Recent Development of Non-π-Conjugated Deep Ultraviolet Nonlinear Optical Materials

Deep-UV nonlinear optical (NLO) materials have attracted much academic and technical attention due to their capability of producing coherent light of wavelengths below 200 nm. As a well-known branch of NLO materials, researchers have obtained many deep-UV NLO crystals with superior properties in π-conjugated system materials (such as borate systems and carbonate systems). However, due to structural and property limitations, the research developments have encountered difficulties in balancing NLO properties. Recently, scientists started to pay attention to the non-π-conjugated system of deep-UV NLO materials. As far as we know, the non-π-conjugated deep-UV NLO materials could be divided into six categories: phosphates, sulfates, borophosphates, borosilicates, fluorides, and silicates. This review focuses on the research advances of deep-UV NLO materials with non-π-conjugated functional groups and summarizes and discusses the deep-UV NLO crystals on the basis of the performance criteria of "deep-UV transmission–large NLO effect–large birefringence". By analyzing the structure–property relationships of these materials, we summarize the microscopic mechanism restricting the balance of properties and new functional groups, which may provide new ideas and help the explorations of deep-UV NLO materials.