Mid-Infrared Nonlinear Optical Halides with Diamond-like Structures: A Theoretical and Experimental Study

As an important family of potential mid-IR nonlinear optical (NLO) crystals, halides exhibit superiority in large optical band gaps and wide IR transparency. However, their second-harmonic generation (SHG) effects are generally small, limiting their frequency conversion efficiency in NLO processes. Diamond-like structures, due to the intrinsic polarization-parallel alignment of tetrahedral frameworks, enable sufficiently large SHG effects while maintaining balanced NLO performance in the mid-IR spectral region. Accordingly, in this study, we focused on the NLO performance mining and assembly of tetrahedral polar motifs in halides with unique diamond-like structures. We screened almost all diamond-like structures in inorganic halide systems and investigated in detail how their balanced NLO performances evolve with ideal and defective diamond-like structures. We predicted that the three existing diamond-like iodides, CuGaI4, Ag2CdI4, and Ag2ZnI4, can satisfy the balanced requirement of strong SHG effects (>3.9 pm/V) and large optical band gaps (>3 eV) by analyzing the microstructural NLO origin. Preliminary experiments also verified our predictions with Ag2CdI4 (1.4 × AgGaS2 and 3.3 eV) and Ag2HgI4 (2.2 × AgGaS2 and 2.4 eV). More importantly, a map with the diamond-like halide family tree was illuminated, unlocking dozens of diamond-like structures with balanced NLO properties, from ternary AgCdI3, Ag2Cd3I8, Ag4CdI6, and AgInI4 to quaternary AgCuCdI4, Ag8HgCd3I16, Ag3HgGaI8, AgCd2InI8, and Ag2CdI2Br2. As long as following this map, one may design and discover potentially promising mid-IR NLO halide crystals with diamond-like structures.