A potential UV nonlinear-optical crystal with a strong second-harmonic response: RbNa2Eu2(BO3)3

Abstract

Ultraviolet (UV) nonlinear optical (NLO) crystals with a second-harmonic generation (SHG) response can efficiently output UV coherent light, and have important applications in UV solid-state lasers. Herein, we designed and synthesized a novel Eu-based borate (NLO) crystal, RbNa₂Eu₂(BO₃)₃ (RNEBO), which introduces Eu into an A₃RE₂(BO₃)₃ (A = alkali metal, RE = rare earth) system applied in the UV region first and exhibits both strong SHG activity and a wide optical transmission window. The RNEBO crystallizes in the non-centrosymmetric (NCS) space group Amm2, featuring a three-dimensional framework of [EuBO₃]_∞ layers with isolated [BO₃]³⁻ groups bridging in a coplanar arrangement. Meanwhile, the alkali metals Rb and Na were incorporated into the void of the anion framework to ensure coordination and charge balance. The distinctive internal structure enables RNEBO to possess a large SHG response (∼3.55 × KDP) and UV transmittance of 6.17% at 200 nm. This finding confirms that RNEBO is a potential UV NLO crystal and provides valuable insights for the exploration of Eu-based borate UV NLO crystals in the future.