Vacancy‐Induced Extraordinary Second Harmonic Generation Response for Diamond‐Like Cu₃PS₄

Improving polarizability is an important strategy for designing high‐performance mid‐infrared (mid‐IR) nonlinear optical (NLO) materials. The substitution of equivalent or aliovalent atoms can manipulate the polarizability by adjusting the symmetry of the polyhedron. Herein, the Li + and Cd 2+ are introduced into the Cu 3 PS 4 as the equivalent and aliovalent dopants for the Cu site. As a result, Li + can significantly improve the bandgap ( E g ) of Li x Cu 3− x PS 4 from 2.38 to 2.88 eV, leading to a higher laser‐induced damage threshold (LIDT) of 4.9 times than AgGaS 2 (AGS) with a comparable second harmonic generation (SHG) response of AGS (26−45 µm). Interestingly, Cd 2+ can improve the SHG response and enlarge the E g simultaneously. As a result, Cd 0.4 Cu 2.2 PS 4 has a large SHG response of 10 × AGS at 2050 nm (26−45 µm) and a LIDT of 2.6 × AGS. Theoretical calculations reveal that lattice vacancies induced by Cd 2+ significantly boost polarizability compared to Li x Cu 3− x PS 4 with no vacancy, leading to a strong NLO response.