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 Cd2+ are introduced into the Cu3PS4 as the equivalent and aliovalent dopants for the Cu site. As a result, Li+ can significantly improve the bandgap (E-g) of LixCu3-xPS4 from 2.38 to 2.88 eV, leading to a higher laser-induced damage threshold (LIDT) of 4.9 times than AgGaS2 (AGS) with a comparable second harmonic generation (SHG) response of AGS (26-45 mu m). Interestingly, Cd2+ can improve the SHG response and enlarge the E-g simultaneously. As a result, Cd0.4Cu2.2PS4 has a large SHG response of 10 x AGS at 2050 nm (26-45 mu m) and a LIDT of 2.6 x AGS. Theoretical calculations reveal that lattice vacancies induced by Cd2+ significantly boost polarizability compared to LixCu3-xPS4 with no vacancy, leading to a strong NLO response.