A3Te(Zn2Ge)Ge2O14 (A = Sr, Ba, and Pb): New Langasite Mid-infrared Nonlinear Optical Materials by Rational Chemical Substitution

Mid-infrared (mid-IR) nonlinear optical (NLO) crystals with excellent comprehensive performances are urgently demanded in the laser technology field. Herein, a series of new langasite members, A3Te(Zn2Ge)Ge2O14 (A = Sr, Ba, and Pb), were designed and synthesized through a rational chemical substitution strategy with classic A3BC3D2O14 langasites as ideal prototypes. These crystals exhibited excellent mid-IR NLO properties including a large second harmonic generation (SHG) response (2.7, 3.2, 13.0 × KDP under 1064 nm and 0.11, 0.12, 0.54 × AGS under 2090 nm), a large band gap (4.03, 3.98, and 3.43 eV), and a wide transparency region from the ultraviolet (UV) to mid-IR region. Compared to those langasites with light D-site cations, the titled crystals highlighted the reduction of vibration frequency by the introduction of heavier Ge4+ cations, which helps extend the transparency range in the mid-IR region. Additionally, theoretical calculation results revealed the significant enhancement of the SHG effect by Pb2+ cations on the A-site. This work confirmed the feasibility of our chemical co-substitution strategy and enriched the langasites.