Mid‐ to Far‐Infrared Nonlinear Optical Crystals: Design Strategies, Structure–Property Relationships and Engineering Prospects

Mid‐ to far‐infrared nonlinear optical (MFIR‐NLO) crystals are indispensable for generating tunable coherent radiation across the 3–25 µm spectral region and underpin a wide range of applications in remote sensing, environmental monitoring, and laser medicine. However, their development is hindered by the difficulty of simultaneously achieving a strong NLO response, broad infrared transparency, and a suitable bandgap. A key limitation arises from two‐phonon absorption, which governs the infrared cut‐off and links spectral coverage to lattice dynamics. This Review discusses MFIR‐NLO crystals from the perspectives of design strategies, crystal chemistry, structure–property relationships, and phonon engineering. We first assess the performance boundaries of commercial chalcopyrite‐type systems and then discuss the emergence of pnictide‐ and chalcogenide‐based crystals, highlighting how crystal‐chemical factors, particularly the distortion and connectivity of coordination polyhedra around metal centers, together with anionic substitution and bonding characteristics, govern phonon frequencies, bandgaps, and NLO activity. Building on the success of ZnGeP 2 in the 3–10 µm region, tellurides are identified as promising candidates for practical 10–25 µm NLO materials. PbIn 6 Te 10 exemplifies this direction, offering broad transparency, strong NLO response, stable phase matching, and favorable crystal growth, thereby providing a benchmark for next‐generation MFIR‐NLO materials.