KRbBPO: Design and Synthesis of a Deep-Ultraviolet Nonlinear Optical Crystal with Balanced Performance Derived from π-Conjugated and Non-π-Conjugated Groups.

A new deep-ultraviolet (DUV) nonlinear optical (NLO) material KRbBPO (KRBPO) has been designed and synthesized by adjusting the B/P molar ratio to 8:1. The KRBPO crystals were synthesized by a flux method and crystallized in noncentrosymmetric (NCS) and polar space group This compound exhibits a double-layer structure in which the A layer is composed of [BO] and [BO] units and the B layer is formed by interconnected [BO] and [BO] groups, and the two layers are connected by [PO] tetrahedron. The theoretical calculations and experiments show that the synergistic interaction of π-conjugated and non-π-conjugated units leads to relatively well-balanced NLO properties of the title compound, including moderated SHG (0.7 × KDP), short UV cutoff edge (λ < 190 nm), and a large band gap of 6.16 eV. Specifically, the coplanar arrangement of B-O groups in double-layer makes the KRBPO display a large birefringence (0.075@532 nm) and enables the shortest phase-matched wavelength to reach an important laser output wavelength of 266 nm.