A promising mid-infrared nonlinear optical crystal with well-balanced properties

Abstract

Nonlinear optical (NLO) crystals have attracted significant interest owing to their ability to extend the frequency of lasers. Here, we report a new germanate NLO crystal, PbZnGeO₄ (PZGO), which features a unique 3D framework built from asymmetric ZnO₄, GeO₄, PbO₃, and PbO₄ units. PZGO exhibits a short UV cutoff edge of 300 nm, corresponding to a large bandgap of 3.7 eV, and a broad transparent range extending up to 5.9 μm, effectively covering the crucial atmospheric transmission window of 3–5 μm. Notably, it achieves a strong phase-matchable second-harmonic generation (SHG) intensity (2.5× KDP), along with a suitable birefringence of 0.04 at 1064 nm, superior to those of most reported germanate NLO crystals. First-principles calculations reveal that the optical activities of PZGO primarily originate from the highly distorted Pb–O units with stereochemically lone-pair electrons.