Design and fabrication of composite structures in ZnSe providing broadband mid-infrared anti-reflection

A comparative study of anti-reflection microstructures (ARMs) and ARMs with Al2O3 coatings (Al2O3/ARMs) on zinc selenide (ZnSe) has been carried out. The transmittance and electric field intensities were analyzed and presented using finite-difference time-domain (FDTD) simulations. ARMs were successfully fabricated on ZnSe surfaces by reactive ion etching and an Al2O3 layer was deposited onto the ARMs by an ion beam sputtering method. The theoretical analysis and experimental results showed that the transmittance of Al2O3/ARMs-treated ZnSe had a close relationship with the thickness of Al2O3 layer and was higher than that of ARMs-treated ZnSe in the 2-5 μm wavelength range, which was due to the fact that the Al2O3/ARMs had a better graded refractive index profile. The angle-dependent reflectance of ZnSe with Al2O3/ARMs was lower than that with ARMs. A difference of the spatial distribution of the electric field intensity between ARMs and Al2O3/ARMs was observed due to the surface Al2O3 coating, which predicted qualitative laser damage thresholds for both structures. The results of this study provide potential applications in high-efficiency IR optoelectronic devices and high power mid-infrared laser systems.