Toward the ultraviolet (UV) or deep-UV nonlinear optical crystals: The combination of π-conjugated planar [XY3] and tetrahedral [XY4]

As the critical devices in laser science and technology, nonlinear optical (NLO) crystals can expand the wavelength to the ultraviolet (UV), or deep-ultraviolet (DUV) region. Generally speaking, a functional UV/DUV NLO material should satisfy the following basic requirements: wide transmittance range; large NLO coefficient (dij > 0.39 pm/V); moderate birefringence (Δn: 0.05–0.10@1064 nm) to achieve the shortest phase-matching (PM) wavelength to the short UV (λPM ≤ 266 nm) or even DUV region (λPM ≤ 200 nm). Nevertheless, it is challenging to satisfy all of the aforementioned conditions with the single π-conjugated or tetrahedra groups. In this review, we will focus on UV or DUV NLO crystals by combining π-conjugated planar [XY3] ([BO3], [CO3], [C(NH2)3]) with tetrahedral [XY4] units ([BO4], [PO4], [SO4], [BeO4/BeO3F], [AlO4/AlO3F], [ZnO4/ZnO3F/ZnO3Cl], [BOxF4-x], etc.), which can eliminate the dangling bonds of π-conjugated groups through tetrahedra and compensate for the birefringence and second-harmonic generation (SHG) through π-conjugated groups. According to this strategy, about 120 compounds were screened, including theoretically predicted, we summarized their structures, linear and nonlinear properties, and classified them into the following main categories according to the different π-conjugated groups in the structure: 1. Borate-based NLO crystals, 2. Carbonate-based NLO crystals, 3. Guanidinium-based NLO crystals. More importantly, by combining π-conjugated [XY3] and tetrahedral [XY4] genes, we anticipate that this review will provide useful implications for the thoughtful design and synthesis of novel NLO materials with the desired features.