Fabrication of antireflection coatings from visible to mid-wave infrared via quantitative control of the refractive index.

The development of ultra-broadband antireflection (AR) coatings for optics in multispectral systems presents a critical yet challenging task. A simple method was proposed for preparing gradient index (GRIN) AR films covering wavelengths from visible to mid-wave infrared. These films exhibit low reflectance, ultra-broadband AR performance, incident-angle insensitivity, and environmental stability. At the core of this approach is quantitative control of the refractive index on the nanograss-like alumina (NGLA). The scattering characteristics and surface-area ratios of NGLA samples with varying initial thicknesses were investigated. Subsequently, a GRIN structure based on NGLA was designed and fabricated on both sides of sapphire substrates, achieving reflectance reductions from 13.92% to 0.72% (420-2000nm) and from 11.73% to 1.55% (3000-5000 nm). To further optimize AR performance and scattering suppression, a dual-layer NGLA stack structure with modulated refractive index was implemented on fused silica and black glass substrates. These dual-layer films maintained reflectance below 1% across 400-2500 nm and demonstrated excellent stability under wide-angle incidence, saltwater immersion, and temperature variations. The proposed broadband AR coating process is simple, cost-effective, and easy to scale up for industrial production, demonstrating significant potential in optical applications.