Optical coatings
A vast majority of scientific and industrial optics have a thin-layer coating to avoid ghost images, back reflections, safety hazard or destroying expensive equipments. But by having a thin-film coating critical properties can be introduced in the optics which uses them. Conventional dielectric optical coatings, which are a key component of almost every optical device, are typically made of layers of transparent material with each layer being at least a quarter wavelength of light in thickness.
Nano coating
Researchers at Harvard University have made an optical coating which can change colour when its thickness is varied by less than 20 nm thick. By changing the thickness the colour of metal surfaces could be customized to the required one.
The new ultrathin optical coatings is nanometre-thick, and nearly opaque, highly light-absorbing dielectric materials, such as semiconductors. By adding a 7 nm layer of germanium to the surface of a gold sample its colour can be changed from gold to pink. Further addition of another 4 nm layer makes it violet, and another 4 nm still turns the coating dark blue.
Phenomenon
The colours appear so due to interference of light waves as they pass through light-absorbing germanium coating below and then reflect back up. Some wavelengths of incident and reflected light constructively interfere with one another and are “boosted”, while others destructively interfere and are absorbed. According to researchers a difference of only a few atoms’ thickness across the coating is enough to produce the dramatic colour shifts. By changing the thickness of the films the interference conditions are changed and thus can control which wavelengths will end up reflected and which will end up absorbed in the thin layer to change the colour of the coating. The researchers have also already tried applying the germanium coating to a silver surface, which then appears gold at certain thicknesses as well as a range of pastel colours. The coating process is fairly simple involving standard lithography and physical vapour deposition techniques.
Applications
The technology has potential applications such as for producing brilliant colours. The colour changing phenomenon can be used for making colour full pretty jewellery. Also, other host of technologically advanced devices, device applications that employ light-absorbing semiconductor layers including ultrathin light detectors and filters, displays, modulators and even solar cells can make use of this technique.