The mirror of the James Webb Space Telescope was plated with gold for a better reflectivity of infrared light. It reflects also red light well but not blue.

Conventional telescope mirrors were plated with aluminium for better reflectivity of visible light.

Would it be possible to combine the reflective properties of gold and aluminium (or silver) for a good reflectivity from infrared to visible light, even ultraviolet? For a future telescope with multiple sensors to cover infrared to ultraviolet light.

The metals may be combined as an alloy or several very thin layers of alternating metals.

  • $\begingroup$ Unfortunately, layers of metal do not work the way layers of dielectrics work. There is next to zero transmission thru even a thin metal layer. $\endgroup$ – Carl Witthoft Jan 13 '20 at 18:40
  • $\begingroup$ @CarlWitthoft layers can be deposited one atom thick, penetration of light will be tens to hundreds of atomic layers depending on the metal, so there's nothing impossible about alternating layers of different metals contributing to the reflectivity. Electric field drops as $$\exp\left(-\frac{2 \pi k}{\lambda} x\right)$$ and $k$ (imaginary part of the index of refraction) of gold and aluminum are roughly between 1 and 10 in the visible, so there's room for plenty of monolayers in one absorption length. $\endgroup$ – uhoh Jan 14 '20 at 0:09
  • $\begingroup$ @uhoh but you will not get Fabry-Perot-type constructive/destructive interference by doing so, so there's no advantage to creating a multilayer all-metal surface so far as optical behavior goes. $\endgroup$ – Carl Witthoft Jan 14 '20 at 12:49
  • $\begingroup$ @CarlWitthoft the part about FP is mostly correct, although FP etalons can be made from transmissive thin metal films lightmachinery.com/optics-catalog/etalons-metal-coated there could me some advantages to making alternating layers of metals; it could modify the conduction bands compared to alloying. $\endgroup$ – uhoh Jan 14 '20 at 13:40
  • $\begingroup$ But those F-Ps have a glass or air gap between the metal layers $\endgroup$ – Carl Witthoft Jan 14 '20 at 14:27

It's the individual atoms of gold that absorb blue light. Adding gold to any metal will rob some of its ability to reflect blue light. Gold already has roughly a 35% reflectivity in the blue and near UV so it is likely that you can bring that up higher by alloying with another metal. I'm not sure if aluminum would be a good choice or if a more soluble metal would be better behaved. Metal mixtures can have weird colors and become eutectic (have low melting points).

But it's not an easy call and I think it would be necessary to get some actual reflectivity data for some alloys to completely answer your question.

enter image description here

From this answer (contains several links worth reading):

In addition to the highly reflective property of the "sea" or "plasma" of conduction electrons of bulk or films of solid or liquid metals, including gold, gold also just happens to have very strong atomic absorption lines in the blue around 460 nm. If you could make gold gas and look at white light through it, the blue would be absorbed by individual gold atoms.

See Wikipedia's colored gold

enter image description here Source


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