Exploration of the deep-ultraviolet nonlinear optical materials in the derivatives of KBe<sub>2</sub>BO<sub>3</sub>F<sub>2</sub>

The use of nonlinear optical crystal materials to extend the limited range of laser sources to the deep-ultraviolet (deep-UV, λ < 200 nm) regions by various frequency conversion techniques, has become an attractive field for generating deep-UV light. However, the lack of nonlinear optics (NLO) crystal materials capable of frequency conversion in the deep-UV light range, limits the development and application of deep-UV all-solid-state lasers. Therefore, scientists all over the world are actively exploring the new generation of deep-UV NLO crystal materials. At present, only the KBe₂BO₃F₂ (KBBF) crystal is capable of generating deep-UV light through the direct sixth harmonic generation of the Nd:YAG laser. The infinite _∞[Be₂BO₃F₂]⁻ single layers, as the brilliant building blocks in the crystal structures of KBBF family, provide a relatively large second harmonic generation coefficient (<i>d</i>₁₁ = 0.47 pm/V) and a sufficient birefringence (Δ<i>n</i> = 0.07@1064 nm). However, the KBBF crystals have insurmountable intrinsic defects, such as the usage of high toxic beryllium oxide, and the serious layer growth habit, which greatly restrict its commercialization process. Since the layered structure of the KBBF crystal is still one of the most brilliant structures for generating deep-UV laser, an effective strategy is to change the interlayer connection mode and develop new NLO materials based on KBBF with less layering growth habit. In this paper, by reviewing the development history of borate deep-UV NLO crystals and the derivatives of KBBF, the relationship between layered structure and optical properties of different interlaminar connections of crystal materials is systematically analyzed. We discuss the main contradictions and solutions of the development of deep-UV NLO crystal materials which are similar to the KBBF structure. In order to provide a reference for the innovative exploration of new materials in the future, several design strategies are also proposed.