Transparent Metal Coatings

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In the article,
    Michael Scalora, Mark J. Bloemer, and Charles M. Bowden, "Metals under a new light", Optics & Photonics News, pp. 23-27 (September 1999).

the authors discuss the design and manufacture of transparent coatings that contain a substantial thickness of metal. Quoting from their article, "Some applications for transparent, conducting films include antennas embedded in windshields, electrodes on flat panel displays, electromagnetic shielding, and solar window panes". The general structure of their coatings is (D M D)^n, which is a periodic structure consisting of dielectric layers D and metal layers M. The substrate is glass (index = 1.52). The authors discuss various designs based on M being silver (Ag) because they seek a low resistance. Below is a plot of the theoretical performance of a 41-layer coating D = 36nm of TiO2, M = 25nm of Ag, and n = 20.

Plot of transmittance

For a coating containing a total of 500nm of silver, it exhibits a remarkable amount of transmittance. The transmittance band can be shifted by changing the thickness of the dielectric layers D. Also, if we allow the thickness of each dielectric layer to be different, the design can be optimized to transmit more light or a specific color light. Allowing the index of the D layer to vary, it can be shown that transmittance is increased if the index of the D layer is increased. However, TiO2 has the highest index of common dielectrics that transmit visible light.

Another design the authors consider is the 13-layer design with D = 36nm of TiO2, M = 16nm of Ag, and n = 6. This coating, containing a total of 96nm of silver, has a higher and broader transmittance, as shown below.

Plot of transmittance

One way of looking at this design is as a stack of simple induced transmission filters. Consider a single D M D period of the stack. The thickness of the M layer is selected and fixed, then the thickness of the two D layers is computed to give the greatest transmission at a single wavelength or a range of wavelengths. In general, more transmission can be "induced" through the metal layer by using more dielectric layers instead of the single D layer used here. As the number of periods n is increased, the amount of metal in the coating increases, but the transmission decreases. The transmission can be increased slightly by adding a few layers between the substrate and the stack.

There are a number of benefits of this type of coating: high conductivity, a single passband (if the metal layers are thick enough), and transmission that is not sensitive to the incident angle. Although this type of coating design is not novel (work on metal-dielectric coatings was done as early as the 1950s), there seem to be many new applications.

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