In modern laser techniques and science, both nonlinear optical (NLO) and linear optical (birefringence) responses are the key properties. However, it is always challenging to reach both strong NLO response and large birefringence in one crystal because of different requirements imposed on the structure.  

In a study published in Angew. Chem. Int. Ed., a research team led by Prof. LUO Junhua and ZHAO Sangen from Fujian Institute of Research on the Structure of Matter of the Chinese Academy of Sciences (CAS) obtained a novel antimony(III) borate, namely SbB₃O₆, by a spontaneous crystallization method in sealed quartz tubes under 10 Pa at a medium temperature of 773 K. Measured on a ZEISS Axio Scope, SbB₃O₆ shows an exceptional birefringence of Δn = 0.290 @ 546 nm, which is the largest among borates.

The researchers first determined the crystal structure of SbB₃O₆ by single-crystal X-ray diffraction analysis. They carried out the powder second-harmonic generation (SHG) tests of SbB₃O₆ by the Kurtz–Perry method with an incident Q-switched Nd:YAG laser of λ = 1064 nm. The result showed a strong phase-matching SHG response of 3.5 times that of the benchmark KH₂PO₄.

Besides, the co-authors from Technical Institute of Physics and Chemistry of CAS performed theoretical calculations based on the first principles. They carried out the (pratial) density of states of the respective species and the electron density distribution, and revealed that the strong linear and NLO response are mainly attributable to the synergistic effect of B-O anionic groups and Sb³⁺ cations with stereochemical active lone pair.

This study provides a rarely explored but excellent source for linear and NLO bi-functional materials that may find applications in integrated optical devices.

One ray of light passes through the crystal and is broken into two refracted light (birefringence of Δn = 0.290 @ 546 nm), as well as one beam of light with a frequency of ω doubles as it passes through the crystal (NLO response of 3.5 × KDP). (Image by Prof. LUO’s group)