Toward Large Second-Harmonic Generation and Deep-UV Transparency in Strongly Electropositive Transition Metal Sulfates

The development of deep-ultraviolet (DUV)/solar-blind UV nonlinear optical (NLO) crystals simultaneously possessing wide UV transparency, strong second-harmonic generation (SHG) response, and suitable birefringence is a major challenge in advanced laser technology. We herein propose a "cation compensation" strategy for strong optical nonlinearity in inorganic solids that is exemplified by the introduction of strongly electropositive transition metals (TMs). Following this strategy, the first d⁰ TM UV-transparent NLO sulfates, MF₂(SO₄) (M = Zr (ZFSO), Hf (HFSO)), have been synthesized. Short UV cutoff edges of 206 nm and below 190 nm are observed for bulk ZFSO and HFSO crystals, respectively, together with the strongest powder SHG responses (3.2 × (ZFSO) and 2.5 × KDP (HFSO)) for solar-blind UV/DUV NLO sulfates, as well as suitable birefringence. This work provides a new and efficient approach to the development of urgently needed high-performance NLO materials for applications in the short-wavelength UV region.