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.