Nonlinear optical materials are essential for the development of solid-state lasers. KBe2BO3F2 (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 NH4B4O6F, which exhibits a wide deep-ultraviolet transparent range and suitable birefringence that enables frequency doubling below 200 nm. NH4B4O6F 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.

Finding the Next Deep-Ultraviolet Nonlinear Optical Material: NH4B4O6F

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 CsB4O6F (CBF), which combines the superior structural properties of two well-known NLO materials, beta-BaB2O4 (BBO) and KBe2BO3F2 (KBBF). CBF exhibits excellent DUV optical properties including a short cutoff edge (155 nm), a large SHG response (approximate to 1.9 X 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.

CsB4O6F: A Congruent-Melting Deep-Ultraviolet Nonlinear Optical Material by Combining Superior Functional Units

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 KBe2BO3F2 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 (BO3F)(4-),(BO2F2)(3-), and (BOF3)(2-) groups in borates could break through the fixed 3DB-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. Li2B6O9F2 is identified as aDUV NLO material with a large second harmonic generation efficiency (0.9 x 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.

Fluorooxoborates: Beryllium-Free Deep-Ultraviolet Nonlinear Optical Materials without Layered Growth.

Fluorooxoborates, benefiting from the large optical band gap, high anisotropy, and ever-greater possibility to form non-centrosymmetric structures activated by the large polarization of [BOx F4-x ](x+1)- building blocks, have been considered as the new fertile fields for searching the ultraviolet (UV) and deep-UV nonlinear optical (NLO) materials. Herein, we report the first asymmetric alkaline-earth metal fluorooxoborate SrB5 O7 F3 , which is rationally designed by taking the classic Sr2 Be2 B2 O7 (SBBO) as a maternal structure. Its [B5 O9 F3 ]6- fundamental building block with near-planar configuration composed by two edge-sharing [B3 O6 F2 ]5- rings in SrB5 O7 F3 has not been reported in any other borates. Solid state 19 F and 11 B magic-angle spinning NMR spectroscopy verifies the presence of covalent B-F bonds in SrB5 O7 F3 . Property characterizations reveal that SrB5 O7 F3 possesses the optical properties required for deep-UV NLO applications, which make SrB5 O7 F3 a promising crystal that could produce deep-UV coherent light by the direct SHG process.

SrB 5 O 7 F 3 Functionalized with [B 5 O 9 F 3 ] 6− Chromophores: Accelerating the Rational Design of Deep‑Ultraviolet Nonlinear Optical Materials

Deep ultraviolet (absorption edge 6.2 eV) nonlinear optical (NLO) materials are of current interest owing to their technological applications and materials design challenges. Technologically, the materials are used in laser systems, atto-second pulse generation, semiconductor manufacturing, and photolithography. Designing and synthesizing a deep UV NLO material requires crystallographic non-centrosymmetry, a wide UV transparency range, a large second-harmonic generating coefficient (dij > 0.39 pm/V), moderate birefringence (Δn ∼ 0.07), chemical stability and resistance to laser damage, and ease in the growth of large high-quality single crystals. This review examines the known deep UV NLO materials with respect to their crystal structure, band gap, SHG efficiency, laser damage threshold, and birefringence. Finally, future directions with respect to new deep UV NLO materials are discussed.

Deep Ultraviolet Nonlinear Optical Materials

The development of new nonlinear optical (NLO) materials for deep-ultraviolet (DUV) applications is in great demand. However, the synthesis of an ideal DUV NLO crystal is a serious challenge. Herein, three new alkali-metal fluorooxoborates, AB4 O6 F (A=K, Rb, and Cs, and a mixed cation between two of them), were successfully synthesized by cation regulation. It is found that all reported compounds exhibit short UV absorption edges (<190 nm), and show second harmonic generation (SHG) responses ranging from 0.8 to 1.9 KH2 PO4 (KDP). Interestingly, by judicious selection of the A-site alkali-metal cations, the arrangement of NLO-active structural units is fine-tuned to an optimal configuration, which contributes to large SHG responses.

Cation‐Tuned Synthesis of Fluorooxoborates: Towards Optimal Deep‐Ultraviolet Nonlinear Optical Materials

The search of new borates with improved functional properties has attracted considerable attention. Herein, a new polar fluorooxoborate, NaB4 O6 F (NBF) was prepared by high-temperature solid-state reaction. NBF belongs to the AB4 O6 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-1 of fluorooxoborates. NBF also shows a second-harmonic generation (SHG) response of 0.9×KH2 PO4 and 0.2×β-BaB2 O4 , 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 [B4 O6 F]∞ layer can be regarded as the "multifunctional unit", which is responsible for the observed properties of NBF.

Bingbing Zhang

Papers

Finding the Next Deep-Ultraviolet Nonlinear Optical Material: NH4B4O6F

CsB4O6F: A Congruent-Melting Deep-Ultraviolet Nonlinear Optical Material by Combining Superior Functional Units

Fluorooxoborates: Beryllium-Free Deep-Ultraviolet Nonlinear Optical Materials without Layered Growth.

Cation‐Tuned Synthesis of Fluorooxoborates: Towards Optimal Deep‐Ultraviolet Nonlinear Optical Materials

Polar Fluorooxoborate, NaB4O6F: A Promising Material for Ionic Conduction and Nonlinear Optics