LixAg1–xGaSe2: Interplay Between Lithium and Silver in Mid‐Infrared Nonlinear Optical Chalcogenides

Abstract AgGaSe 2 and LiGaSe 2 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 Li x Ag 1– x GaSe 2 crystals is systematically investigated. It reveals that, with the increase of Li content, Li x Ag 1– x GaSe 2 almost keeps the same tetragonal symmetry with AgGaSe 2 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 Li x Ag 1– x GaSe 2 would be as large as five‐fold of AgGaSe 2 . 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.