Unique Nonlinear Optical Mechanism induced by Monovalent Gallium and Indium Cations in Heptachlorodigallate AGa2Cl7 (A = Ga+, In+)

The same anionic group basically has similar NLO properties even in different systems, so it is regarded as the “gene” of NLO materials. Isolated monovalent Ga+/In+ cations in the existing AGa2Cl7 (A = Ga, In) system carry lone-pair-electronic nonbonding orbitals, which are free between the energy bands near the gap and are not substantially hybridized with the orbitals of anionic [Ga2Cl7]- dimer.

Surprisingly, this kind of band structure can significantly enhance the NLO polarizability of both Ga+/In+ cations and [Ga2Cl7]- anionic groups, differing from conventional mechanism of the anionic group theory. First-principles calculations demonstrate that AGa2Cl7 (A = Ga, In) indeed exhibits greatly enhanced second harmonic responses over the analogous KGa2Cl7. The underlying contribution to this unusual enhancement stems not only from the anisotropic distribution of photo-responsive nonbonding electron pairs on Ga+/In+, but also from the synergistic charge transfer effect between cations and anionic groups.