Nowadays, frequency conversion technology has been developed to expand the fields of laser application. Nonlinear optical (NLO) materials, as one of efficient frequency-shifting devices, have attracted increasing attention.

A series of self-defects such as difficulties in obtaining single crystal with high optical quality, low laser damage threshold and severe two-photon absorption hinder infrared (IR) NLO materials' commercial application. Therefore, exploration of new IR NLO materials with excellent properties (broad IR transparency range, large second harmonic generation (SHG) response, and high damage resistance) is extremely urgent.

A research group led by Prof. PAN Shilie at Xinjiang Technical Institute of Physics & Chemistry (XTIPC) of Chinese Academy of Sciences (CAS) developed two new potential Mid-IR nonlinear optical crystals, M₂LiVO₄ (M = Rb, Cs). The title compounds with noncentrosymmetric structures (Orthorhombic, Cmc21) were synthesized by high-temperature solution method. This study was published in Scientific Reports.

Researchers investigated a complete survey of linear and nonlinear optical properties for title compounds. The structure of title compounds consists of the VO₄ and LiO₄ tetrahedra, forming 2D [LiVO₄]_∞ layers that are separated by the Rb⁺/Cs⁺ cations to maintain charge balance. Large size crystals of Cs₂LiVO₄ (CLVO) have been grown by the top-seeded solution growth method.

The results indicated that the title compounds exhibit not only relatively large Eg (3.8 and 3.7 eV, respectively) and wide transparent region (0.33–6.0 μm for CLVO) among the known Mid-IR NLO crystals, but also achieve the suitable balance between large SHG response (4 and 5 that of KDP, respectively) and high LDT (about 28 × AgGaS₂ for Rb₂LiVO₄ and Cs₂LiVO₄).

Besides, researchers used the first-principles studies to explore the relationship between the electronic structure and optical properties. The studies revealed that the NLO response of title compounds mainly comes from the VO₄ anionic groups, and it also paved the way for exploiting Mid-IR NLO materials based on vanadates.

This work was financially supported by the West Light Foundation of the CAS, National Natural Science Foundation of China, the Xinjiang International Science & Technology Cooperation Program, etc.