KMg(H₂O)PO₄: A Deep-Ultraviolet Transparent Nonlinear Optical Material Derived from KTiOPO₄

Abstract

Discovery of new nonlinear optical (NLO) crystals, especially crystals that can produce coherent light in the deep-ultraviolet (DUV) region is essential for the development of diverse fields. Here, we successfully discovered and synthesized a new DUV transparent NLO crystal KMg(H2O)PO4 (KMP) using KTiOPO4 (KTP) as a template through substituting TiO6 by the MgO5(H2O) group. Its structure constructed by 2D [MgPO5]∞ infinite layers in which PO4 groups arrange orderly leads to KMP exhibiting a large second harmonic generation (SHG) efficiency (≈1.14 × KDP) which is similar to KBe2BO3F2 (1.2 × KDP). Moreover, the ultraviolet cut-off edge of KMP is blue-shifted to <200 nm from 350 nm of KTP after replacing the transition metal cations Ti4+ by alkaline-earth metal cations Mg2+. First principle calculations reveal that the SHG efficiency of KMP was significantly contributed by PO4 groups, and contributions from cations are negligibly small which agrees well with the anion group theory.