Promising UV/DUV nonlinear optical materials with X-membered (X = multiples of 6) rings in anhydrous inorganic borates

Ultraviolet (UV) and deep ultraviolet (DUV) nonlinear optical (NLO) materials have garnered significant research traction because of their extensive practical applications and the complexities. The design and synthesis of NLO materials require crystallographic non-centrosymmetry (NCS), wide transparency range, large second harmonic generation (SHG) response, moderate birefringence, high laser-induced damage threshold (LIDT), and physical-chemical stability. Borates have been considered as an effective system for NLO materials in view of their rich structural composition. In today's period of rapid technological advancement, there is great aspiration for generating new borates with excellent properties and novel structures. To provide an innovative approach to enhance the advancement of borate NLO materials, this paper reviews a number of borates containing isolated 6-membered rings (6-MRs), interconnected 6-MRs, 12-MRs, 18-MRs and rare 24-MRs with NLO effects. The structural-property correlations governing their optical characteristics undergo systematic investigation from a new perspective. More notably, this review not only provides researchers with an understanding of the structural diversity of borates but also offers a conceptual framework for researchers exploring borate compounds in NLO applications. • This article presents a new perspective on more than 100 borate NLO crystals. • The structures and optical properties of these excellent borates with X-MRs are summarized. • We outline key structural design strategies for developing advanced borate UV/DUV NLO materials. • This article also explores the opportunities and challenges regarding borate UV/DUV NLO materials.