The primary goal of this review is to present a clear chemical perspective of borates in order to stimulate and facilitate the discovery of new borate-based optical materials. These materials, which exhibit structures as varied as they are complex, are needed to meet the urgent technological milestones. In the current period of rapid sociotechnological breakthroughs, the need for the rational design and discovery of novel borates with superior performance is greater than ever before. Through the sustained efforts of chemists and material scientists, more than 3900 boron-containing compounds, including borate minerals and synthetic borates, have been documented in the scientific literature. This review provides a survey of all the reported anhydrous borates and an analysis of their complex structural chemistry. State-of-the-art progress related to technological advances in borate-based nonlinear optical, birefringent, and self-frequency-doubling materials is surveyed, with special emphasis on the relationships between structural architectures and optical properties. More importantly, this review serves both as a scientific introduction for graduates and post-doctoral researchers to the chemical richness of solid-state borates and as a comprehensive reference for researchers interested in borate-based optical 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 .

Coherent light radiation down to the deepultraviolet spectral range (λ < 200 nm) produced by common laser sources is extensively used in diverse fields ranging from ultrahighresolution photolithography to photochemical synthesis to highprecision microprocessing. Actually, it is hard to immediately obtain certain wavelengths, deep-ultraviolet coherent light in particular, from commercial laser sources. However, the direct second harmonic generation process governed in part by nonlinear optical crystals is a feasible and effective approach to generate deep-ultraviolet coherent light, which motivates chemists and materials scientists to find potential deep-ultraviolet nonlinear optical materials that can practically meet the scientific requirements. The research progress required to go from a new singlecrystal structure to final device applications involves many pivotal steps and is highly time-consuming and challenging, and therefore, it is necessary to commence systematic studies aimed at shortening the research cycle and accelerating the rational design of deep-ultraviolet nonlinear optical materials. In this Account, we choose borates as raw materials because they have ever-greater possibilities to form desired noncentrosymmetric structures, wide optical transparency windows, rich structural chemistry, and also large polarizabilities to guarantee the coexistence of large second-order nonlinear optical coefficients and suitable birefringence. Besides, the effects of fluorine atoms on the structural chemistry and optical properties of borates have been summarized and analyzed. On the basis of these favorable influences, three specific rational design strategies, including experimental and theoretical methods, have been proposed in order to shorten the investigational cycle of discovering the new expected compounds with high physicochemical performances required for practical applications. In this way, the progress of searching for candidates for the next generation of deep-ultraviolet nonlinear optical materials was accelerated from borates to borate fluorides to fluorooxoborates with three effective strategies: (1) expansion of the frontier from borates to borate fluorides with the introduction of fluorine to achieve enhanced optical performance; (2) computer-assisted design of new deep-ultraviolet nonlinear optical materials with a newly introduced systematic global structure optimization method; and (3) expansion of the frontier from borate fluorides to fluorooxoborates by proposed functionalized oxyfluoride [BO x F 4−x ] (x+1)− (x = 1, 2, 3) chromophores to balance multiple criteria. The preliminary development of fluorooxoborates exhibiting high performance as a new fertile field to search for deepultraviolet nonlinear optical materials is highly encouraging and inspiring and can guide chemists and materials scientists with new directions and thoughts aimed at finding the next generation of practical deep-ultraviolet nonlinear optical materials.