Deep-ultraviolet nonlinear optical materials: Development trend and innovative exploration

With the rapid development of all-solid-state laser technology in the field of optical communication, optical processing and optical storage, deep-ultraviolet (deep-UV, wavelengths below 200 nm) nonlinear optical (NLO) materials have become a hot topic at home and abroad. Coherent radiation in the deep-UV is possible with excimer lasers, e.g., F2 excimer at 157 nm. However, solid-state lasers in these wavelength ranges are often preferred owing to handling ease, narrow bandwidth, tunability, high energy density, and high peak power density. An excellent manner to generate coherent deep-UV light is through solid-state lasers using cascaded frequency conversion with NLO materials. Deep-UV NLO crystals, which can double the frequency of incident light to the deep-UV region, are essential for all-solid-state laser. The prerequisites for deep-UV NLO crystals are crystallographically noncentrosymmetric (NCS), wide optical transparency window, large second harmonic generation (SHG) response, phase- matching capability, and easy growth of large (centimeter size) high quality single crystals. For years, there has been a “200 nm wall”, that is, no material is available for deep-UV SHG. Over the past decades, great efforts have been made on chemical design and synthesis of deep-UV NLO materials. Currently, only KBe2BO3F2 (KBBF) crystal is capable for generating deep-UV light through direct sixth harmonic generation of the Nd:YAG laser. The infinite ∞[Be2BO3F2]− single layers in KBBF provide a relatively large SHG coefficient ( d 11=0.47 pm/V) and a sufficient birefringence (Δ n =0.07@1064 nm). However, the KBBF crystals have insurmountable intrinsic defects, such as the high toxicity of the beryllium oxide, and the serious layer growth habit, which greatly restrict its commercialization and application process. Therefore, researchers are actively exploring the next generation of deep-UV NLO materials. In this review, we will first briefly discuss the history of deep-UV NLO crystals. The main factors that restrict the development of deep-UV NLO crystals were highlighted, emphasizing one of the greatest challenges after the material is synthesized is its large single-crystal growth. In the second section, we will focus on progress and trend of deep-UV NLO materials in recent ten years and divide them into different categories according to different material systems and structural features. Five groups of materials are given—beryllium borates, beryllium-free borates, carbonate-fluorides, phosphates and fluorooxoborates. The absorption edge, powder SHG efficiency and birefringence of these materials are summarized in detail. In this section, the development of fluorooxoborates is discussed and the important achievements are reviewed systematically. In addition, the major contradictions among bandgap, NLO coefficient and birefringence were discussed and the methods of theoretical structure prediction algorithms for solving these problems were proposed in order to provide reference for discovering new deep-UV NLO materials.