Reduction of reflection losses in ZnGeP2 using motheye antireflection surface relief structures

We report the reduction of surface reflection losses in zinc germanium phosphide (ZnGeP2, or ZGP) crystals by fabricating an antireflection (AR) structure in the substrate itself using subwavelength motheye surface patterns. The motheye AR patterning works by creating a region of gradually varying effective refractive index between air and the ternary nonlinear crystal. Motheye structures were created using interference lithography and reactive-ion etching in a SiCl4 plasma. The ZGP crystal with motheye patterning on the output surface reached a transmittance of ∼67% at a cutoff wavelength of 3.8 μm (close to the theoretical maximum of 73%), with negligible surface contamination from the motheye etching process. The motheye patterning technique could be applied to other nonlinear crystals where surface reflection losses are a concern.