How can I change the background of my mirror images from yellow color ( because of Au thin film coating) to colorless?
If you can alloy gold MAu (M: alkali metal), then you might have a colorless film, since they show a metal (Au) to semiconductor transition[1-4] (MAu).
1.a Electronic structure of CsAu http://www.sciencedirect.com/science/article/pii/0038109883905161
1.b Preparation and characterization of thin CsAu films http://www.sciencedirect.com/science/article/pii/0040609091905242
2. Optical properties of RbAu and CsAu https://journals.aps.org/prb/abstract/10.1103/PhysRevB.12.3008
3..a Chemical trends in gold alkali alloys... Part I... http://www.sciencedirect.com/science/article/pii/0925838896800302 3..b Chemical trends in gold alkali alloys... Part II... http://www.sciencedirect.com/science/article/pii/0925838896800314
4. Modulation of the gold particle–plasmon resonance by the metal–semiconductor ... http://iopscience.iop.org/article/10.1088/1464-4258/10/5/055202/meta
(I learn something new every time I try to answer a question...)
The responses from Ioannis and U. Dreher are very instructive, but I am not sure that a mirror made with these materials would give the reflectance value that you need.
If the Au coating is really required for its high reflectance in the the near and mid-IR, then I am afraid that there is not easy solution to remove the yellowish reflectance in the visible. You could adda multilayer dielectric coating on top of the Au to increase its reflectance of blue, but it would certainly lead to a reduction of the reflectance in some portions of the near and mid-IR. The optimization of the coating would thus depend on where in the IR you don't need a high reflectance.
Also, if the yellowish color is undesirable from a 'cosmetic' point of view (on photographs or films taken using the Au mirror), maybe you can change the camera settings, or use a software to post-process the images by changing the color and illumination.
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