Trade-Off for Better Balanced Nonlinear Optical Performance with Disordered Si in ZnGeP₂

ZnGeP2 (ZGP) possesses prominent mid-far infrared nonlinear optical (NLO) performance but suffers from a narrow bandgap. A comparative analysis revealed that the bandgap could be enhanced by substituting Si for Ge in ZGP, successfully synthesizing a series of ZnSixGe1–xP2 crystals with enlarged bandgaps compared with that of ZGP. Interestingly, when the x varies from 0 to 0.5, in addition to obtaining improved bandgaps, the second-harmonic generation (SHG) responses are enhanced simultaneously, which is abnormal because they are usually inversely proportional. Theoretical calculations with special quasirandom structures show that the NLO coefficient decreases gradually with increasing Si content, but the decrease is tiny in the 0–50% range and acute in the 50–100% range of Si. The transmission experiment of the ZnSi0.25Ge0.75P2 wafer indicates that the introduction of silicon effectively improves the transmittance in the 1–2 μm range. The enhanced transmittance and maintained NLO coefficients result in a better macroscopic SHG response. Combined with an increased laser-induced damage threshold, the introduction of Si becomes a practical approach to improve performance and extend the application of ZGP crystals.