Mid-infrared nonlinear optical materials for ultrafast sources: opinion

High-performance nonlinear optical (NLO) materials are essential for shifting ultrafast laser sources deep into the mid-infrared, especially for spectroscopic applications aimed at identifying species in the molecular fingerprint region (6.7–20 μm). Traditional oxide NLO crystals are limited to output wavelengths below 5 μm, but non-oxide semiconductors like ZGP, CSP, OP-GaAs, and OP-GaP offer deep infrared transparency, high nonlinearities, and broadband tunability. We review essential property requirements, compare materials of choice in two distinct classes - bulk birefringent crystals and quasi-phase-matched semiconductors - cite key ultrafast laser demonstrations, and identify emerging new materials and future directions in the field.