First-principles prediction of an enhanced optical second-harmonic susceptibility of low-dimensional alkali-metal chalcogenides

Fully first-principles calculations for the second-harmonic susceptibilities of recently synthesized alkali-metal chalcogenides such as KPSe₆, K₂P₂Se₆, LiAsS₂, and NaAsSe₂ predict a record-breaking second-harmonic generation coefficient among materials with band gaps larger than 1.0 eV, with the highest value being that for NaAsSe₂, namely, 324.6 pm/V. A detailed analysis of their highly precise full-potential linearized augmented plane-wave electronic structures suggests that it is a quasi-one-dimensional structural anisotropy with a strong covalent character that yields the very large second-harmonic coefficients.