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Optical thin-film materials with low refractive index for broadband elimination of Fresnel reflection


In 1880, by studying light passing through Earth's atmosphere, Lord Rayleigh mathematically demonstrated that graded-refractive-index layers have broadband antireflection characteristics1. Graded-index coatings with different index profiles have been investigated for broadband antireflection properties, particularly with air as the ambient medium2,3,4. However, because of the unavailability of optical materials with very low refractive indices that closely match the refractive index of air, such broadband antireflection coatings have not been realizable. Here we report the fabrication of TiO2 and SiO2 graded-index films deposited by oblique-angle deposition, and, for the first time, we demonstrate their potential for antireflection coatings by virtually eliminating Fresnel reflection from an AlN–air interface over a broad range of wavelengths. This is achieved by controlling the refractive index of the TiO2 and SiO2 nanorod layers, down to a minimum value of n = 1.05 in the case of the latter, the lowest value so far reported.

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Figure 1: Low-n material.
Figure 2: Different index profiles for graded-index coating.
Figure 3: TiO2SiO2 graded-index coating.
Figure 4: Reflectivity of graded-index coating.


  1. Rayleigh, J. S. On reflection of vibrations at the confines of two media between which the transition is gradual. Proc. London Math. Soc. 11, 51–56 (1880).

    MathSciNet  MATH  Google Scholar 

  2. Southwell, W. H. Gradient-index antireflection coatings. Opt. Lett. 8, 584–586 (1983).

    Article  ADS  Google Scholar 

  3. Dobrowolski, J. A., Poitras, D., Ma, P., Vakil, H. & Acree, M. Toward perfect antireflection coatings: numerical investigation. Appl. Opt. 41, 3075–3083 (2002).

    Article  ADS  Google Scholar 

  4. Poitras, D. & Dobrowolski, J. A. Toward perfect antireflection coatings. 2. Theory. Appl. Opt. 43, 1286–1295 (2004).

    Article  ADS  Google Scholar 

  5. Vollgraff, J. A. Snellius' notes on the reflection and refraction of rays. Osiris 1, 718–725 (1936).

    Article  Google Scholar 

  6. Boutry, G. A. Augustin Fresnel: His time, life and work 1788–1827. Science Progress 36, 587–604 (1948).

    Google Scholar 

  7. Xi, J.-Q. et al. Internal high reflectivity omni-directional reflectors. Appl. Phys. Lett. 87, 031111 (2005).

    Article  ADS  Google Scholar 

  8. Xi, J.-Q. et al. Omnidirectional reflector using nanoporous SiO2 as a low-refractive-index material. Opt. Lett. 30, 1518–1520 (2005).

    Article  ADS  Google Scholar 

  9. Sharma, R., Haberer, E. D., Meier, C., Hu, E. L. & Nakamura, S. Vertically oriented GaN-based air-gap distributed Bragg reflector structure fabricated using band-gap-selective photoelectrochemical etching. Appl. Phys. Lett. 87, 051107 (2005).

    Article  ADS  Google Scholar 

  10. Ho, S.-T. et al. High index contrast mirrors for optical microcavities. Appl. Phys. Lett. 57, 1387–1389 (1990).

    Article  ADS  Google Scholar 

  11. Xu, Q., Almeida, V. R., Panepucci, R. R. & Lipson, M. Experimental demonstration of guiding and confining light in nanometer-size low-refractive-index material. Opt. Lett. 29, 1626–1628 (2004).

    Article  ADS  Google Scholar 

  12. Kim, J. K. et al. GaInN light-emitting diode with conductive omnidirectional reflector having a low-refractive-index indium-tin oxide layer. Appl. Phys. Lett. 88, 013501 (2006).

    Article  ADS  Google Scholar 

  13. Jain, A. et al. Porous silica materials as low-k dielectrics for electronic and optical interconnects. Solid Thin Films 398–399, 513–522 (2001).

    Article  ADS  Google Scholar 

  14. Xi, J.-Q., Kim, J. K. & Schubert, E. F., Silica nanorod-array films with very low refractive indices. Nano Lett. 5, 1385–1387 (2005).

