Poly(difluorophosphazene) as the First Deep‐Ultraviolet Nonlinear Optical Polymer: A First‐Principles Prediction

Wide-Band Gap Binary Semiconductor P3N5 with Highly Anisotropic Optical Linearity and Nonlinearity

Wide-band gap binary semiconductors find extensive use in advanced optoelectronic devices due to their exceptional electronic, optical, and defect properties. This paper systematically investigates the linear and nonlinear optical and defect properties of two P3N5 structures as wide-band gap binary semiconductors and evaluates their responses to external pressure modulation using first-principles calculations. The research demonstrates that the high-pressure phase of P3N5 has a broad UV solar-blind band gap (Eg ∼ 4.9 eV) and displays highly anisotropic optical linearity and nonlinearity, including a significant second harmonic generation effect (d24 ∼ 1.8 pm/V) and large birefringence (Δn ∼ 0.12), exhibiting a relatively small change in amplitude against pressure due to unique lattice incompressibility. This material enables birefringent phase-matched second harmonic coherent output at a much shorter wavelength (down to 286 nm) than currently known wide-band gap binary semiconductors such as SiC, GaN, AlN, Ga2O3, and Si3N4. An in-depth study of the defect properties of P3N5 in relation to its UV optical properties is also provided. These results are important references for utilizing the optoelectronic functions of this binary material system.

Deep-ultraviolet nonlinear optical crystals: concept development and materials discovery

Deep-ultraviolet (DUV, wavelength λ < 200 nm) nonlinear optical (NLO) crystal is the core component of frequency conversion to generate DUV laser, which plays an important role in cutting-edge laser technology and fundamental science. Significant progress has been made in both experimental exploration and theoretical design in the field of DUV NLO crystals over the past three decades. In-depth insight into "structure-property correlations", in particular, allows for rigorous and precise identification of DUV NLO crystals. In this article, we reviewed the current experimental and theoretical research progress while elucidating the core concepts and stringent criteria of qualified DUV phase-matched second-harmonic generation crystals. We also discussed the development of the DUV NLO "structure-property correlations" from first principles and how it has sparked interest in related materials, as well as future directions for obtaining potential DUV NLO crystals.

Periodic B- and N-doped phenalenyl π-aggregates: unexpected nonlinear optical properties by tuning pancake π-π bonding

Phenalenyl (PLY) and its derivatives could form one-dimensional π-aggregates through pancake π-π bonding, which would lead to exotic optoelectronic properties. We will highlight the key aspects of the PLY derivatives from the design strategies to exploration of the electronic properties. Here, we primarily construct alternating boron (B)- and nitrogen (N)-doped PLY π-aggregates: dimer[12], trimer[12-1], trimer[12-2], tetramer[12]₂, pentamer[12]₂-1, pentamer[12]₂-2, and hexamer[12]₃. The geometric and electronic structures show that the short intermolecular distances of the π-aggregates drive the formation of pancake π-π bonding. Significantly, the molecular structures show periodic changes in the π-aggregates, but the first hyperpolarizabilities (β_tot) present unexpected changes, which are found to increase sharply with increasing even layer thickness due to intermolecular charge transfer. The β_tot value of hexamer[12]₃ (5.72 × 10⁴ a.u.) is 6.4 times that of tetramer[12]₂ (8.95 × 10³ a.u.), and is 22.4 times that of dimer[12] (2.55 × 10³ a.u.). Thus, constructing π-aggregates can significantly improve the second-order NLO response, which is mainly due to intermolecular charge transfer through pancake π-π bonding of the interlayers.

Novel van der Waals Deep-UV Nonlinear Optical Materials

Van der Waals (vdW) deep-UV (DUV) nonlinear optical (NLO) crystal is an important material system recently developed. Herein, we review its concept and original intention, and then summarized the discovery process of related materials, including the role of A-site cations and the resulting two-/one-dimensional vdW DUV NLO systems. Finally, we evaluate the practical DUV NLO performance and prospected the opportunities and challenges.

