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

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 AGa 2 Cl 7 (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 [Ga 2 Cl 7 ] – dimer. Surprisingly, this kind of band structure can significantly enhance the NLO polarizability of both Ga + /In + cations and [Ga 2 Cl 7 ] – anionic groups, differing from conventional mechanism of the anionic group theory. First-principles calculations demonstrate that AGa 2 Cl 7 (A = Ga + , In + ) indeed exhibits greatly enhanced second harmonic generation (SHG) responses over the analogous KGa 2 Cl 7 . 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 [Ga 2 Cl 7 ] – anionic group exhibits significantly different SHG effects in different AGa 2 Cl 7 (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 AgGaS 2 , the unique AGa 2 Cl 7 (A = Ga + , In + ) are predicted as potential mid-infrared NLO materials. We have predicted the existing AGa 2 Cl 7 (A = Ga + , In + ) systems to exhibit a unique second harmonic generation (SHG) mechanism, differing from conventional origin of the anionic group theory. The unique SHG mechanism induced by monovalent Ga + /In + cations enables AGa 2 Cl 7 (A = Ga + , In + ) to possess potential mid-infrared nonlinear optical properties including enhanced SHG effects, enlarged birefringence, good thermal properties and wide optical transparency. • Unique band structure induced by isolated monovalent Ga + /In + cations can dramatically enhance nonlinear optical polarizability. • The same [Ga 2 Cl 7 ] - anionic group exhibits significantly different SHG responses in different AGa 2 Cl 7 (A = K, Ga, In) systems. • AGa 2 Cl 7 (A = Ga + , In + ) are predicted to exhibit potential mid-infrared nonlinear optical properties.