Size effect and identified superior functional units enhancing second harmonic generation responses on the II-IV-V-2 type nonlinear optical crystals

[[abstract]]The II-IV-V2 type nonlinear optical (NLO) materials with chalcopyrite structures have wide applications in infrared (IR) frequency conversion, optical parametric oscillation, and terahertz wave producing, which can be as references to explore IR NLO systems. The template effect, mainly the size effect of five II-IV-V2 type compounds (ZnGeP2, ZnSiP2, ZnSnP2, CdGeP2 and CdGeAs2) on the optical properties is analyzed in terms of microscopic structures and electronic structures. The mechanism for significant differences in the second harmonic generation (SHG) intensities from 2.8 to 15 × AgGaS2 (the IR benchmark) is clarified via the density functional theory, the real space atom-cutting method and SHG-weight density analysis. It is found that the size effect of ions in this family of crystals is mainly responsible for the decreases of band gaps, then leads to the enhancement of SHG intensities. Moreover, the real space atom-cutting calculations show that the [SiP4], [GeP4], [SnP4] and [GeAs4] dominate the contributions to SHG response, which are identified as the superior functional units and are conducive to enhancing the SHG effects. Therefore, one can select those NLO superior functional units that enlarge the cell size and SHG responses to obtain new IR NLO materials.[[notice]]補正完