A New Nonlinear Optical Selenide Crystal AgLiGa2Se4 with Good Comprehensive Performance in Mid-Infrared Region

Mid‐infrared (mid‐IR) nonlinear optical (NLO) crystals are indispensable for the mid‐IR lasers generation with tunable wavelengths from 3 to 20 µm. AgGaSe₂ is a commercial mid‐IR NLO crystal with the highest figures of merit, but suffers low laser damage threshold (LDT). To achieve the balance of optical transmission, NLO effect, and LDT, it is proposed to molecularly modify the AgGaSe₂ structure by introducing the [LiSe₄] tetrahedra, and successfully grow large crystals of a new selenide AgLiGa₂Se₄. The replacement of half of the heavy Ag⁺ cations with light Li⁺ increases the band gap to 2.2 eV (vs. 1.7 eV in AgGaSe₂). The LDT value in AgLiGa₂Se₄ increases five times compared to that in AgGaSe₂, while keeping a relatively large NLO susceptibility of 26 pm V⁻¹. Moreover, the thermal expansion coefficients in AgLiGa₂Se₄ are approximately two times lower in absolute value compared with AgGaSe₂, which is beneficial to the large crystal growth. All these advantages would make AgLiGa₂Se₄ a new promising NLO crystal for mid‐IR laser applications.