of K elements by Li+, Na+, Mg2+, and Ca2+ in KH2PO4 (KDP) crystals are investigated in depth. This study provides insight into the structural properties of crystals, with particular emphasis on internal chemical bonding effects. It further discusses the impact of H and O defects on the bandgap and optical properties of KDP crystals. Detailed analysis of structural characteristics and comprehensive examination of the effects of chemical bonds contribute to a deeper understanding of the effects of defects on the lattice and electronic structure, revealing potential alterations and their underlying mechanisms. Our investigation focuses on the regulatory role of different defects on the bandgap and optical properties. These comprehensive analyses provide a more thorough understanding of the impact of defects on the properties of KDP crystals, establishing an essential research foundation for further exploration of nonlinear optics crystal potential applications. [ABSTRACT FROM AUTHOR]

Infrared nonlinear optical crystals are indispensable functional materials in modern laser science. In order to avoid the inherent defects of traditional materials such as two-photon absorption and low laser damage threshold, there is still an urgent need to find infrared nonlinear optical crystals with excellent properties. In this paper, we combine the machine learning (ML) algorithm with the features that only contain crystal chemical components, train the nonlinear optical crystal database containing more than 400 materials, and optimize the ML classification model. The random forest classifier with synthetic minority oversampling technique (SMOTE) sampling shows good classification ability. An external test is used to test the generalizability of the model, and the result shows that the model meets the requirements. The combination of the ML classification model and high-throughput computing is helpful to accelerate the discovery of new crystals. [ABSTRACT FROM AUTHOR]

[Display omitted] The Potassium dihydrogen phosphate (KH 2 PO 4 : KDP) of ferroelectric phase is a polarized structure that can be used for nonlinear optical transformation. Using the generalized gradient approximate (GGA), we study and predict the crystal structural trends, electronic structures, charge densities, and optical properties of KDP crystals with paraelectric (PE) and ferroelectric (FE) phases under pressures ranging from 0 to 20 GPa. We have studied how pressure affects the structural and electronic properties of KDP. It is found that under high pressure, KDP undergoes a transition from Fdd 2 structure to I -42 d structure. The calculations indicate that when the hydrostatic structural pressure of the ferroelectric KDP is applied to 11.36 GPa, the KDP has a paraelectric phase structure. The 11.36 GPa is the critical pressure value at which the structure undergoes phase transition. We further showed that as the pressure increases, the lattice size and ion displacement undergo a strong distortion. Surprisingly, the band gap of KDP greatly increases with the increase of pressure. The results show that high pressure enhances the compactness of the KDP system and enhance the laser damage threshold, which can open a new way for high pressure manipulation of frequency doubling materials in optoelectronic applications. [ABSTRACT FROM AUTHOR]

[Display omitted] The electronic structure and optical properties of Na 2 Bi 2 (SeO 3) 3 F 2 crystal are calculated under uniaxial, epitaxial, and hydrostatic stress based on the first principles of density functional theory. The crystal structures of the materials are composed of isolated [BiO x F y ] polyhedrons and [SeO 3 ] tetrahedrons. The stability of the crystal was confirmed by the cohesive energies and Ab initio molecular dynamics (AIMD). The band gap is 3.93 eV without strain, the maximum band gap is 4.04 eV and the minimum band gap is 2.33 eV under epitaxial stress of −2.61 % and hydrostatic pressure of −9 GPa, respectively. It is found that there is a band gap transition from direct to indirect when uniaxial strain and hydrostatic pressure are applied. This change has been explained by analyzing the near-band-edge orbitals energy. The strain also affects the optical properties of the crystal Na 2 Bi 2 (SeO 3) 3 F 2. In particular, hydrostatic pressure of −9 GPa causes a minimum increase of transmittance of about 10 % and a maximum increase of about 30 %. The optical properties of the crystal can be effectively improved by applying strain, which can be universal for other crystals. [ABSTRACT FROM AUTHOR]

• The electronic structures and optical properties of Tm:YAG, Tm:LuYAG, and Tm:LuAG crystals are calculated based on first principles. • The reasons for the variation of the output wavelength of three crystals are analyzed by dielectric function. • The output wavelength of the Tm:LuYAG crystal can be effectively changed by adjusting the content of Lu element in the mixed crystal. Thulium-doped mixed yttrium aluminum garnet Tm:(Lu x Y 1-x) 3 AG crystals are popular laser crystals, with the Tm:YAG terminal compound being widely applied. However, it is unclear how the content of Lu3+ affects the properties of Tm:YAG. Based on the first principles of density functional theory, we have studied the electronic structure and optical properties of Tm:(Lu x Y 1-x) 3 AG crystals. The two terminal compounds Tm:YAG and Tm:LuAG have the same crystal structure as Tm:LuYAG and the band gap values change to 4.50 eV, 4.57 eV, and 4.95 eV with Lu3+ doping concentration of 0, 50 %, and 100 %. The calculation of optical properties shows that with the increase of Lu3+ doping concentration, the overall dielectric function peak occurs redshift, which is effectively proved by the output wavelength Tm:YAG (2013.84 nm) Tm:LuYAG (2018.95 nm) Tm:LuAG (2025.23 nm) of the three crystals in the experiment. In the ultraviolet, visible and infrared bands (0–6 eV), the optical properties of the three crystals, such as reflectivity, refractive index, and transmittance, are the same. This theoretical analysis can provide insights for studying the variation of the output wavelength of other laser crystals. [ABSTRACT FROM AUTHOR]