High-performance antireflection coatings utilizing nanoporous layers
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duction One of the most important quantities in all of optics is the refractive index. This is particularly true in the subfield of optical coatings, where the refractive index appears in virtually every equation as a figure of merit. However, in the design of optical coatings, the choices of optically transparent materials with a wide range of refractive index values are quite limited. Gases are materials with the lowest refractive index (nair ≈ 1.0), but they completely lack structural stability. MgF2, CaF2, and SiO2, which have refractive indices nMgF2 = 1.39, nCaF2 = 1.44, nSiO2 = 1.46, respectively, are materials with refractive indices among the lowest available for conventional, dense materials. The limited availability of materials across a wide range of refractive index values and the unavailability of very low-n materials for optical thin-film applications can strongly limit the performance of optical coatings. Recently, nanoporous materials with both tailorable- and low-refractive index properties have been explored for their
customizable optical characteristics.1–8 By controlling the porosity of such nanoporous films, the effective refractive index of the grown or deposited film can be controlled. The refractive index may be tailored within a wide range of values, from the value of the dense bulk material down to values close to the index of air. These two properties—tailorability of the refractive index and ability to attain very low-refractive index values— make nanoporous coatings ideally suited for achieving the highest possible performance in optical coatings.
Oblique-angle deposition One particularly promising method to fabricate nanoporous films with highly desirable optical properties is oblique-angle deposition. Films deposited using oblique-angle methods have shown excellent control over the morphology and thickness of the deposited layers.1–4,9–11 By controlling the vapor-flux incident angle during deposition, the porosity and thus the refractive index of the nanoporous film can be arbitrarily tailored.
David J. Poxson, Rensselaer Polytechnic Institute, Troy, NY 12180, USA; [email protected] Mei-Ling Kuo, Rensselaer Polytechnic Institute, Troy, NY 12180, USA; [email protected] Frank W. Mont, Raydex Technology, Inc., Cambridge MA 02138; [email protected] Y.-S. Kim, Rensselaer Polytechnic Institute, Troy, NY 12180, USA; [email protected] Xing Yan, Rensselaer Polytechnic Institute, Troy, NY 12180, USA; [email protected] Roger E. Welser, Magnolia Solar, Inc.; [email protected] Ashok K. Sood, Magnolia Solar Inc.; [email protected] Jaehee Cho, Rensselaer Polytechnic Institute, Troy, NY 12180, USA; [email protected] Shawn-Yu Lin, Rensselaer Polytechnic Institute, Troy, NY 12180, USA; [email protected] E. Fred Schubert, Rensselaer Polytechnic Institute, Troy, NY 12180, USA; [email protected] DOI: 10.1557/mrs.2011.110
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MRS BULLETIN • VOLUME 36 • JUNE 2011 • www.mrs.org/bulletin
© 2011 Materials Research Society
HIGH-PERFORMANCE ANTIREFLECTION COATINGS UTILIZING NANOPOROUS LAYERS
Oblique-angle deposition
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