Maximizing the optical anisotropyi nb irefringent materials has emerged as an efficient route for modulating the polarization-dependent light propagation. Currently,t he generation of deep-ultraviolet (deep-UV) polarized light below 200 nm is essential but challenging due to the interdisciplinary significance and insufficiency of high-performing birefringent crystals.H erein, by introducing multiple heteroanionic units, the first sodium difluorodihydroxytriborate-boric acid Na-[B 3 O 3 F 2 (OH) 2 ]•[B(OH) 3 ]h as been characterized as an ovel deep-UV birefringent crystal. Tw or are heteroanionic units, [B 3 O 3 F 2 (OH) 2 ]and [B(OH) 3 ], optimally align to induce large optical anisotropyand also the dangling bonds are eliminated with hydrogens,w hichr esults in an extremely large birefringence and band gap.T he well-ordered OH/F anions in [B 3 O 3 F 2 (OH) 2 ]a nd [B(OH) 3 ]w ere identified and confirmed by various approaches,a nd also the origin of large birefringence was theoretically discussed. These results confirm the feasibility of utilizing hydrogen involved heteroanionic units to design crystals with large birefringence,a nd also expand the alternative system of deep-UV birefringent crystals with new hydroxyfluorooxoborates.

In crystal engineering, it is an effective and controllable approach to modify the electronic band structure and optimize crystal performances using rational chemical cosubstitution in a classic structure model. Herein, the noncentrosymmetric (NCS) rare-earth borate fluoride La2B5O9F3 was designed and synthesized successfully based on the extraordinarily stable M2B5O9X (M = Ca, Sr, Ba, Sn, Pb, and Eu; X = Cl, Br, and I) template. Moreover, all 70 rare-earth borate halides were discussed, and the ratio of crystallization in NCS group is only 17.1%, much lower than 34.9% in all anhydrous borates. Benefiting from the substitution of [MOX] by [LaOF] polyhedra with improved hyperpolarizability and anisotropy of polarizability, compared with the M2B5O9X family, La2B5O9F3 with optimized band structure exhibits the suitable SHG response (1.2 × KH2PO4 (KDP) @ 1064 nm), large band gap (6.58 eV), and moderate birefringence, which well achieves the optimal balance among the three critical parameters mentioned above for nonlinear optical (NLO) applications in the short-wavelength region. This work expands the research field of NLO materials to rare-earth borate fluorides and can lead to a better understanding of the role of rare-earth metal cations.

Nonlinear optical crystals lie at the core of ultrafast laser science and quantum communication technology. The emergence of two‐dimensional (2D) materials provides a revolutionary potential for nonlinear optical crystals due to their exceptionally high nonlinear coefficients. However, uncontrolled stacking orders generally induce the destructive nonlinear response due to the optical phase deviation in different 2D layers. Therefore, conversion efficiency of 2D nonlinear crystals is typically limited to less than 0.01% (far below the practical criterion of >1%). Here, we controllably synthesize crystalline films of rhombohedral boron nitride (rBN) with parallel stacked layers. This success is realized by the utilization of vicinal FeNi (111) single crystal, where both the unidirectional arrangement of BN grains into a single‐crystal monolayer and the continuous precipitation of (B, N) source for thick layers are guaranteed. The preserved in‐plane inversion asymmetry in rBN films keeps the in‐phase second harmonic generation field in every layer and leads to a record‐high conversion efficiency of 1% in the whole family of 2D materials within the coherence thickness of only 1.6 μm. Our work provides a route for designing ultrathin nonlinear optical crystals from 2D materials, and will promote the on‐demand fabrication of integrated photonic and compact quantum optical devices.This article is protected by copyright. All rights reserved