Golden Age of Thin Films

Golden colors catch the eye with their warm, metallic glow. Effect arises from the interaction of light with material but not all that glitters is gold. Understanding how surfaces create these golden hues reveals fascinating physics and practical applications in technology and art.

Interference Color

Above UV cutoff frequency light can travel in dielectric material. The speed of light and its refraction in medium is described by the refractive index. Thin films are layers of material just a few nanometers to micrometers thick. When light hits these transparent films, some of it reflects off the top surface, and some penetrates and reflects off the bottom surface while phase changes can take place at interfaces. The two reflected waves overlap and interfere with each other. This interference can be constructive or destructive depending on the film’s thickness, the light’s wavelength, and the angle of viewing. Fully dielectric interference filters can contain dozens of precisely tuned layers in some applications.

Constructive interference amplifies certain wavelengths, making those colors more visible. For golden colors, the film thicknesses and refractive indexes are just right to enhance wavelengths in the yellow to red range of the visible golden spectrum.

Materials That Reflect Golden Glory Inherently

Elements and molecules have unique signatures of reflectance. Light from the sun is white but the sky is blue due to scattering from nitrogen and oxygen molecules. Gold’s yellow color is essentially due to special relativity. When the charge of nucleus is, at it is, the electrons must have a certain velocity, at around 58% from the speed of light, and they show up at certain energy levels with certain band gaps, that are able to absorb blue and violet wavelengths. If the speed of light would be, say 10% less, gold would look like dark matte black or deep purple solid. If the speed of light would double, gold would be a silvery liquid like mercury. Common materials that produce golden colors naturally, with our speed of light, when their internal structure interacts with EM radiation at the visible light spectrum include:

• Gold Ascendant.  

• Titanium nitride (TiN) Commonly used to improve the hardness and scratch resistance of steel components.

• Zirconium nitride (ZrN) Brass colored

• Copper Based Alloys Brass alloys like CuZn37 and more recently developed aluminium containing alloys which are commonly used to mint coins. Similar to real gold but often described as slightly duller or with a distinct yellow-orange hue. Economical, quite resistant to tarnishing and wear. Hypoallergenic and shows antimicrobial properties.

• Ternary Au- Ag- Cu- System Basis of the most common gold jewellery, often with alloying elements such as nickel, palladium and manganese.

  
  

Ternary diagram of the Au-Cu-Ag system

Colour Measuring System

Color of the sample depends on illuminant, the sample itself and the observer. Colorimeter or spectrometer use artificial light source that simulates natural light and computer as the observer. Commonly used CIELAB method espresses the colour as three-dimensional coordinates: Luminance or brightness (L*), intensity of green-red (a*), and intensity of blue-yellow (b*).

CIELAB color space

Final Thoughts on Golden Thin Films

Physical Vapor Deposition (PVD) has been for decades a well accepted by manufacturers, allowing materials to be deposited at low temperature, virtually to any solid substrates. Additional protection and functionality can be applied to by plasma assisted chemical vapor deposition. In golden age of thin films, we can help you in NextGen Surfaces Finland Oy.