Design and Fabrication of Antireflective Coatings with Enhanced Transmittance and High Laser-Induced Damage Threshold for ZnGeP₂ Mid-Infrared Crystals

Zinc germanium phosphide (ZnGeP₂, ZGP), a key mid-infrared nonlinear crystal for optical parametric oscillation (OPO) applications, suffers from limited transmittance (∼60%) at pump (2090 nm) and generated (3600-4800 nm) wavelengths, hindering high-energy laser applications. This study overcomes this limitation by designing and experimentally validating highly effective antireflective coatings (ARCs) using Ta₂O₅/SiO₂ multilayers. An optimized Air|(1.44L/1.32H)|ZGP|(1.32H/1.44L)|Air structure exhibited three key improvements: (1) Simulated average transmittance >99% across both bands; (2) Minimized electric field intensity at critical interfaces, enhancing the laser-induced damage threshold (LIDT); (3) The design exhibits excellent thickness tolerance, maintaining >94% transmittance in the case of -45 nm to +50 nm film thickness variation, facilitating reliable fabrication via magnetron sputtering deposition. Fabricated coatings demonstrated outstanding performance: 97.0% transmittance at 2090 nm and 96.3% average across 3600-4800 nm─a ∼66% relative improvement over bare ZGP. Furthermore, under high-frequency laser irradiation conditions (16 kHz, 2090 nm), the LIDT of the coated sample reached at least 1.39 J/cm², demonstrating robust durability in demanding operational environments. This ARC solution effectively addresses the critical limitations of ZGP crystals while maintaining compatibility with high-frequency laser operation requirements.