Nonlinear optical (NLO) material is one of the important parts of solid-state lasers. It is largely used in laser frequency conversion, optical parameter oscillators, other optical and photonic devices.

Designing and synthesizing a deep UV NLO material requires crystallographic non-centrosymmetry, a wide UV transparency range, a large second-harmonic generating coefficient, moderate birefringence, chemical stability and resistance to laser damage, and ease in the growth of large high-quality single crystals. For example, β-BaB₂O₄, LiB₃O₅ and CsLiB₆O₁₀ are successful example of NLO material. Recently, metal phosphates have attracted considerable interests for designing new DUV NLO materials.  

A research group led by Prof. PAN Shilie at Xinjiang Technical Institute of Physics & Chemistry (XTIPC) of the Chinese Academy of Sciences developed three new NLO crystals  APb₂(PO₃)₅ (A = K, Rb and Cs).

The three compounds are isostructural and belong to the non-centrosymmetric space group of Pn (No. 7). The structures of APb₂(PO₃)₅ (A = K, Rb and Cs) exhibit a 3D network which consists of the PbO_n (n = 6, 8), KO₇/RbO₈/CsO₁₀ and PO₄ units.  

Researchers found UV-vis-NIR transmittance spectrum result shows that KPb₂(PO₃)₅ has a short deep-UV cutoff edge of 177 nm. It is the shortest UV cut-off edge among Pb-containing NLO crystals.

Second harmonic generation (SHG) result indicate that KPb₂(PO₃)₅ and RbPb₂(PO₃)₅ are display moderate powder SHG intensities (0.5 × KDP and 0.3× KDP, respectively) in type I phase matching behaviors. Compared with RbBa₂(PO₃)₅, KPb₂(PO₃)₅ has a noticeable enhancement in birefringence (experimental value of 0.03 at 589.3 nm).  

To better understand the blue-shift mechanism and enhancement of birefringence in KPb₂(PO₃)₄, researchers employed the theoretical calculations by density functional theory.

As a result, there is almost no hybridization between Pb-6p and O-2p orbitals at the top of valence bands, and therefore, the stereochemistry activity of the lead atom in KPb₂(PO₃)₅ is weak according to the stereochemistry activity mechanism, which causes a blue-shift of the band gap in KPb₂(PO₃)₅. 

The result was published in J. Mater. Chem. C as a front cover. This article is part of themed collection: 2016 Journal of Materials Chemistry C Hot Papers. 

This work was supported by the National Natural Science Foundation of China. 

Figure1：The DUV transmittance spectrum and the photograph of the KPb₂(PO₃)₅ crystal plate. (b) NLO properties of KPb₂(PO₃)₅ and RbPb₂(PO₃)₅ with commercial KDP as a reference(Image by XTIPC)