Engineers from University of Michigan found they could be used to obscure objects so that they appeared to be nothing more than a flat black sheet. The team suggest “forests” of the material may one day be used to cloak spacecraft in deep space.
The group says the technology works because the nanotubes’ “index of refraction [is] very close to that of air”.
This means they slow down light to a similar degree.
As a result there is very little scattering of light as it passes from the air into the layer of nanotubes.
A tank etched out of silicon viewed without carbon nanotube coating (left) and with the coating
By absorbing instead of scattering light, carbon nanotube coatings could cloak an object against a black background, such as that of deep space, the researchers note. In such cases the carbon nanotube forest “acts as a perfect magic black cloth that can completely conceal the 3-D structure of the object,” the researchers write.
We demonstrate broadband, near perfect absorption with a conformal coating of a multi-walled carbon nanotube (CNT) forest on an arbitrarily shaped surface. The complex refractive index of such a CNT forest is retrieved from the measured transmission and reflection spectra using Kramers-Kronig constrained variational analysis, which gives a typical value of neff = 1.04 + 0.01i at visible wavelengths. Therefore, when used as a conformal coating on an object, a thick layer of the CNT forest can provide an excellent impedance match to air and near perfect absorption, preventing any detectable light reflection and scattering from the object.