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Nanometre optical coatings based on strong interference effects in highly absorbing media

Nature Materials volume 12, pages 20–24 (2013)Cite this article

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Abstract

Optical coatings, which consist of one or more films of dielectric or metallic materials, are widely used in applications ranging from mirrors to eyeglasses and photography lenses1,2. Many conventional dielectric coatings rely on Fabry–Perot-type interference, involving multiple optical passes through transparent layers with thicknesses of the order of the wavelength to achieve functionalities such as anti-reflection, high-reflection and dichroism. Highly absorbing dielectrics are typically not used because it is generally accepted that light propagation through such media destroys interference effects. We show that under appropriate conditions interference can instead persist in ultrathin, highly absorbing films of a few to tens of nanometres in thickness, and demonstrate a new type of optical coating comprising such a film on a metallic substrate, which selectively absorbs various frequency ranges of the incident light. These coatings have a low sensitivity to the angle of incidence and require minimal amounts of absorbing material that can be as thin as 5–20 nm for visible light. This technology has the potential for a variety of applications from ultrathin photodetectors and solar cells to optical filters, to labelling, and even the visual arts and jewellery.

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Figure 1: Schematic of incident light from medium 1 (air) being reflected from a structure comprising dielectric medium 2 with thickness h and metallic medium 3.
Figure 2: Optical properties of the thin films.
Figure 3: Reflectivity spectra.
Figure 4: Wide variety of colours formed by coating Au with nanometre films of Ge.
Figure 5: Spectrum of colours resulting from coating Ag with nanometre films of Ge.
Figure 6: Photograph of colour images generated using multi-step patterning of ultrathin Ge films, with the edge of a United States penny included for size comparison.

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Acknowledgements

We acknowledge helpful discussions with J. Lin, N. Yu and J. Choy, and thank J. Deng and R. Sher for assistance with the measurements. We also thank L. Liu and E. Grinnell for assistance in photography. The fabrication and some of the measurements were performed at the Harvard Center for Nanoscale Systems, which is a member of the National Nanotechnology Infrastructure Network. We thank E. Mazur for access to his spectrophotometer. This research is supported in part by the Air Force Office of Scientific Research under grant number FA9550-12-1-0289. M. Kats is supported by the National Science Foundation through a Graduate Research Fellowship.

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  1. School of Engineering and Applied Sciences, Harvard University, Cambridge, Massachusetts 02138, USA

    Mikhail A. Kats, Romain Blanchard, Patrice Genevet & Federico Capasso

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  1. Mikhail A. Kats
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  2. Romain Blanchard
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  3. Patrice Genevet
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Contributions

M.A.K. developed the concept, performed the calculations and fabricated the samples. M.A.K. and R.B. characterized the samples and performed the measurements. M.A.K., R.B., P.G. and F.C. analysed and interpreted the data and implications. M.A.K., R.B. and F.C. wrote the manuscript. F.C. supervised the research.

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Correspondence to Federico Capasso.

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The authors declare no competing financial interests.

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Kats, M., Blanchard, R., Genevet, P. et al. Nanometre optical coatings based on strong interference effects in highly absorbing media. Nature Mater 12, 20–24 (2013). https://doi.org/10.1038/nmat3443

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