Gold films colored with nanometer-thick layers of germanium.
Solar cells, photodetectors and even jewelry could benefit from a new type of absorptive semiconductor-film optical coating developed by researchers in Federico Capasso’s group at Harvard University (Nature Mater., doi:10.1038/nmat3443). A layer of this coating just a few atoms thick can change the apparent color of the underlying material.
Many modern optical coatings consist of carefully optimized layers of dielectric and metallic thin films. The Harvard group, led by graduate student Mikhail Kats, modified the usual reflectivity of gold surfaces by spraying them with electron-beam-evaporated germanium to create layers between 7 and 25 nm deep—a fraction of the wavelength of visible light.
The researchers gave the gold pieces four different colors with various thicknesses of germanium: light pink (7 nm), purple (11 nm), dark blue (15 nm) and light blue (25 nm). Similar experiments with silver yielded a variety of pastel tones.
Germanium is a lossy dielectric, meaning it strongly absorbs the incoming radiation. In such a lossy material, the reflection and transmission phase shifts at the boundaries of the material do not always equal 0 or π, allowing researchers to manipulate the resulting absorption resonance into the design of the object they are building.
Unlike conventional coatings, the germanium coatings have a low sensitivity to the angle of incidence of the impinging light, so the coated materials will seem to have the same color to observers at different positions.
Over the past few years, optical scientists have devised several new methods for coloring metals, including femtosecond laser ablation. Unlike that technique, the Harvard group’s method is reversible. Also, the ultrathin films, if used in new types of detectors, would take less semiconductor material than other coatings, thus resulting in cost savings.