Deep Ultraviolet-Transparent Materials with Strong Second-Harmonic Response

Deep ultraviolet (UV) nonlinear optical (NLO) materials with excellent performances are particularly important in laser technology. However, the conflict between large second-harmonic generation (SHG) responses and wide band gaps makes the rational design and syntheses of deep UV NLO materials remain a great challenge. Herein, we report five new and polar deep UV NLO crystals with enhanced SHG responses, Ba3La4O4(BO3)3X (X = F, Cl, and Br) and Sr5La(BO3)4X (X = Cl and Br), which were synthesized by introducing rare-earth cations for constructing oxyborates and borates. The property measurements show that they exhibit not only short UV cut-off edges (λcut-off < 190 nm) but also large phase-matching SHG responses (2.3–6.4 × KDP). Especially for Ba3La4O4(BO3)3Br, its SHG response (6.4 × KDP) is the largest in all the reported BO3-type deep UV borates. Systematic studies reveal that the large SHG responses of these compounds can be attributed to the large electronic populations and non-symmetry localization of the electron density of La–O polyhedra in oxyborates, which will provide some new insights into the origin of enhanced SHG responses beyond the anionic group theory.