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

Journal name:
Nature Materials
Volume:
12,
Pages:
20–24
Year published:
DOI:
doi:10.1038/nmat3443
Received
Accepted
Published online

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.

At a glance

Figures

  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 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.

    a, The case of a perfect electric conductor (PEC) and a lossless dielectric. As there is no absorption and no penetration into the metal, the reflectivity equals unity at all wavelengths. Structures approaching this limit can be used as phase-shifting elements, which are known as Gires–Tournois etalons. b, An absorbing dielectric on a PEC substrate supports an absorption resonance for h ~ mλ/4n2assuming that the losses (k2) are relatively small and m is an odd integer. No resonance exists for h smaller than λ/4n2. c, A lossless dielectric on a substrate with finite optical conductivity (for example, Au at visible frequencies) can support a resonance for hλ/4n2 owing to the non-trivial phase shifts at the interface between medium 2 and medium 3, but the total absorption is small because the only loss mechanism is the one associated with the finite reflectivity of the metal. d, An ultrathin (hλ/4n2) absorbing dielectric on Au at visible frequencies can support a strong and widely tailorable absorption resonance.

  2. Optical properties of the thin films.
    Figure 2: Optical properties of the thin films.

    a, Real and imaginary parts of the complex refractive indices of Au and Ge, obtained by variable-angle spectroscopic ellipsometry. b, Near-normal incidence (7°) reflection spectra of thick Au coated with 7, 10, 15, 20 and 25 nm of Ge. Inset: schematic of the Ge film on a Au substrate, showing a partial wave decomposition. c, Calculated reflection spectra using equation (1) and the optical constants in a corresponding to the measurement in b. d, Calculated fraction of the total incident light that is absorbed within the Ge layer.

  3. Reflectivity spectra.
    Figure 3: Reflectivity spectra.

    a,b, Experimental reflectivity spectra for s- and p-polarization, respectively, for angles of incidence from 20° to 80° for an Au film coated with 15 nm of Ge (the value of reflectivity is indicated by the colour bars). c,d, The calculated spectra corresponding to those in a,b using equation (1).

  4. Wide variety of colours formed by coating Au with nanometre films of Ge.
    Figure 4: Wide variety of colours formed by coating Au with nanometre films of Ge.

    ah, 0, 3, 5, 7, 10, 15, 20 and 25 nm of Ge deposited on optically thick Au, which was deposited on polished silicon. The clip marks from mounting in the electron-beam evaporator are visible. ik, 0, 10 and 20 nm of Ge deposited over 150 nm of Au, on a rough (unpolished) silicon substrate.

  5. Spectrum of colours resulting from coating Ag with nanometre films of Ge.
    Figure 5: Spectrum of colours resulting from coating Ag with nanometre films of Ge.

    ae, Thick Ag films coated with 0, 3, 5, 7 and 10 nm of evaporated Ge, respectively.

  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.
    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.

    Five steps of photolithography with alignment are used to selectively deposit an optically thick layer of Au on a glass slide, followed by Ge layers of either 7, 11, 15 or 25 nm. This yields light pink, purple, dark blue and light blue colours, respectively. Among the demonstrated patterns are the logo and shield of the School of Engineering and Applied Sciences; these are a trademark of Harvard University, and are protected by copyright; they are used in this research with permission.

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Affiliations

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

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

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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The authors declare no competing financial interests.

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