Recent progress of newly developed functional building units and corresponding nonlinear optical materials

Nonlinear optical (NLO) crystals are key materials for solid-state lasers, which play an important role in modern science and technology. However, owing to the intrinsic limitation of functional building units (FBUs) in conventional compounds, it is difficult to surpass the performance of existing materials. Therefore, it is necessary to continuously explore new NLO-active FBUs to overcome the recent stagnant situation. Recently, a lot of new FBUs were discovered as NLO-active units in different wavelength ranges. Their contributions are confirmed by the performance of corresponding crystals. In this review, newly identified NLO-active units and corresponding NLO crystals are summarized and discussed in terms of crystal structure, NLO performance, and structure-property relationship. One can find that the heteroleptic coordinated tetrahedra and organic π-conjugated planar groups are the main source of new FUBs that will be a research hotspot in further research work. This review concludes the feasible directions on the exploration of new NLO crystals to satisfy the urgent requirements.