Abstract
We present a new type of nanoporous antireflection (AR) coating based on grass-like alumina with a graded refractive index profile. The grass-like alumina AR coating is fabricated using atomic layer deposition (ALD) of alumina and immersion in heated deionized water. Optical transmittance of 99.5% at 500 nm was achieved with average transmittance of 99.0% in the range of 350-800 nm at normal incidence for double-sided coated glass. Angular spectral transmittance (0-80°) of the double-sided AR coated glass was also measured in the range of 350-800 nm and found to have mean spectral transmittance of 94.0% at 60°, 85.0% at 70°, and 53.1% at 80° angles of incidence, respectively. The grass-like alumina AR coating is suitable for mass production with the presented technique: even hundreds of optical components can be coated in parallel. Furthermore, as an ALD-based technique, the coating can be deposited conformally on surfaces with extreme topography, unlike many spin-coating, physical vapor deposition or glancing angle deposition-based coatings used today.
Highlights
Antireflection (AR) coatings are perhaps the most universal feature in all of optics, as they are used everywhere from camera lenses to high-performance solar cells
We present a new type of nanoporous antireflection (AR) coating based on grass-like alumina with a graded refractive index profile
Nanoporous materials have been studied vigorously as AR coatings for glass and other materials.[2−11] Often nanoporous AR coatings are manufactured with a gradient refractive index profile[5−10] due to the wellknown broadband and omnidirectional AR properties a gradient-index layer provides.[12,13]
Summary
Antireflection (AR) coatings are perhaps the most universal feature in all of optics, as they are used everywhere from camera lenses to high-performance solar cells. With a single AR layer, the Fresnel reflection at normal incidence is minimized when the refractive index of the AR layer is nAR = nsubstratenair , and the optical thickness of the layer is chosen to be a quarter of the wavelength This means that glass, the most widely used optical material, with refractive index of approximately 1.5 should have an AR coating with refractive index of 1.22. This grass-like alumina is extremely interesting as a nanoporous AR coating due to its scalability, ability to coat all surfaces with one run, excellent optical transmittance, and gradient refractive index, but has not so far been used in this capacity, porous anodic alumina and sol−gel-based aluminas (some with “flower-like” structures using the hot water treatment) have been.[2,20−25] Neither porous anodic alumina nor sol−gel-based aluminas produce conformal coatings,[26] and the presented grass-like alumina AR coating is the first conformal nanoporous AR coating made from alumina and the first nanoporous AR coating made using DIW immersion of ALD alumina
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