Deep-Ultraviolet Nonlinear-Optical van-der-Waals Beryllium Borates*

Van-der-Waals (vdW) deep-ultraviolet (DUV) nonlinear-optical (NLO) materials hold great potential to extend DUV NLO applications to two dimensions, but they are rare in nature. In this study, we propose a design principle to realize vdW DUV NLO materials via structural evolution from the non-vdW (BO₃ )-(BeO₃ F) layers in KBe₂ BO₃ F₂ (KBBF) to the vdW (BO₃ )-(BeO₄ H) layers in berborite Be₂ BO₅ H₃ (BBH) and the vdW (BO₄ )-(BeO₄ ) layers in beryllium metaborate BeB₂ O₄ (BEBO). Based on first-principles calculations, the fundamental NLO properties of BBH and BEBO demonstrate that a balanced DUV NLO performance can be achieved in these two systems. Importantly, BBH, a layered material existing in nature, can achieve an available DUV phase-matched output with strong second harmonic generation (SHG) for 177.3/193.7 nm DUV lasers, which is almost identical to that of KBBF. Remarkably, BEBO shows an excellent DUV SHG capacity and an even shorter phase-matching wavelength than KBBF. Therefore, the newly discovered vdW BBH and BEBO, once verified by experiments, could provide an ideal platform to study DUV NLO effects in three dimensions and two dimensions.

Sr2Pb(BeB5O10)(BO3): An Excellent Ultraviolet Nonlinear-Optical Beryllium Borate by the Pb-Modified Construction of a Conjugated System and Lone-Pair Effect

The design of material by chemical and/or crystalline modification of a classic structure model benefits not only the optimized physical properties but also the controllability and efficiency. Herein, a new nonlinear-optical (NLO) beryllium borate crystal, Sr₂Pb(BeB₅O₁₀)(BO₃) (SPBBO), is successfully designed and synthesized by chemical and crystalline modification of the perovskite-like K₃B₆O₁₀Cl NLO crystal. SPBBO displays a 3D BeB₅O₁₀^3- open-framework structure composed of interconnecting BeB₅O₁₃ groups with filled cationic Sr/Pb and anionic BO₃ groups, which exhibits the striking enhancement of the second-harmonic-generation (SHG) response (8 × KDP) and birefringence (0.10) compared to the parent model. Replacement of K by Sr and Pb with a lone pair and replacement of Cl by conjugated BO₃ result in the synergistic conjugation of Pb with host BeB₅O₁₀^3- and filled BO₃ groups, contributing to the striking enhancement of the SHG and birefringence. Single-crystal measurements show that SPBBO has a short UV absorption edge of 280 nm with a wide energy band gap of 4.35 eV and an outstanding laser-induced resistant behavior with a remarkably high laser-induced damage threshold of 2100 MW cm^-2. The excellent properties indicate that the SPBBO crystal is a very promising UV NLO functional material.

Tuning the first hyperpolarizability of hexaphyrins with different connections of mislinked pyrrole units: a theoretical study

In the satisfactory design and synthesis of high-performance nonlinear optical (NLO) materials, for meeting the rapidly expanding demands of optoelectronic devices, a deeper understanding of the relationship between the structures and NLO properties has become a key issue. Herein, five novel mislinked expanded hexaphyrins with different connections of pyrrole units are selected to study the relationship between the structures and NLO properties. These five mislinked expanded hexaphyrins are neo-fused, neo-confused hexaphyrins, singly, doubly, and triply N-confused hexaphyrins. From theoretical results, the order of the static first hyperpolarizability (β₀) values is found to be: neo-fused hexaphyrin (β₀ = 4163 a.u.) < neo-confused hexaphyrin (β₀ = 5494 a.u.) < singly N-confused hexaphyrin (β₀ = 6510 a.u.) < doubly N-confused hexaphyrin (β₀ = 15 130 a.u.) < triply N-confused hexaphyrin (β₀ = 26 095 a.u.). Furthermore, β₀ values of the doubly and triply N-confused hexaphyrins are improved 2.1 and 3.7 times over that of their usual parent hexaphyrin (β₀ = 7120 a.u.), respectively. It is worth noting that increasing mislinked connection numbers and changing mislinked connection ways of the pyrrole units in these mislinked expanded hexaphyrins plays a crucial role in the tune of their second-order NLO responses.

