Ultrashort phase-matching wavelength and strong second-harmonic generation in deep-UV-transparent oxyfluorides by covalency reduction

The development of urgently-needed ultraviolet (UV)/deep-UV nonlinear optical (NLO) materials has been hindered by contradictory requirements of the microstructure, in particular the need for a strong second-harmonic generation (SHG) response as well as a short phase-matching (PM) wavelength. We herein employ a "de-covalency" band gap engineering strategy to adjust the optical linearity and nonlinearity. This has been achieved by assembling two types of transition-metal (TM) polyhedra ([TaO${}_2$F${}_4$] and [TaF${}_7$]), affording the first tantalum-based deep-UV-transparent NLO materials, A${}_5$Ta${}_3$OF${}_{18}$ (A = K (KTOF), Rb (RTOF)). Experimental and theoretical studies reveal that the highly ionic bonds and strong electropositivity of tantalum in the two oxyfluorides induce record short PM wavelengths (238 (KTOF) and 240 (RTOF) nm) for d${}^0$-TM-centered oxides, in addition to strong SHG responses 2.8 × KH${}_2$PO${}_4$ (KTOF) and 2.6 × KH${}_2$PO${}_4$ (RTOF)), and sufficient birefringences (0.092 (KTOF) and 0.085 (RTOF) at 546 nm). These results not only broaden the available strategies for achieving deep-UV NLO materials by exploiting the currently neglected d${}^0$-TMs, but also push the shortest PM wavelength into the short-wavelength UV region. (© 2023 Wiley‐VCH GmbH)