Nonlinear optical (NLO) materials, which can expand the wavelength range provided by common laser sources through second harmonic generation (SHG), have been at the forefront of laser science and technology. However, two main drawbacks weigh seriously against the applications of the commercially available infrared (IR) NLO crystals (e.g., AgGaQ₂ (Q = S, Se) and ZnGeP₂), including low laser damage threshold (LDT) and difficulty to grow high-quality crystals. To search for new IR NLO crystals with both high LDT and moderate SHG response that can also be easily grown is still a challenge.

A research group led by Prof. PAN Shilie at Xinjiang Technical Institute of Physics & Chemistry of Chinese Academy of Sciences and American scientists from Northwestern University developed a promising IR NLO crystal, Pb₁₇O₈Cl₁₈ (POC). Both polycrystalline samples and single crystals of POC were synthesized in an open system.

Researchers introduced the Cl^– anion with a large electronegativity and Pb²⁺ as a relatively heavy element in POC which gave rise to the extension of its UV and IR cutoff edges to 0.34 and 13.9 μm. Besides, Chlorine could increase the band gap and obtain higher laser damage threshold, and SHG response was enhanced by introduction of lone-pair cations Pb²⁺. POC was thermally stable up to about 550 °C, which was quite high among halide-based IR NLO materials.

The SHG properties of the PbCl₂–PbO system were proposed for the first time. The crystal structure of POC consisted of only the targeted Pb–O and Pb–Cl bonds, which are beneficial to enlarge its IR and UV cutoff edges, respectively. POC crystallized in the space group Fmm2 (No. 42) of the orthorhombic system with the unit cell parameters a = 35.4963(14) Å, b = 5.8320(2) Å, c = 16.0912(6) Å, V = 3331.1(2) ų, and Z = 3. The structure was solved with the SHELXL program package and refined to R₁ = 0.0557 for 1729 reflections with F_o> 4σ(F_o) and wR₂ = 0.1321. 

Researchers found that POC could effectively overcome the two main drawbacks of the commercially available IR NLO materials, and it showed high LDT of 408 MW/cm², which is 12.8 times that of the benchmark IR NLO AgGaS₂ materials. They obtained transparent POC single crystals with 7 mm × 2 mm × 2 mm in an open system. The POC overcame the difficulty to grow high-quality crystals by the conventional Bridgman−Stockbarger technique. 

The study was published in J. Am. Chem. Soc.

This work was financially supported by the National Natural Science Foundation of China, the Western Light of Chinese Academy of Sciences, 973 Program of China, etc.

Figure: Photograph of as grown crystal in an open system and the linear and nonlinear optical properties of Pb₁₇O₈Cl₁₈. (Image by XTIPC)