Nonlinear optical materials are essential for the development of solid-state lasers. KBeBOF (KBBF) is a unique nonlinear optical material for generation of deep-ultraviolet coherent light; however, its industrial application is limited. Here, we report a new material NHBOF, which exhibits a wide deep-ultraviolet transparent range and suitable birefringence that enables frequency doubling below 200 nm. NHBOF possesses large nonlinear coefficients about 2.5 times that of KBBF. In addition, it is easy to grow bulk crystals and does not contain toxic elements.

The discovery of new nonlinear optical (NLO) materials for coherent light generation in the deep-ultraviolet (DUV, wavelength below 200 nm) region is essential for the development of laser technologies. Herein, we report a new material CsB O F (CBF), which combines the superior structural properties of two well-known NLO materials, β-BaB O (BBO) and KBe BO F (KBBF). CBF exhibits excellent DUV optical properties including a short cutoff edge (155 nm), a large SHG response (≈1.9×KDP), and a suitable birefringence that enables frequency doubling down to 171.6 nm. Remarkably, CBF melts congruently and shows an improved growth habit. In addition, our rational design strategy will contribute to the discovery of DUV NLO materials.

Deep-ultraviolet nonlinear optical (DUV NLO) crystals are the key materials to extend the output range of solid-state lasers to below 200 nm. The only practical material KBe BO F suffers high toxicity through beryllium and strong layered growth. Herein, we propose a beryllium-free material design and synthesis strategy for DUV NLO materials. Introducing the (BO F) , (BO F ) , and (BOF ) groups in borates could break through the fixed 3D B-O network that would produce a larger birefringence without layering and simultaneously keep a short cutoff edge down to DUV. The theoretical and experimental studies on a series of fluorooxoborates confirm this strategy. Li B O F is identified as a DUV NLO material with a large second harmonic generation efficiency (0.9×KDP) and a large predicted birefringence (0.07) without layering. This study provides a feasible way to break down the DUV wall for NLO materials.

Owing to their vital role in creating and controlling polarized light, birefringent materials are used extensively in various advanced optical systems which in turn impact a large, rapidly increasing range of applications in science and technology. Currently, the fairly small birefringence of MgF 2 and the low transmittance of α-BaB 2 O 4 (α-BBO) hinder their efficient application for wavelength below 200 nm. For example, deepultraviolet (DUV) birefringent materials for light polarization are urgently needed for DUV lithography. Here we demonstrate based on computational and experimental results that parallel chains of corner-connected planar sp 2 -hybridized BO 3 groups found in Ca(BO 2 ) 2 effectively produce large birefringence. Ca(BO 2 ) 2 achieves three vital "best" properties including the shortest UV cutoff edge, the largest birefringence, and the highest laser-induced damage threshold (LIDT) compared to all the reported borate birefringent materials. On the basis of a Ca(BO 2 ) 2 single crystal, a DUV Glan polarizer has been realized and is more efficient than one constructed with commercially available MgF 2 .

The search of new borates with improved functional properties has attracted considerable attention. Herein, a new polar fluorooxoborate, NaB O F (NBF) was prepared by high-temperature solid-state reaction. NBF belongs to the AB O F family (A=alkali metal or ammonium), a series of compounds that undergoes significant cation-dependent structural changes. NBF is of particular interest owing to the special cation position. Temperature-dependent ionic conductivity measurements show that NBF is a solid ionic conductor, and it has the lowest active energy of 32.5 kJ mol of fluorooxoborates. NBF also shows a second-harmonic generation (SHG) response of 0.9×KH PO and 0.2×β-BaB O , at 1064 and 532 nm, respectively, and it has a short UV cutoff edge below 180 nm. Based on bond valence (BV) concepts, symmetry analysis, and the first principles calculation, the unique [B O F] layer can be regarded as the "multifunctional unit", which is responsible for the observed properties of NBF.