Inorganic planar π-conjugated groups in nonlinear optical crystals: Review and outlook
The nonlinear optical (NLO) materials have been playing significant roles in laser science and technology for converting laser wavelengths to spectral regions where the normal lasers operated poorly. Inorganic planar π-conjugated groups possessing both large second harmonic generation (SHG) responses and optical anisotropy are one of the most desirable fundamental building units (FBUs) for NLO materials. Up to now, inorganic planar π-conjugated groups including [BO3], [B3O6], [CO3], [NO3], [HxC3N3O3] (x varying from 0 to 3), [BS3], [B3S6], [HgSe3], [AgSe3], and [B2P5] units are reported as NLO-active groups, which strongly enhance the structural diversity of NLO materials. In this review, the NLO materials containing inorganic planar π-conjugated groups are classified according to their shortest transparent regions, i.e. deep-ultraviolet (DUV, λ < 200 nm), ultraviolet (UV, 200 nm < λ < 380 nm), visible (VIS, 380 nm < λ < 780 nm), and infrared (IR, λ > 780 nm) regions. On the basis of available experimental and calculated data, the structure-property relationships and potential applied wave ranges for different inorganic planar π-conjugated groups are discussed.