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.
AgGaSe2 and LiGaSe2 are two famous mid‐infrared nonlinear optical (NLO) materials with similar chemical formula but different structural symmetry. The former material has relatively larger NLO effect and birefringence but rather small energy band gap, while the latter is the opposite. Aiming at achieving a good balance of NLO properties, here the substitution between silver and lithium ions on the evolution of structural and optical properties in a new series of LixAg1–xGaSe2 crystals is systematically investigated. It reveals that, with the increase of Li content, LixAg1–xGaSe2 almost keeps the same tetragonal symmetry with AgGaSe2 until x ≈ 0.9. The NLO effects and birefringence values vary with respect to x with the largest variation at x = 0.8–0.9. The optimal combination of birefringence (0.03–0.025) and nonlinear parameters (26–30 pm V−1) is achieved at x = 0.4–0.5. As the energy band gap increases with the increase of x, the maximal value of 2.2 eV for chalcopyrite structure suggests that the laser‐induced damage threshold of LixAg1–xGaSe2 would be as large as five‐fold of AgGaSe2. This study provides a good example to show that the rational substitution between Li and Ag can significantly improve the balance of NLO properties in chalcogenides.
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