Nanostructured Antireflection Coatings for Optical Detection and Sensing Applications
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Nanostructured Antireflection Coatings for Optical Detection and Sensing Applications Gopal G. Pethuraja1,2, Roger E. Welser1, John W. Zeller1, Yash R. Puri1, Ashok K. Sood1, Harry Efstathiadis2, Pradeep Haldar2, Nibir K. Dhar3 and Priyalal Wijewarnasuriya4 1
Magnolia Optical Technologies Inc., 52-B Cummings Park, Suite 314, Woburn, MA 01801
2
Energy and Environmental Technology Applications Center (E2TAC), College of Nanoscale Science and Engineering, Albany, NY 12203 3
DARPA/MTO, 675 North Randolph Street, Arlington, VA 22203
4
U.S. Army Research Laboratory, 2800 Powder Mill Road, Adelphi, MD 20783
ABSTRACT Optical components such as lenses, glass windows, and prisms are subject to Fresnel reflection due to the mismatch between the refractive indices of the air and glass. An optical interface layer, i.e., antireflection (AR) layer, is needed to eliminate this unwanted reflection at the air/glass interface. Nanostructured broadband and wide-angle AR structures have been developed using a scalable self-assembly process. Ultra-high performance of the nanostructured AR coatings has been demonstrated on various substrates such as quartz, sapphire, polymer, and other materials typically employed in optical lenses. AR coatings on polycarbonate lead to optical transmittance enhancement from approximately 90% to almost 100% for the entire visible, and part of the near-infrared (NIR), band. The AR coatings have also been demonstrated on curved surfaces. AR coatings on n-BK7 lenses enable ultra-high light transmittance for the entire visible, and most of the NIR, spectrum. Nanostructured oxide layers with step-graded index profiles, deposited onto the optical elements of an optical system, can significantly increase sensitivity, and hence improve the overall performance of the system. INTRODUCTION Incident light on optical windows and lenses will partially undergo Fresnel reflection due to the mismatch between the refractive indices of the air and glass. For normal light incidence, the Fresnel reflection at the interface of two mediums having refractive indices n1 and n2 is given by: − = + The reflection loss at the air/glass-lens interface is typically around 4% from the lens surface (~8% from both surface interfaces of the lens) at normal light incidence. This loss can be greater than 40% for off-angle light incidence. An optical interface layer with intermediate refractive indices at the air/glass interface can eliminate this unwanted reflection. Conventionally, a singlelayer coating with optical thickness equal to one-quarter of the wavelength (λ/4) of interest has been used as an antireflection (AR) coating. Ideally, such single-layer AR coating should have a refractive index, nλ/4, as given by: × / =
where ns and na are the t refractivee indices of the substratee and air, respectively. Often, due tto the unavailab bility of matterials with desired d preccise refractivve index valuues, the perfformance off such λ/4 AR coatings c dev viates from th he optimum m. This is esppecially the case for low w refractive
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