The film itself selectively reflects or absorbs incident light, and its color is the result of the optical properties of the film. The color of thin films is generated by reflected light, so two aspects need to be considered, namely the intrinsic color generated by the absorption characteristics of non transparent thin film materials for visible light spectrum, and the interference color generated by multiple reflections of transparent or slightly absorbing thin film materials.
1.Intrinsic color
The absorption characteristics of opaque thin film materials to the visible light spectrum lead to the appearance of intrinsic colors, and the most important process is the transition of photon energy absorbed by electrons. For conductive materials, the electrons absorb photon energy in the partially filled valence band to transition to an unfilled high energy state above the Fermi level, which is called an in band transition. For semiconductors or insulating materials, there is an energy gap between the valence band and the conduction band. Only electrons with absorbed energy greater than the width of the energy gap can cross the gap and transition from the valence band to the conduction band, known as interband transition. No matter what kind of transition, it will cause inconsistency between reflected light and absorbed light, which causes the material to exhibit its intrinsic color. Materials with bandgap widths greater than the visible ultraviolet limit, such as those greater than 3.5eV, are transparent to the human eye. The bandgap width of narrow bandgap materials is less than the infrared limit of the visible spectrum, and if it is less than 1.7eV, it appears black. Materials with bandwidths in the middle area can exhibit characteristic colors. Doping can cause interband transitions in materials with wide energy gaps. Doping elements create an energy level between the energy gaps, dividing them into two smaller energy intervals. Electrons that absorb lower energy may also undergo transitions, resulting in the original transparent material presenting color.
1.Interference color
Transparent or slightly absorbing thin film materials exhibit interference colors due to their multiple reflections of light. Interference is the change in amplitude that occurs after the superposition of waves. In life, if there is an oil film on the surface of the water puddle, it can be observed that the oil film presents Iridescence, which is the color produced by typical film interference. Depositing a thin layer of transparent oxide film on a metal substrate can obtain many novel colors through interference. If a single wavelength of light is incident from the atmosphere onto the surface of the transparent layer, a portion of it is reflected on the surface of the thin film and directly returns to the atmosphere; The other part undergoes refraction through the transparent film and reflects at the film substrate interface. Then continue to transmit the transparent film and refract at the interface between the film and the atmosphere before returning to the atmosphere. The two will result in optical path difference and superimposed interference.
Post time: Jun-30-2023