Two Covalent Ultraviolet Nonlinear Optical Crystals

Nonlinear optical (NLO) crystals are the vital components of laser science and technology, as they can convert lasers in common wavelengths into new wavelength bands for ultraviolet (UV), IR, and even terahertz laser output. Known UV NLO crystals mainly focus on crystals containing cations, but covalent crystals have rarely been reported. Here we report two covalent NLO crystals, B₂ O₃ I and B₂ O₃ II. According to the first-principles calculations, B₂ O₃ I and II have extremely short absorption edges of about 134 nm and 141 nm, large NLO coefficients of d₂₂ =1.38 pm/V and d₂₄ =0.702 pm/V, as well as sufficient birefringences of 0.037 and 0.031, respectively. Notably, the absorption edges are almost the shortest among NLO crystals. Meanwhile, the NLO coefficients are evidently larger than that of another well-known covalent NLO crystal α-SiO₂ and are comparable to those of the commercial UV NLO crystal LiBO₃ with Li⁺ cation. Furthermore, the birefringences are significantly larger than that of α-SiO₂ , which are favorable to the phase matching for both crystals. These results reveal that B₂ O₃ I and B₂ O₃ II are excellent candidates for UV NLO applications. In-depth calculations are carried out to reveal the origin of excellent NLO properties. These covalent crystals provide a new direction for the research of UV NLO crystals.

Prediction of ternary fluorooxoborates with coplanar triangular units [BOxF3−x]x− from first-principles

From first-principles prediction, we got all the basic structural units of fluorooxoborates, namely, tetrahedral elements [BO_xF_4−x] (x = 1,2,3) like [BO₄] and triangular elements [BO_xF_3−x] (x = 1,2) like [BO₃].

LiMII (IO3 )3 (MII =Zn and Cd): Two Promising Nonlinear Optical Crystals Derived from a Tunable Structure Model of α-LiIO3

Excellent nonlinear optical materials simultaneously meet the requirements of large SHG response, phase-matching capability, wide transparency windows, considerable energy band-gap, good thermal stability and structure stability. Herein, two new promising nonlinear optical (NLO) crystals LiM^II (IO₃ )₃ (M^II =Zn and Cd) are rationally designed by the aliovalent substitution strategy from the commercialized α-LiIO₃ with the perfect parallel alignment of IO₃ groups. Compared with parent α-LiIO₃ and related A^I ₂ M^IV (IO₃ )₆ , the title compounds exhibit more stable covalent 3D structure, and overcome the racemic twinning problem of A^I ₂ M^IV (IO₃ )₆ . More importantly, both compounds inherit NLO-favorable structure merits of α-LiIO₃ and show larger SHG response (≈14× and ≈12×KDP), shorter absorption edge (294 and 297 nm) with wider energy band-gap (4.21 and 4.18 eV), good thermal stability (460 and 430 °C), phase-matching behaviors, wider optical transparency window and good structure stability, achieving an excellent balance of NLO properties.

LiMg(IO3)3: an excellent SHG material designed by single-site aliovalent substitution

An excellent second harmonic generation (SHG) material, LiMg(IO₃)₃ (LMIO), has been elaborately designed from Li₂M^IV(IO₃)₆ (M^IV = Ti, Sn, and Ge) by aliovalent substitution of the central M^IV cation followed by Wyckoff position exchange. The new structure sustains the ideal-alignment of (IO₃)^- groups. Importantly, LMIO exhibits an extremely strong SHG effect of roughly 24 × KH₂PO₄ (KDP) under 1064 nm laser radiation or 1.5 × AgGaS₂ (AGS) under 2.05 μm laser radiation, which is larger than that of α-LiIO₃ (18 × KDP). The replacement of M^IV with Mg²⁺ without d-d electronic transitions induces an obviously larger band gap (4.34 eV) with a short absorption edge (285 nm). This study shows that single-site aliovalent substitution provides a new synthetic route for designing SHG materials.