Mid-infrared nonlinear optical heptachlorodigallate AGa2Cl7 (A = Ga+, In+) with a unique second harmonic generation mechanism induced by monovalent cations

Abstract

Nonlinear optical (NLO) anionic group theory holds that the anionic group mainly determines the NLO effect of the materials. 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 generation (SHG) 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. Due to the anisotropic induction of Ga+/In+, the same [Ga2Cl7]– anionic group exhibits significantly different SHG effects in different AGa2Cl7 (A = K, Ga, In) systems. This phenomenon is rare and satisfies a unique SHG mechanism, providing insights into NLO crystal physics and novel structural design. Importantly, except for enhanced SHG effects, monovalent Ga+/In+ cations can induce enlarged birefringence, larger than regular tetrahedra-based halides and more conducive to the phase matching. Furthermore, given wide optical transparency and better thermal properties as compared with the infrared NLO benchmark material AgGaS2, the unique AGa2Cl7 (A = Ga+, In+) are predicted as potential mid-infrared NLO materials.