Wideband antireflection coatings by combining ... - OSA Publishing

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interference multilayers with structured top .... with a low-index gradient layer on the top. ... designs were evaluated using OptiLayer software, Version 5.22. .... underlying layer stack was obtained by needle optimization with a target reflectance ...
Wideband antireflection coatings by combining interference multilayers with structured top layers U. Schulz Fraunhofer Institute of Applied Optics and Precision Engineering, A.-Einstein-Str. 7, 07745 Jena, Germany [email protected]

Abstract: The residual reflectance obtained for a broad wavelength range depends mainly on the refractive index of the last layer. Using interference layer stacks composed of naturally available low- and high-index materials, the residual reflection for a broad range cannot be adjusted below a certain limit. However, nanostructured (gradient) and porous layers are effective media with a refractive index lower than that of natural materials. Results demonstrate that an interference layer stack combined with a structured layer as the last layer yields better antireflection properties owing to the low effective index of the structure. © 2009 Optical Society of America OCIS codes: (310.1219) Antireflection; (220.4000) Microstructure fabrication; (310.1860) Deposition and Fabrication; (310.1620) Interference coatings

References and links 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13. 14. 15.

S. Pongratz, and A. Zöller, “Plasma ion assisted deposition: A promising technique for optical coatings,” J. Vac. Sci. Technol. A 10(4), 1897–1904 (1992). A. Macleod, Thin-Film Optical Filters, 3rd edition (Institute of Physics Publishing, 2001). P. G. Verly, J. A. Dobrowolski, and R. R. Willey, “Fourier-transform method for the design of wideband antireflection coatings,” Appl. Opt. 31(19), 3836–3846 (1992). R. Willey, “Predicting achievable design performance of broadband antireflection coatings,” Appl. Opt. 32(28), 5447–5451 (1993). A. V. Tikhonravov, M. K. Trubetskov, T. V. Amotchkina, and J. A. Dobrowolski, “Estimation of the average residual reflectance of broadband antireflection coatings,” Appl. Opt. 47(13), C124–C130 (2008). M. Minot, “The angular reflectance of single-layer gradient refractive-index films,” J. Opt. Soc. Am. 67(8), 1046–1050 (1977). W. H. Southwell, “Pyramid-array surface-relief structures producing antireflection index matching on optical surfaces,” J. Opt. Soc. Am. A 8(3), 549–553 (1991). D. H. Raguin, and G. M. Morris, “Antireflection structured surfaces for the infrared spectral region,” Appl. Opt. 32(7), 1154–1167 (1993). J. A. Dobrowolski, D. Poitras, P. Ma, H. Vakil, and M. Acree, “Toward perfect antireflection coatings: numerical investigation,” Appl. Opt. 41(16), 3075–3083 (2002). A. Gombert, W. Glaubitt, K. Rose, J. Dreibholz, B. Bläsi, A. Heinzel, D. Sporn, W. Döll, and V. Wittwer, “Subwavelength-structured antireflective surfaces on glass,” Thin Solid Films 351(1-2), 73–78 (1999). S. Walheim, E. Schäffer, J. Mlynek, and U. Steiner, “Nanophase-separated polymer films as high-performance antireflection coatings,” Science 283(5401), 520–522 (1999). A. Kaless, P. Munzert, U. Schulz, and N. Kaiser, “Nano-motheye antireflection pattern by plasma treatment of polymers,” Surf. Coat. Tech. 20, 58–61 (2004). U. Schulz, P. Munzert, R. Leitel, I. Wendling, N. Kaiser, and A. Tünnermann, “Antireflection of transparent polymers by advanced plasma etching procedures,” Opt. Express 15(20), 13108–13111 (2007). R. Leitel, U. Schulz, N. Kaiser, and A. Tünnermann, “Stochastic subwavelength structures on poly(methyl methacrylate) surfaces for antireflection generated by plasma treatment,” Appl. Opt. 47(13), C143–C146 (2008). S. Wilbrandt, O. Stenzel, N. Kaiser, M. K. Trubetskov, and A. V. Tikhonravov, “In situ optical characterization and reengineering of interference coatings,” Appl. Opt. 47(13), C49–C54 (2008).

1. Introduction The generation of antireflective (AR) properties is a basic requirement for optical surfaces. The effects of low-index treatments and coatings on antireflection have been known since the first experiments by Joseph Fraunhofer in 1817. AR coatings have been applied in optical #108153 - $15.00 USD

(C) 2009 OSA

Received 6 Mar 2009; revised 30 Apr 2009; accepted 5 May 2009; published 11 May 2009

25 May 2009 / Vol. 17, No. 11 / OPTICS EXPRESS 8704

instruments for more than 70 years. Super wideband AR coatings (ratio of the upper wavelength limit λU to the lower wavelength limit λL of g = 2 and higher) are required for optical parts used over wide incidence ranges. These coatings are also useful in providing antireflection over an enhanced visible range for sharp curved substrates. The development of super wideband AR coatings, especially for low-index substrates, remains a challenge because of the limited availability of low-index thin-film materials and the technological difficulties in producing complex multilayers. The aim of the present study was to combine different strategies to obtain better antireflective properties. Better antireflection over a broad spectral range was demonstrated theoretically and in practice by combining interference multilayers with a low-index gradient layer on the top. This paper discusses suitable designs and the problems associated with practical implementation. 2. Methods An interference stack was deposited by electron beam evaporation of SiO2 and Ta2O5. PMMA granules (7N, Evonik) were dissolved and spin-coated on top of the interference layer. Coating and etching processes were carried out in an APS904 vacuum deposition chamber (Leybold-Optics) equipped with an advanced plasma source [1]. Oxygen used as the reactive gas was partly ionized by the argon plasma emitted from the plasma source. Argon and oxygen ions were accelerated by a self-bias voltage to impinge on the substrate with energy of up to approximately 120 eV. For the etching step the bias voltage was kept at 80 V. A Phillips XL40 scanning electron microscope (SEM) was used to visualize the nanostructures. A thin Au layer (