    Article  ADS  Google Scholar 

  15. Xi, J.-Q. et al. Very low-refractive-index optical thin films consisting of an array of SiO2 nanorods. Opt. Lett. 31, 601–603 (2006).

    Article  ADS  Google Scholar 

  16. Hodgkinson, I. J., Horowitz, F., Macleod, H. A., Sikkens, M. & Wharton, J. J. Measurement of the principal refractive indices of thin films deposited at oblique incidence. J. Opt. Soc. Am. A 2, 1693–1697 (1985).

    Article  ADS  Google Scholar 

  17. Southwell, W. H. Pyramid-array surface-relief structures producing antireflection index matching on optical surfaces. J. Opt. Soc. Am. A 8, 549–553 (1991).

    Article  ADS  Google Scholar 

  18. Minot, M. J. Single-layer, gradient refractive index antireflection films effective from 0.35 µm to 2.5 µm. J. Opt. Soc. Am. 66, 515–519 (1976).

    Article  ADS  Google Scholar 

  19. Asahara, Y. & Izumitani, T. The properties of gradient index antireflection layer on the phase separable glass. J. Non-Crys. Solids 42, 269–279 (1980).

    Article  ADS  Google Scholar 

  20. Yoldas, B. E. & Partlow, D. P. Wide spectrum antireflective coating for fused silica and other glasses. Appl. Opt. 23, 1418–1424 (1984).

    Article  ADS  Google Scholar 

  21. Maffitt, K. N., Brueckner, H. U. & Lowrey, D. R. Polymeric optical element having antireflecting surface. US Patent 4,153,654, 8 May 1979.

  22. Wilson, S. J. & Hutley, M. C. The optical properties of “moth eye” antireflection surfaces. Opt. Acta 7, 993–1009 (1982).

    Article  ADS  Google Scholar 

  23. Wu, G. et al. Preparation and properties of scratch-resistant nano porous broadband AR silica films derived by a two-step catalytic sol-gel process. Proc. SPIE 4086, 807–810 (2000).

    Article  ADS  Google Scholar 

  24. Robbie, K. & Brett, M. J. Sculptured thin films and glancing angle deposition: Growth mechanics and applications. J. Vac. Sci. Technol. A 15, 1460–1465 (1997).

    Article  ADS  Google Scholar 

  25. Robbie, K. et al. Ultrahigh vacuum glancing angle deposition system for thin films with controlled three-dimensional nanoscale structure. Rev. Sci. Instrum. 75, 1089–1097 (2004).

    Article  ADS  Google Scholar 

  26. Kennedy, S. R. & Brett, M. J. Porous broadband antireflection coating by glancing angle deposition. Appl. Opt. 42, 4573–4579 (2003).

    Article  ADS  Google Scholar 

  27. Robbie, K., Sit, J. C. & Brett, M. J. Advanced techniques for glancing angle deposition. J. Vac. Sci. Technol. B 16, 1115–1122 (1998).

    Article  Google Scholar 

  28. Abelmann, L. & Lodder, C. Oblique evaporation and surface diffusion. Thin Solid Films 305, 1–21 (1997).

    Article  ADS  Google Scholar 

  29. Harris, K. D., Westra, K. L. & Brett, M. J. Fabrication of perforated thin films with helical and chevron pore shapes. Electrochem. Solid-State Lett. 4, C39–C42 (2001).

    Article  Google Scholar 

  30. Pulker, H. K., Paesold, G. & Ritter, E. Refractive indices of TiO2 films produced by reactive evaporation of various titanium-oxygen phases. Appl. Opt. 15, 2986–2991 (1976).

    Article  ADS  Google Scholar 

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The authors gratefully acknowledge support from Sandia National Laboratories (USA), Crystal IS Corporation (USA), Samsung Advanced Institute of Technology (Korea), the Army Research Office (USA), the New York State Office of Science, Technology and Academic Research (USA), the National Science Foundation (USA) and the Department of Energy (USA).

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Correspondence to E. Fred Schubert.

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Xi, JQ., Schubert, M., Kim, J. et al. Optical thin-film materials with low refractive index for broadband elimination of Fresnel reflection. Nature Photon 1, 176–179 (2007).

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